libgo: update to go1.9

[official-gcc.git] / libgo / go / database / sql / sql.go

// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package sql provides a generic interface around SQL (or SQL-like)
// databases.
//
// The sql package must be used in conjunction with a database driver.
// See https://golang.org/s/sqldrivers for a list of drivers.
//
// Drivers that do not support context cancelation will not return until
// after the query is completed.
//
// For usage examples, see the wiki page at
// https://golang.org/s/sqlwiki.
package sql

import (
        "context"
        "database/sql/driver"
        "errors"
        "fmt"
        "io"
        "reflect"
        "runtime"
        "sort"
        "sync"
        "sync/atomic"
        "time"
)

var (
        driversMu sync.RWMutex
        drivers   = make(map[string]driver.Driver)
)

// nowFunc returns the current time; it's overridden in tests.
var nowFunc = time.Now

// Register makes a database driver available by the provided name.
// If Register is called twice with the same name or if driver is nil,
// it panics.
func Register(name string, driver driver.Driver) {
        driversMu.Lock()
        defer driversMu.Unlock()
        if driver == nil {
                panic("sql: Register driver is nil")
        }
        if _, dup := drivers[name]; dup {
                panic("sql: Register called twice for driver " + name)
        }
        drivers[name] = driver
}

func unregisterAllDrivers() {
        driversMu.Lock()
        defer driversMu.Unlock()
        // For tests.
        drivers = make(map[string]driver.Driver)
}

// Drivers returns a sorted list of the names of the registered drivers.
func Drivers() []string {
        driversMu.RLock()
        defer driversMu.RUnlock()
        var list []string
        for name := range drivers {
                list = append(list, name)
        }
        sort.Strings(list)
        return list
}

// A NamedArg is a named argument. NamedArg values may be used as
// arguments to Query or Exec and bind to the corresponding named
// parameter in the SQL statement.
//
// For a more concise way to create NamedArg values, see
// the Named function.
type NamedArg struct {
        _Named_Fields_Required struct{}

        // Name is the name of the parameter placeholder.
        //
        // If empty, the ordinal position in the argument list will be
        // used.
        //
        // Name must omit any symbol prefix.
        Name string

        // Value is the value of the parameter.
        // It may be assigned the same value types as the query
        // arguments.
        Value interface{}
}

// Named provides a more concise way to create NamedArg values.
//
// Example usage:
//
//     db.ExecContext(ctx, `
//         delete from Invoice
//         where
//             TimeCreated < @end
//             and TimeCreated >= @start;`,
//         sql.Named("start", startTime),
//         sql.Named("end", endTime),
//     )
func Named(name string, value interface{}) NamedArg {
        // This method exists because the go1compat promise
        // doesn't guarantee that structs don't grow more fields,
        // so unkeyed struct literals are a vet error. Thus, we don't
        // want to allow sql.NamedArg{name, value}.
        return NamedArg{Name: name, Value: value}
}

// IsolationLevel is the transaction isolation level used in TxOptions.
type IsolationLevel int

// Various isolation levels that drivers may support in BeginTx.
// If a driver does not support a given isolation level an error may be returned.
//
// See https://en.wikipedia.org/wiki/Isolation_(database_systems)#Isolation_levels.
const (
        LevelDefault IsolationLevel = iota
        LevelReadUncommitted
        LevelReadCommitted
        LevelWriteCommitted
        LevelRepeatableRead
        LevelSnapshot
        LevelSerializable
        LevelLinearizable
)

// TxOptions holds the transaction options to be used in DB.BeginTx.
type TxOptions struct {
        // Isolation is the transaction isolation level.
        // If zero, the driver or database's default level is used.
        Isolation IsolationLevel
        ReadOnly  bool
}

// RawBytes is a byte slice that holds a reference to memory owned by
// the database itself. After a Scan into a RawBytes, the slice is only
// valid until the next call to Next, Scan, or Close.
type RawBytes []byte

// NullString represents a string that may be null.
// NullString implements the Scanner interface so
// it can be used as a scan destination:
//
//  var s NullString
//  err := db.QueryRow("SELECT name FROM foo WHERE id=?", id).Scan(&s)
//  ...
//  if s.Valid {
//     // use s.String
//  } else {
//     // NULL value
//  }
//
type NullString struct {
        String string
        Valid  bool // Valid is true if String is not NULL
}

// Scan implements the Scanner interface.
func (ns *NullString) Scan(value interface{}) error {
        if value == nil {
                ns.String, ns.Valid = "", false
                return nil
        }
        ns.Valid = true
        return convertAssign(&ns.String, value)
}

// Value implements the driver Valuer interface.
func (ns NullString) Value() (driver.Value, error) {
        if !ns.Valid {
                return nil, nil
        }
        return ns.String, nil
}

// NullInt64 represents an int64 that may be null.
// NullInt64 implements the Scanner interface so
// it can be used as a scan destination, similar to NullString.
type NullInt64 struct {
        Int64 int64
        Valid bool // Valid is true if Int64 is not NULL
}

// Scan implements the Scanner interface.
func (n *NullInt64) Scan(value interface{}) error {
        if value == nil {
                n.Int64, n.Valid = 0, false
                return nil
        }
        n.Valid = true
        return convertAssign(&n.Int64, value)
}

// Value implements the driver Valuer interface.
func (n NullInt64) Value() (driver.Value, error) {
        if !n.Valid {
                return nil, nil
        }
        return n.Int64, nil
}

// NullFloat64 represents a float64 that may be null.
// NullFloat64 implements the Scanner interface so
// it can be used as a scan destination, similar to NullString.
type NullFloat64 struct {
        Float64 float64
        Valid   bool // Valid is true if Float64 is not NULL
}

// Scan implements the Scanner interface.
func (n *NullFloat64) Scan(value interface{}) error {
        if value == nil {
                n.Float64, n.Valid = 0, false
                return nil
        }
        n.Valid = true
        return convertAssign(&n.Float64, value)
}

// Value implements the driver Valuer interface.
func (n NullFloat64) Value() (driver.Value, error) {
        if !n.Valid {
                return nil, nil
        }
        return n.Float64, nil
}

// NullBool represents a bool that may be null.
// NullBool implements the Scanner interface so
// it can be used as a scan destination, similar to NullString.
type NullBool struct {
        Bool  bool
        Valid bool // Valid is true if Bool is not NULL
}

// Scan implements the Scanner interface.
func (n *NullBool) Scan(value interface{}) error {
        if value == nil {
                n.Bool, n.Valid = false, false
                return nil
        }
        n.Valid = true
        return convertAssign(&n.Bool, value)
}

// Value implements the driver Valuer interface.
func (n NullBool) Value() (driver.Value, error) {
        if !n.Valid {
                return nil, nil
        }
        return n.Bool, nil
}

// Scanner is an interface used by Scan.
type Scanner interface {
        // Scan assigns a value from a database driver.
        //
        // The src value will be of one of the following types:
        //
        //    int64
        //    float64
        //    bool
        //    []byte
        //    string
        //    time.Time
        //    nil - for NULL values
        //
        // An error should be returned if the value cannot be stored
        // without loss of information.
        Scan(src interface{}) error
}

// Out may be used to retrieve OUTPUT value parameters from stored procedures.
//
// Not all drivers and databases support OUTPUT value parameters.
//
// Example usage:
//
//   var outArg string
//   _, err := db.ExecContext(ctx, "ProcName", sql.Named("Arg1", Out{Dest: &outArg}))
type Out struct {
        _Named_Fields_Required struct{}

        // Dest is a pointer to the value that will be set to the result of the
        // stored procedure's OUTPUT parameter.
        Dest interface{}

        // In is whether the parameter is an INOUT parameter. If so, the input value to the stored
        // procedure is the dereferenced value of Dest's pointer, which is then replaced with
        // the output value.
        In bool
}

// ErrNoRows is returned by Scan when QueryRow doesn't return a
// row. In such a case, QueryRow returns a placeholder *Row value that
// defers this error until a Scan.
var ErrNoRows = errors.New("sql: no rows in result set")

// DB is a database handle representing a pool of zero or more
// underlying connections. It's safe for concurrent use by multiple
// goroutines.
//
// The sql package creates and frees connections automatically; it
// also maintains a free pool of idle connections. If the database has
// a concept of per-connection state, such state can only be reliably
// observed within a transaction. Once DB.Begin is called, the
// returned Tx is bound to a single connection. Once Commit or
// Rollback is called on the transaction, that transaction's
// connection is returned to DB's idle connection pool. The pool size
// can be controlled with SetMaxIdleConns.
type DB struct {
        driver driver.Driver
        dsn    string
        // numClosed is an atomic counter which represents a total number of
        // closed connections. Stmt.openStmt checks it before cleaning closed
        // connections in Stmt.css.
        numClosed uint64

        mu           sync.Mutex // protects following fields
        freeConn     []*driverConn
        connRequests map[uint64]chan connRequest
        nextRequest  uint64 // Next key to use in connRequests.
        numOpen      int    // number of opened and pending open connections
        // Used to signal the need for new connections
        // a goroutine running connectionOpener() reads on this chan and
        // maybeOpenNewConnections sends on the chan (one send per needed connection)
        // It is closed during db.Close(). The close tells the connectionOpener
        // goroutine to exit.
        openerCh    chan struct{}
        closed      bool
        dep         map[finalCloser]depSet
        lastPut     map[*driverConn]string // stacktrace of last conn's put; debug only
        maxIdle     int                    // zero means defaultMaxIdleConns; negative means 0
        maxOpen     int                    // <= 0 means unlimited
        maxLifetime time.Duration          // maximum amount of time a connection may be reused
        cleanerCh   chan struct{}
}

// connReuseStrategy determines how (*DB).conn returns database connections.
type connReuseStrategy uint8

const (
        // alwaysNewConn forces a new connection to the database.
        alwaysNewConn connReuseStrategy = iota
        // cachedOrNewConn returns a cached connection, if available, else waits
        // for one to become available (if MaxOpenConns has been reached) or
        // creates a new database connection.
        cachedOrNewConn
)

// driverConn wraps a driver.Conn with a mutex, to
// be held during all calls into the Conn. (including any calls onto
// interfaces returned via that Conn, such as calls on Tx, Stmt,
// Result, Rows)
type driverConn struct {
        db        *DB
        createdAt time.Time

        sync.Mutex  // guards following
        ci          driver.Conn
        closed      bool
        finalClosed bool // ci.Close has been called
        openStmt    map[*driverStmt]bool

        // guarded by db.mu
        inUse      bool
        onPut      []func() // code (with db.mu held) run when conn is next returned
        dbmuClosed bool     // same as closed, but guarded by db.mu, for removeClosedStmtLocked
}

func (dc *driverConn) releaseConn(err error) {
        dc.db.putConn(dc, err)
}

func (dc *driverConn) removeOpenStmt(ds *driverStmt) {
        dc.Lock()
        defer dc.Unlock()
        delete(dc.openStmt, ds)
}

func (dc *driverConn) expired(timeout time.Duration) bool {
        if timeout <= 0 {
                return false
        }
        return dc.createdAt.Add(timeout).Before(nowFunc())
}

// prepareLocked prepares the query on dc. When cg == nil the dc must keep track of
// the prepared statements in a pool.
func (dc *driverConn) prepareLocked(ctx context.Context, cg stmtConnGrabber, query string) (*driverStmt, error) {
        si, err := ctxDriverPrepare(ctx, dc.ci, query)
        if err != nil {
                return nil, err
        }
        ds := &driverStmt{Locker: dc, si: si}

        // No need to manage open statements if there is a single connection grabber.
        if cg != nil {
                return ds, nil
        }

        // Track each driverConn's open statements, so we can close them
        // before closing the conn.
        //
        // Wrap all driver.Stmt is *driverStmt to ensure they are only closed once.
        if dc.openStmt == nil {
                dc.openStmt = make(map[*driverStmt]bool)
        }
        dc.openStmt[ds] = true
        return ds, nil
}

// the dc.db's Mutex is held.
func (dc *driverConn) closeDBLocked() func() error {
        dc.Lock()
        defer dc.Unlock()
        if dc.closed {
                return func() error { return errors.New("sql: duplicate driverConn close") }
        }
        dc.closed = true
        return dc.db.removeDepLocked(dc, dc)
}

func (dc *driverConn) Close() error {
        dc.Lock()
        if dc.closed {
                dc.Unlock()
                return errors.New("sql: duplicate driverConn close")
        }
        dc.closed = true
        dc.Unlock() // not defer; removeDep finalClose calls may need to lock

        // And now updates that require holding dc.mu.Lock.
        dc.db.mu.Lock()
        dc.dbmuClosed = true
        fn := dc.db.removeDepLocked(dc, dc)
        dc.db.mu.Unlock()
        return fn()
}

func (dc *driverConn) finalClose() error {
        var err error

        // Each *driverStmt has a lock to the dc. Copy the list out of the dc
        // before calling close on each stmt.
        var openStmt []*driverStmt
        withLock(dc, func() {
                openStmt = make([]*driverStmt, 0, len(dc.openStmt))
                for ds := range dc.openStmt {
                        openStmt = append(openStmt, ds)
                }
                dc.openStmt = nil
        })
        for _, ds := range openStmt {
                ds.Close()
        }
        withLock(dc, func() {
                dc.finalClosed = true
                err = dc.ci.Close()
                dc.ci = nil
        })

        dc.db.mu.Lock()
        dc.db.numOpen--
        dc.db.maybeOpenNewConnections()
        dc.db.mu.Unlock()

        atomic.AddUint64(&dc.db.numClosed, 1)
        return err
}

// driverStmt associates a driver.Stmt with the
// *driverConn from which it came, so the driverConn's lock can be
// held during calls.
type driverStmt struct {
        sync.Locker // the *driverConn
        si          driver.Stmt
        closed      bool
        closeErr    error // return value of previous Close call
}

// Close ensures dirver.Stmt is only closed once any always returns the same
// result.
func (ds *driverStmt) Close() error {
        ds.Lock()
        defer ds.Unlock()
        if ds.closed {
                return ds.closeErr
        }
        ds.closed = true
        ds.closeErr = ds.si.Close()
        return ds.closeErr
}

// depSet is a finalCloser's outstanding dependencies
type depSet map[interface{}]bool // set of true bools

// The finalCloser interface is used by (*DB).addDep and related
// dependency reference counting.
type finalCloser interface {
        // finalClose is called when the reference count of an object
        // goes to zero. (*DB).mu is not held while calling it.
        finalClose() error
}

// addDep notes that x now depends on dep, and x's finalClose won't be
// called until all of x's dependencies are removed with removeDep.
func (db *DB) addDep(x finalCloser, dep interface{}) {
        //println(fmt.Sprintf("addDep(%T %p, %T %p)", x, x, dep, dep))
        db.mu.Lock()
        defer db.mu.Unlock()
        db.addDepLocked(x, dep)
}

func (db *DB) addDepLocked(x finalCloser, dep interface{}) {
        if db.dep == nil {
                db.dep = make(map[finalCloser]depSet)
        }
        xdep := db.dep[x]
        if xdep == nil {
                xdep = make(depSet)
                db.dep[x] = xdep
        }
        xdep[dep] = true
}

// removeDep notes that x no longer depends on dep.
// If x still has dependencies, nil is returned.
// If x no longer has any dependencies, its finalClose method will be
// called and its error value will be returned.
func (db *DB) removeDep(x finalCloser, dep interface{}) error {
        db.mu.Lock()
        fn := db.removeDepLocked(x, dep)
        db.mu.Unlock()
        return fn()
}

func (db *DB) removeDepLocked(x finalCloser, dep interface{}) func() error {
        //println(fmt.Sprintf("removeDep(%T %p, %T %p)", x, x, dep, dep))

        xdep, ok := db.dep[x]
        if !ok {
                panic(fmt.Sprintf("unpaired removeDep: no deps for %T", x))
        }

        l0 := len(xdep)
        delete(xdep, dep)

        switch len(xdep) {
        case l0:
                // Nothing removed. Shouldn't happen.
                panic(fmt.Sprintf("unpaired removeDep: no %T dep on %T", dep, x))
        case 0:
                // No more dependencies.
                delete(db.dep, x)
                return x.finalClose
        default:
                // Dependencies remain.
                return func() error { return nil }
        }
}

// This is the size of the connectionOpener request chan (DB.openerCh).
// This value should be larger than the maximum typical value
// used for db.maxOpen. If maxOpen is significantly larger than
// connectionRequestQueueSize then it is possible for ALL calls into the *DB
// to block until the connectionOpener can satisfy the backlog of requests.
var connectionRequestQueueSize = 1000000

// Open opens a database specified by its database driver name and a
// driver-specific data source name, usually consisting of at least a
// database name and connection information.
//
// Most users will open a database via a driver-specific connection
// helper function that returns a *DB. No database drivers are included
// in the Go standard library. See https://golang.org/s/sqldrivers for
// a list of third-party drivers.
//
// Open may just validate its arguments without creating a connection
// to the database. To verify that the data source name is valid, call
// Ping.
//
// The returned DB is safe for concurrent use by multiple goroutines
// and maintains its own pool of idle connections. Thus, the Open
// function should be called just once. It is rarely necessary to
// close a DB.
func Open(driverName, dataSourceName string) (*DB, error) {
        driversMu.RLock()
        driveri, ok := drivers[driverName]
        driversMu.RUnlock()
        if !ok {
                return nil, fmt.Errorf("sql: unknown driver %q (forgotten import?)", driverName)
        }
        db := &DB{
                driver:       driveri,
                dsn:          dataSourceName,
                openerCh:     make(chan struct{}, connectionRequestQueueSize),
                lastPut:      make(map[*driverConn]string),
                connRequests: make(map[uint64]chan connRequest),
        }
        go db.connectionOpener()
        return db, nil
}

func (db *DB) pingDC(ctx context.Context, dc *driverConn, release func(error)) error {
        var err error
        if pinger, ok := dc.ci.(driver.Pinger); ok {
                withLock(dc, func() {
                        err = pinger.Ping(ctx)
                })
        }
        release(err)
        return err
}

// PingContext verifies a connection to the database is still alive,
// establishing a connection if necessary.
func (db *DB) PingContext(ctx context.Context) error {
        var dc *driverConn
        var err error

        for i := 0; i < maxBadConnRetries; i++ {
                dc, err = db.conn(ctx, cachedOrNewConn)
                if err != driver.ErrBadConn {
                        break
                }
        }
        if err == driver.ErrBadConn {
                dc, err = db.conn(ctx, alwaysNewConn)
        }
        if err != nil {
                return err
        }

        return db.pingDC(ctx, dc, dc.releaseConn)
}

// Ping verifies a connection to the database is still alive,
// establishing a connection if necessary.
func (db *DB) Ping() error {
        return db.PingContext(context.Background())
}

// Close closes the database, releasing any open resources.
//
// It is rare to Close a DB, as the DB handle is meant to be
// long-lived and shared between many goroutines.
func (db *DB) Close() error {
        db.mu.Lock()
        if db.closed { // Make DB.Close idempotent
                db.mu.Unlock()
                return nil
        }
        close(db.openerCh)
        if db.cleanerCh != nil {
                close(db.cleanerCh)
        }
        var err error
        fns := make([]func() error, 0, len(db.freeConn))
        for _, dc := range db.freeConn {
                fns = append(fns, dc.closeDBLocked())
        }
        db.freeConn = nil
        db.closed = true
        for _, req := range db.connRequests {
                close(req)
        }
        db.mu.Unlock()
        for _, fn := range fns {
                err1 := fn()
                if err1 != nil {
                        err = err1
                }
        }
        return err
}

const defaultMaxIdleConns = 2

func (db *DB) maxIdleConnsLocked() int {
        n := db.maxIdle
        switch {
        case n == 0:
                // TODO(bradfitz): ask driver, if supported, for its default preference
                return defaultMaxIdleConns
        case n < 0:
                return 0
        default:
                return n
        }
}

// SetMaxIdleConns sets the maximum number of connections in the idle
// connection pool.
//
// If MaxOpenConns is greater than 0 but less than the new MaxIdleConns
// then the new MaxIdleConns will be reduced to match the MaxOpenConns limit
//
// If n <= 0, no idle connections are retained.
func (db *DB) SetMaxIdleConns(n int) {
        db.mu.Lock()
        if n > 0 {
                db.maxIdle = n
        } else {
                // No idle connections.
                db.maxIdle = -1
        }
        // Make sure maxIdle doesn't exceed maxOpen
        if db.maxOpen > 0 && db.maxIdleConnsLocked() > db.maxOpen {
                db.maxIdle = db.maxOpen
        }
        var closing []*driverConn
        idleCount := len(db.freeConn)
        maxIdle := db.maxIdleConnsLocked()
        if idleCount > maxIdle {
                closing = db.freeConn[maxIdle:]
                db.freeConn = db.freeConn[:maxIdle]
        }
        db.mu.Unlock()
        for _, c := range closing {
                c.Close()
        }
}

// SetMaxOpenConns sets the maximum number of open connections to the database.
//
// If MaxIdleConns is greater than 0 and the new MaxOpenConns is less than
// MaxIdleConns, then MaxIdleConns will be reduced to match the new
// MaxOpenConns limit
//
// If n <= 0, then there is no limit on the number of open connections.
// The default is 0 (unlimited).
func (db *DB) SetMaxOpenConns(n int) {
        db.mu.Lock()
        db.maxOpen = n
        if n < 0 {
                db.maxOpen = 0
        }
        syncMaxIdle := db.maxOpen > 0 && db.maxIdleConnsLocked() > db.maxOpen
        db.mu.Unlock()
        if syncMaxIdle {
                db.SetMaxIdleConns(n)
        }
}

// SetConnMaxLifetime sets the maximum amount of time a connection may be reused.
//
// Expired connections may be closed lazily before reuse.
//
// If d <= 0, connections are reused forever.
func (db *DB) SetConnMaxLifetime(d time.Duration) {
        if d < 0 {
                d = 0
        }
        db.mu.Lock()
        // wake cleaner up when lifetime is shortened.
        if d > 0 && d < db.maxLifetime && db.cleanerCh != nil {
                select {
                case db.cleanerCh <- struct{}{}:
                default:
                }
        }
        db.maxLifetime = d
        db.startCleanerLocked()
        db.mu.Unlock()
}

// startCleanerLocked starts connectionCleaner if needed.
func (db *DB) startCleanerLocked() {
        if db.maxLifetime > 0 && db.numOpen > 0 && db.cleanerCh == nil {
                db.cleanerCh = make(chan struct{}, 1)
                go db.connectionCleaner(db.maxLifetime)
        }
}

func (db *DB) connectionCleaner(d time.Duration) {
        const minInterval = time.Second

        if d < minInterval {
                d = minInterval
        }
        t := time.NewTimer(d)

        for {
                select {
                case <-t.C:
                case <-db.cleanerCh: // maxLifetime was changed or db was closed.
                }

                db.mu.Lock()
                d = db.maxLifetime
                if db.closed || db.numOpen == 0 || d <= 0 {
                        db.cleanerCh = nil
                        db.mu.Unlock()
                        return
                }

                expiredSince := nowFunc().Add(-d)
                var closing []*driverConn
                for i := 0; i < len(db.freeConn); i++ {
                        c := db.freeConn[i]
                        if c.createdAt.Before(expiredSince) {
                                closing = append(closing, c)
                                last := len(db.freeConn) - 1
                                db.freeConn[i] = db.freeConn[last]
                                db.freeConn[last] = nil
                                db.freeConn = db.freeConn[:last]
                                i--
                        }
                }
                db.mu.Unlock()

                for _, c := range closing {
                        c.Close()
                }

                if d < minInterval {
                        d = minInterval
                }
                t.Reset(d)
        }
}

// DBStats contains database statistics.
type DBStats struct {
        // OpenConnections is the number of open connections to the database.
        OpenConnections int
}

// Stats returns database statistics.
func (db *DB) Stats() DBStats {
        db.mu.Lock()
        stats := DBStats{
                OpenConnections: db.numOpen,
        }
        db.mu.Unlock()
        return stats
}

// Assumes db.mu is locked.
// If there are connRequests and the connection limit hasn't been reached,
// then tell the connectionOpener to open new connections.
func (db *DB) maybeOpenNewConnections() {
        numRequests := len(db.connRequests)
        if db.maxOpen > 0 {
                numCanOpen := db.maxOpen - db.numOpen
                if numRequests > numCanOpen {
                        numRequests = numCanOpen
                }
        }
        for numRequests > 0 {
                db.numOpen++ // optimistically
                numRequests--
                if db.closed {
                        return
                }
                db.openerCh <- struct{}{}
        }
}

// Runs in a separate goroutine, opens new connections when requested.
func (db *DB) connectionOpener() {
        for range db.openerCh {
                db.openNewConnection()
        }
}

// Open one new connection
func (db *DB) openNewConnection() {
        // maybeOpenNewConnctions has already executed db.numOpen++ before it sent
        // on db.openerCh. This function must execute db.numOpen-- if the
        // connection fails or is closed before returning.
        ci, err := db.driver.Open(db.dsn)
        db.mu.Lock()
        defer db.mu.Unlock()
        if db.closed {
                if err == nil {
                        ci.Close()
                }
                db.numOpen--
                return
        }
        if err != nil {
                db.numOpen--
                db.putConnDBLocked(nil, err)
                db.maybeOpenNewConnections()
                return
        }
        dc := &driverConn{
                db:        db,
                createdAt: nowFunc(),
                ci:        ci,
        }
        if db.putConnDBLocked(dc, err) {
                db.addDepLocked(dc, dc)
        } else {
                db.numOpen--
                ci.Close()
        }
}

// connRequest represents one request for a new connection
// When there are no idle connections available, DB.conn will create
// a new connRequest and put it on the db.connRequests list.
type connRequest struct {
        conn *driverConn
        err  error
}

var errDBClosed = errors.New("sql: database is closed")

// nextRequestKeyLocked returns the next connection request key.
// It is assumed that nextRequest will not overflow.
func (db *DB) nextRequestKeyLocked() uint64 {
        next := db.nextRequest
        db.nextRequest++
        return next
}

// conn returns a newly-opened or cached *driverConn.
func (db *DB) conn(ctx context.Context, strategy connReuseStrategy) (*driverConn, error) {
        db.mu.Lock()
        if db.closed {
                db.mu.Unlock()
                return nil, errDBClosed
        }
        // Check if the context is expired.
        select {
        default:
        case <-ctx.Done():
                db.mu.Unlock()
                return nil, ctx.Err()
        }
        lifetime := db.maxLifetime

        // Prefer a free connection, if possible.
        numFree := len(db.freeConn)
        if strategy == cachedOrNewConn && numFree > 0 {
                conn := db.freeConn[0]
                copy(db.freeConn, db.freeConn[1:])
                db.freeConn = db.freeConn[:numFree-1]
                conn.inUse = true
                db.mu.Unlock()
                if conn.expired(lifetime) {
                        conn.Close()
                        return nil, driver.ErrBadConn
                }
                return conn, nil
        }

        // Out of free connections or we were asked not to use one. If we're not
        // allowed to open any more connections, make a request and wait.
        if db.maxOpen > 0 && db.numOpen >= db.maxOpen {
                // Make the connRequest channel. It's buffered so that the
                // connectionOpener doesn't block while waiting for the req to be read.
                req := make(chan connRequest, 1)
                reqKey := db.nextRequestKeyLocked()
                db.connRequests[reqKey] = req
                db.mu.Unlock()

                // Timeout the connection request with the context.
                select {
                case <-ctx.Done():
                        // Remove the connection request and ensure no value has been sent
                        // on it after removing.
                        db.mu.Lock()
                        delete(db.connRequests, reqKey)
                        db.mu.Unlock()
                        select {
                        default:
                        case ret, ok := <-req:
                                if ok {
                                        db.putConn(ret.conn, ret.err)
                                }
                        }
                        return nil, ctx.Err()
                case ret, ok := <-req:
                        if !ok {
                                return nil, errDBClosed
                        }
                        if ret.err == nil && ret.conn.expired(lifetime) {
                                ret.conn.Close()
                                return nil, driver.ErrBadConn
                        }
                        return ret.conn, ret.err
                }
        }

        db.numOpen++ // optimistically
        db.mu.Unlock()
        ci, err := db.driver.Open(db.dsn)
        if err != nil {
                db.mu.Lock()
                db.numOpen-- // correct for earlier optimism
                db.maybeOpenNewConnections()
                db.mu.Unlock()
                return nil, err
        }
        db.mu.Lock()
        dc := &driverConn{
                db:        db,
                createdAt: nowFunc(),
                ci:        ci,
                inUse:     true,
        }
        db.addDepLocked(dc, dc)
        db.mu.Unlock()
        return dc, nil
}

// putConnHook is a hook for testing.
var putConnHook func(*DB, *driverConn)

// noteUnusedDriverStatement notes that ds is no longer used and should
// be closed whenever possible (when c is next not in use), unless c is
// already closed.
func (db *DB) noteUnusedDriverStatement(c *driverConn, ds *driverStmt) {
        db.mu.Lock()
        defer db.mu.Unlock()
        if c.inUse {
                c.onPut = append(c.onPut, func() {
                        ds.Close()
                })
        } else {
                c.Lock()
                fc := c.finalClosed
                c.Unlock()
                if !fc {
                        ds.Close()
                }
        }
}

// debugGetPut determines whether getConn & putConn calls' stack traces
// are returned for more verbose crashes.
const debugGetPut = false

// putConn adds a connection to the db's free pool.
// err is optionally the last error that occurred on this connection.
func (db *DB) putConn(dc *driverConn, err error) {
        db.mu.Lock()
        if !dc.inUse {
                if debugGetPut {
                        fmt.Printf("putConn(%v) DUPLICATE was: %s\n\nPREVIOUS was: %s", dc, stack(), db.lastPut[dc])
                }
                panic("sql: connection returned that was never out")
        }
        if debugGetPut {
                db.lastPut[dc] = stack()
        }
        dc.inUse = false

        for _, fn := range dc.onPut {
                fn()
        }
        dc.onPut = nil

        if err == driver.ErrBadConn {
                // Don't reuse bad connections.
                // Since the conn is considered bad and is being discarded, treat it
                // as closed. Don't decrement the open count here, finalClose will
                // take care of that.
                db.maybeOpenNewConnections()
                db.mu.Unlock()
                dc.Close()
                return
        }
        if putConnHook != nil {
                putConnHook(db, dc)
        }
        added := db.putConnDBLocked(dc, nil)
        db.mu.Unlock()

        if !added {
                dc.Close()
        }
}

// Satisfy a connRequest or put the driverConn in the idle pool and return true
// or return false.
// putConnDBLocked will satisfy a connRequest if there is one, or it will
// return the *driverConn to the freeConn list if err == nil and the idle
// connection limit will not be exceeded.
// If err != nil, the value of dc is ignored.
// If err == nil, then dc must not equal nil.
// If a connRequest was fulfilled or the *driverConn was placed in the
// freeConn list, then true is returned, otherwise false is returned.
func (db *DB) putConnDBLocked(dc *driverConn, err error) bool {
        if db.closed {
                return false
        }
        if db.maxOpen > 0 && db.numOpen > db.maxOpen {
                return false
        }
        if c := len(db.connRequests); c > 0 {
                var req chan connRequest
                var reqKey uint64
                for reqKey, req = range db.connRequests {
                        break
                }
                delete(db.connRequests, reqKey) // Remove from pending requests.
                if err == nil {
                        dc.inUse = true
                }
                req <- connRequest{
                        conn: dc,
                        err:  err,
                }
                return true
        } else if err == nil && !db.closed && db.maxIdleConnsLocked() > len(db.freeConn) {
                db.freeConn = append(db.freeConn, dc)
                db.startCleanerLocked()
                return true
        }
        return false
}

// maxBadConnRetries is the number of maximum retries if the driver returns
// driver.ErrBadConn to signal a broken connection before forcing a new
// connection to be opened.
const maxBadConnRetries = 2

// PrepareContext creates a prepared statement for later queries or executions.
// Multiple queries or executions may be run concurrently from the
// returned statement.
// The caller must call the statement's Close method
// when the statement is no longer needed.
//
// The provided context is used for the preparation of the statement, not for the
// execution of the statement.
func (db *DB) PrepareContext(ctx context.Context, query string) (*Stmt, error) {
        var stmt *Stmt
        var err error
        for i := 0; i < maxBadConnRetries; i++ {
                stmt, err = db.prepare(ctx, query, cachedOrNewConn)
                if err != driver.ErrBadConn {
                        break
                }
        }
        if err == driver.ErrBadConn {
                return db.prepare(ctx, query, alwaysNewConn)
        }
        return stmt, err
}

// Prepare creates a prepared statement for later queries or executions.
// Multiple queries or executions may be run concurrently from the
// returned statement.
// The caller must call the statement's Close method
// when the statement is no longer needed.
func (db *DB) Prepare(query string) (*Stmt, error) {
        return db.PrepareContext(context.Background(), query)
}

func (db *DB) prepare(ctx context.Context, query string, strategy connReuseStrategy) (*Stmt, error) {
        // TODO: check if db.driver supports an optional
        // driver.Preparer interface and call that instead, if so,
        // otherwise we make a prepared statement that's bound
        // to a connection, and to execute this prepared statement
        // we either need to use this connection (if it's free), else
        // get a new connection + re-prepare + execute on that one.
        dc, err := db.conn(ctx, strategy)
        if err != nil {
                return nil, err
        }
        return db.prepareDC(ctx, dc, dc.releaseConn, nil, query)
}

// prepareDC prepares a query on the driverConn and calls release before
// returning. When cg == nil it implies that a connection pool is used, and
// when cg != nil only a single driver connection is used.
func (db *DB) prepareDC(ctx context.Context, dc *driverConn, release func(error), cg stmtConnGrabber, query string) (*Stmt, error) {
        var ds *driverStmt
        var err error
        defer func() {
                release(err)
        }()
        withLock(dc, func() {
                ds, err = dc.prepareLocked(ctx, cg, query)
        })
        if err != nil {
                return nil, err
        }
        stmt := &Stmt{
                db:    db,
                query: query,
                cg:    cg,
                cgds:  ds,
        }

        // When cg == nil this statement will need to keep track of various
        // connections they are prepared on and record the stmt dependency on
        // the DB.
        if cg == nil {
                stmt.css = []connStmt{{dc, ds}}
                stmt.lastNumClosed = atomic.LoadUint64(&db.numClosed)
                db.addDep(stmt, stmt)
        }
        return stmt, nil
}

// ExecContext executes a query without returning any rows.
// The args are for any placeholder parameters in the query.
func (db *DB) ExecContext(ctx context.Context, query string, args ...interface{}) (Result, error) {
        var res Result
        var err error
        for i := 0; i < maxBadConnRetries; i++ {
                res, err = db.exec(ctx, query, args, cachedOrNewConn)
                if err != driver.ErrBadConn {
                        break
                }
        }
        if err == driver.ErrBadConn {
                return db.exec(ctx, query, args, alwaysNewConn)
        }
        return res, err
}

// Exec executes a query without returning any rows.
// The args are for any placeholder parameters in the query.
func (db *DB) Exec(query string, args ...interface{}) (Result, error) {
        return db.ExecContext(context.Background(), query, args...)
}

func (db *DB) exec(ctx context.Context, query string, args []interface{}, strategy connReuseStrategy) (Result, error) {
        dc, err := db.conn(ctx, strategy)
        if err != nil {
                return nil, err
        }
        return db.execDC(ctx, dc, dc.releaseConn, query, args)
}

func (db *DB) execDC(ctx context.Context, dc *driverConn, release func(error), query string, args []interface{}) (res Result, err error) {
        defer func() {
                release(err)
        }()
        if execer, ok := dc.ci.(driver.Execer); ok {
                var dargs []driver.NamedValue
                dargs, err = driverArgs(dc.ci, nil, args)
                if err != nil {
                        return nil, err
                }
                var resi driver.Result
                withLock(dc, func() {
                        resi, err = ctxDriverExec(ctx, execer, query, dargs)
                })
                if err != driver.ErrSkip {
                        if err != nil {
                                return nil, err
                        }
                        return driverResult{dc, resi}, nil
                }
        }

        var si driver.Stmt
        withLock(dc, func() {
                si, err = ctxDriverPrepare(ctx, dc.ci, query)
        })
        if err != nil {
                return nil, err
        }
        ds := &driverStmt{Locker: dc, si: si}
        defer ds.Close()
        return resultFromStatement(ctx, dc.ci, ds, args...)
}

// QueryContext executes a query that returns rows, typically a SELECT.
// The args are for any placeholder parameters in the query.
func (db *DB) QueryContext(ctx context.Context, query string, args ...interface{}) (*Rows, error) {
        var rows *Rows
        var err error
        for i := 0; i < maxBadConnRetries; i++ {
                rows, err = db.query(ctx, query, args, cachedOrNewConn)
                if err != driver.ErrBadConn {
                        break
                }
        }
        if err == driver.ErrBadConn {
                return db.query(ctx, query, args, alwaysNewConn)
        }
        return rows, err
}

// Query executes a query that returns rows, typically a SELECT.
// The args are for any placeholder parameters in the query.
func (db *DB) Query(query string, args ...interface{}) (*Rows, error) {
        return db.QueryContext(context.Background(), query, args...)
}

func (db *DB) query(ctx context.Context, query string, args []interface{}, strategy connReuseStrategy) (*Rows, error) {
        dc, err := db.conn(ctx, strategy)
        if err != nil {
                return nil, err
        }

        return db.queryDC(ctx, nil, dc, dc.releaseConn, query, args)
}

// queryDC executes a query on the given connection.
// The connection gets released by the releaseConn function.
// The ctx context is from a query method and the txctx context is from an
// optional transaction context.
func (db *DB) queryDC(ctx, txctx context.Context, dc *driverConn, releaseConn func(error), query string, args []interface{}) (*Rows, error) {
        if queryer, ok := dc.ci.(driver.Queryer); ok {
                dargs, err := driverArgs(dc.ci, nil, args)
                if err != nil {
                        releaseConn(err)
                        return nil, err
                }
                var rowsi driver.Rows
                withLock(dc, func() {
                        rowsi, err = ctxDriverQuery(ctx, queryer, query, dargs)
                })
                if err != driver.ErrSkip {
                        if err != nil {
                                releaseConn(err)
                                return nil, err
                        }
                        // Note: ownership of dc passes to the *Rows, to be freed
                        // with releaseConn.
                        rows := &Rows{
                                dc:          dc,
                                releaseConn: releaseConn,
                                rowsi:       rowsi,
                        }
                        rows.initContextClose(ctx, txctx)
                        return rows, nil
                }
        }

        var si driver.Stmt
        var err error
        withLock(dc, func() {
                si, err = ctxDriverPrepare(ctx, dc.ci, query)
        })
        if err != nil {
                releaseConn(err)
                return nil, err
        }

        ds := &driverStmt{Locker: dc, si: si}
        rowsi, err := rowsiFromStatement(ctx, dc.ci, ds, args...)
        if err != nil {
                ds.Close()
                releaseConn(err)
                return nil, err
        }

        // Note: ownership of ci passes to the *Rows, to be freed
        // with releaseConn.
        rows := &Rows{
                dc:          dc,
                releaseConn: releaseConn,
                rowsi:       rowsi,
                closeStmt:   ds,
        }
        rows.initContextClose(ctx, txctx)
        return rows, nil
}

// QueryRowContext executes a query that is expected to return at most one row.
// QueryRowContext always returns a non-nil value. Errors are deferred until
// Row's Scan method is called.
// If the query selects no rows, the *Row's Scan will return ErrNoRows.
// Otherwise, the *Row's Scan scans the first selected row and discards
// the rest.
func (db *DB) QueryRowContext(ctx context.Context, query string, args ...interface{}) *Row {
        rows, err := db.QueryContext(ctx, query, args...)
        return &Row{rows: rows, err: err}
}

// QueryRow executes a query that is expected to return at most one row.
// QueryRow always returns a non-nil value. Errors are deferred until
// Row's Scan method is called.
// If the query selects no rows, the *Row's Scan will return ErrNoRows.
// Otherwise, the *Row's Scan scans the first selected row and discards
// the rest.
func (db *DB) QueryRow(query string, args ...interface{}) *Row {
        return db.QueryRowContext(context.Background(), query, args...)
}

// BeginTx starts a transaction.
//
// The provided context is used until the transaction is committed or rolled back.
// If the context is canceled, the sql package will roll back
// the transaction. Tx.Commit will return an error if the context provided to
// BeginTx is canceled.
//
// The provided TxOptions is optional and may be nil if defaults should be used.
// If a non-default isolation level is used that the driver doesn't support,
// an error will be returned.
func (db *DB) BeginTx(ctx context.Context, opts *TxOptions) (*Tx, error) {
        var tx *Tx
        var err error
        for i := 0; i < maxBadConnRetries; i++ {
                tx, err = db.begin(ctx, opts, cachedOrNewConn)
                if err != driver.ErrBadConn {
                        break
                }
        }
        if err == driver.ErrBadConn {
                return db.begin(ctx, opts, alwaysNewConn)
        }
        return tx, err
}

// Begin starts a transaction. The default isolation level is dependent on
// the driver.
func (db *DB) Begin() (*Tx, error) {
        return db.BeginTx(context.Background(), nil)
}

func (db *DB) begin(ctx context.Context, opts *TxOptions, strategy connReuseStrategy) (tx *Tx, err error) {
        dc, err := db.conn(ctx, strategy)
        if err != nil {
                return nil, err
        }
        return db.beginDC(ctx, dc, dc.releaseConn, opts)
}

// beginDC starts a transaction. The provided dc must be valid and ready to use.
func (db *DB) beginDC(ctx context.Context, dc *driverConn, release func(error), opts *TxOptions) (tx *Tx, err error) {
        var txi driver.Tx
        withLock(dc, func() {
                txi, err = ctxDriverBegin(ctx, opts, dc.ci)
        })
        if err != nil {
                release(err)
                return nil, err
        }

        // Schedule the transaction to rollback when the context is cancelled.
        // The cancel function in Tx will be called after done is set to true.
        ctx, cancel := context.WithCancel(ctx)
        tx = &Tx{
                db:          db,
                dc:          dc,
                releaseConn: release,
                txi:         txi,
                cancel:      cancel,
                ctx:         ctx,
        }
        go tx.awaitDone()
        return tx, nil
}

// Driver returns the database's underlying driver.
func (db *DB) Driver() driver.Driver {
        return db.driver
}

// ErrConnDone is returned by any operation that is performed on a connection
// that has already been committed or rolled back.
var ErrConnDone = errors.New("database/sql: connection is already closed")

// Conn returns a single connection by either opening a new connection
// or returning an existing connection from the connection pool. Conn will
// block until either a connection is returned or ctx is canceled.
// Queries run on the same Conn will be run in the same database session.
//
// Every Conn must be returned to the database pool after use by
// calling Conn.Close.
func (db *DB) Conn(ctx context.Context) (*Conn, error) {
        var dc *driverConn
        var err error
        for i := 0; i < maxBadConnRetries; i++ {
                dc, err = db.conn(ctx, cachedOrNewConn)
                if err != driver.ErrBadConn {
                        break
                }
        }
        if err == driver.ErrBadConn {
                dc, err = db.conn(ctx, cachedOrNewConn)
        }
        if err != nil {
                return nil, err
        }

        conn := &Conn{
                db: db,
                dc: dc,
        }
        return conn, nil
}

type releaseConn func(error)

// Conn represents a single database session rather a pool of database
// sessions. Prefer running queries from DB unless there is a specific
// need for a continuous single database session.
//
// A Conn must call Close to return the connection to the database pool
// and may do so concurrently with a running query.
//
// After a call to Close, all operations on the
// connection fail with ErrConnDone.
type Conn struct {
        db *DB

        // closemu prevents the connection from closing while there
        // is an active query. It is held for read during queries
        // and exclusively during close.
        closemu sync.RWMutex

        // dc is owned until close, at which point
        // it's returned to the connection pool.
        dc *driverConn

        // done transitions from 0 to 1 exactly once, on close.
        // Once done, all operations fail with ErrConnDone.
        // Use atomic operations on value when checking value.
        done int32
}

func (c *Conn) grabConn(context.Context) (*driverConn, releaseConn, error) {
        if atomic.LoadInt32(&c.done) != 0 {
                return nil, nil, ErrConnDone
        }
        c.closemu.RLock()
        return c.dc, c.closemuRUnlockCondReleaseConn, nil
}

// PingContext verifies the connection to the database is still alive.
func (c *Conn) PingContext(ctx context.Context) error {
        dc, release, err := c.grabConn(ctx)
        if err != nil {
                return err
        }
        return c.db.pingDC(ctx, dc, release)
}

// ExecContext executes a query without returning any rows.
// The args are for any placeholder parameters in the query.
func (c *Conn) ExecContext(ctx context.Context, query string, args ...interface{}) (Result, error) {
        dc, release, err := c.grabConn(ctx)
        if err != nil {
                return nil, err
        }
        return c.db.execDC(ctx, dc, release, query, args)
}

// QueryContext executes a query that returns rows, typically a SELECT.
// The args are for any placeholder parameters in the query.
func (c *Conn) QueryContext(ctx context.Context, query string, args ...interface{}) (*Rows, error) {
        dc, release, err := c.grabConn(ctx)
        if err != nil {
                return nil, err
        }
        return c.db.queryDC(ctx, nil, dc, release, query, args)
}

// QueryRowContext executes a query that is expected to return at most one row.
// QueryRowContext always returns a non-nil value. Errors are deferred until
// Row's Scan method is called.
// If the query selects no rows, the *Row's Scan will return ErrNoRows.
// Otherwise, the *Row's Scan scans the first selected row and discards
// the rest.
func (c *Conn) QueryRowContext(ctx context.Context, query string, args ...interface{}) *Row {
        rows, err := c.QueryContext(ctx, query, args...)
        return &Row{rows: rows, err: err}
}

// PrepareContext creates a prepared statement for later queries or executions.
// Multiple queries or executions may be run concurrently from the
// returned statement.
// The caller must call the statement's Close method
// when the statement is no longer needed.
//
// The provided context is used for the preparation of the statement, not for the
// execution of the statement.
func (c *Conn) PrepareContext(ctx context.Context, query string) (*Stmt, error) {
        dc, release, err := c.grabConn(ctx)
        if err != nil {
                return nil, err
        }
        return c.db.prepareDC(ctx, dc, release, c, query)
}

// BeginTx starts a transaction.
//
// The provided context is used until the transaction is committed or rolled back.
// If the context is canceled, the sql package will roll back
// the transaction. Tx.Commit will return an error if the context provided to
// BeginTx is canceled.
//
// The provided TxOptions is optional and may be nil if defaults should be used.
// If a non-default isolation level is used that the driver doesn't support,
// an error will be returned.
func (c *Conn) BeginTx(ctx context.Context, opts *TxOptions) (*Tx, error) {
        dc, release, err := c.grabConn(ctx)
        if err != nil {
                return nil, err
        }
        return c.db.beginDC(ctx, dc, release, opts)
}

// closemuRUnlockCondReleaseConn read unlocks closemu
// as the sql operation is done with the dc.
func (c *Conn) closemuRUnlockCondReleaseConn(err error) {
        c.closemu.RUnlock()
        if err == driver.ErrBadConn {
                c.close(err)
        }
}

func (c *Conn) txCtx() context.Context {
        return nil
}

func (c *Conn) close(err error) error {
        if !atomic.CompareAndSwapInt32(&c.done, 0, 1) {
                return ErrConnDone
        }

        // Lock around releasing the driver connection
        // to ensure all queries have been stopped before doing so.
        c.closemu.Lock()
        defer c.closemu.Unlock()

        c.dc.releaseConn(err)
        c.dc = nil
        c.db = nil
        return err
}

// Close returns the connection to the connection pool.
// All operations after a Close will return with ErrConnDone.
// Close is safe to call concurrently with other operations and will
// block until all other operations finish. It may be useful to first
// cancel any used context and then call close directly after.
func (c *Conn) Close() error {
        return c.close(nil)
}

// Tx is an in-progress database transaction.
//
// A transaction must end with a call to Commit or Rollback.
//
// After a call to Commit or Rollback, all operations on the
// transaction fail with ErrTxDone.
//
// The statements prepared for a transaction by calling
// the transaction's Prepare or Stmt methods are closed
// by the call to Commit or Rollback.
type Tx struct {
        db *DB

        // closemu prevents the transaction from closing while there
        // is an active query. It is held for read during queries
        // and exclusively during close.
        closemu sync.RWMutex

        // dc is owned exclusively until Commit or Rollback, at which point
        // it's returned with putConn.
        dc  *driverConn
        txi driver.Tx

        // releaseConn is called once the Tx is closed to release
        // any held driverConn back to the pool.
        releaseConn func(error)

        // done transitions from 0 to 1 exactly once, on Commit
        // or Rollback. once done, all operations fail with
        // ErrTxDone.
        // Use atomic operations on value when checking value.
        done int32

        // All Stmts prepared for this transaction. These will be closed after the
        // transaction has been committed or rolled back.
        stmts struct {
                sync.Mutex
                v []*Stmt
        }

        // cancel is called after done transitions from 0 to 1.
        cancel func()

        // ctx lives for the life of the transaction.
        ctx context.Context
}

// awaitDone blocks until the context in Tx is canceled and rolls back
// the transaction if it's not already done.
func (tx *Tx) awaitDone() {
        // Wait for either the transaction to be committed or rolled
        // back, or for the associated context to be closed.
        <-tx.ctx.Done()

        // Discard and close the connection used to ensure the
        // transaction is closed and the resources are released.  This
        // rollback does nothing if the transaction has already been
        // committed or rolled back.
        tx.rollback(true)
}

func (tx *Tx) isDone() bool {
        return atomic.LoadInt32(&tx.done) != 0
}

// ErrTxDone is returned by any operation that is performed on a transaction
// that has already been committed or rolled back.
var ErrTxDone = errors.New("sql: Transaction has already been committed or rolled back")

// close returns the connection to the pool and
// must only be called by Tx.rollback or Tx.Commit.
func (tx *Tx) close(err error) {
        tx.cancel()

        tx.closemu.Lock()
        defer tx.closemu.Unlock()

        tx.releaseConn(err)
        tx.dc = nil
        tx.txi = nil
}

// hookTxGrabConn specifies an optional hook to be called on
// a successful call to (*Tx).grabConn. For tests.
var hookTxGrabConn func()

func (tx *Tx) grabConn(ctx context.Context) (*driverConn, releaseConn, error) {
        select {
        default:
        case <-ctx.Done():
                return nil, nil, ctx.Err()
        }

        // closeme.RLock must come before the check for isDone to prevent the Tx from
        // closing while a query is executing.
        tx.closemu.RLock()
        if tx.isDone() {
                tx.closemu.RUnlock()
                return nil, nil, ErrTxDone
        }
        if hookTxGrabConn != nil { // test hook
                hookTxGrabConn()
        }
        return tx.dc, tx.closemuRUnlockRelease, nil
}

func (tx *Tx) txCtx() context.Context {
        return tx.ctx
}

// closemuRUnlockRelease is used as a func(error) method value in
// ExecContext and QueryContext. Unlocking in the releaseConn keeps
// the driver conn from being returned to the connection pool until
// the Rows has been closed.
func (tx *Tx) closemuRUnlockRelease(error) {
        tx.closemu.RUnlock()
}

// Closes all Stmts prepared for this transaction.
func (tx *Tx) closePrepared() {
        tx.stmts.Lock()
        defer tx.stmts.Unlock()
        for _, stmt := range tx.stmts.v {
                stmt.Close()
        }
}

// Commit commits the transaction.
func (tx *Tx) Commit() error {
        if !atomic.CompareAndSwapInt32(&tx.done, 0, 1) {
                return ErrTxDone
        }
        select {
        default:
        case <-tx.ctx.Done():
                return tx.ctx.Err()
        }
        var err error
        withLock(tx.dc, func() {
                err = tx.txi.Commit()
        })
        if err != driver.ErrBadConn {
                tx.closePrepared()
        }
        tx.close(err)
        return err
}

// rollback aborts the transaction and optionally forces the pool to discard
// the connection.
func (tx *Tx) rollback(discardConn bool) error {
        if !atomic.CompareAndSwapInt32(&tx.done, 0, 1) {
                return ErrTxDone
        }
        var err error
        withLock(tx.dc, func() {
                err = tx.txi.Rollback()
        })
        if err != driver.ErrBadConn {
                tx.closePrepared()
        }
        if discardConn {
                err = driver.ErrBadConn
        }
        tx.close(err)
        return err
}

// Rollback aborts the transaction.
func (tx *Tx) Rollback() error {
        return tx.rollback(false)
}

// PrepareContext creates a prepared statement for use within a transaction.
//
// The returned statement operates within the transaction and will be closed
// when the transaction has been committed or rolled back.
//
// To use an existing prepared statement on this transaction, see Tx.Stmt.
//
// The provided context will be used for the preparation of the context, not
// for the execution of the returned statement. The returned statement
// will run in the transaction context.
func (tx *Tx) PrepareContext(ctx context.Context, query string) (*Stmt, error) {
        dc, release, err := tx.grabConn(ctx)
        if err != nil {
                return nil, err
        }

        stmt, err := tx.db.prepareDC(ctx, dc, release, tx, query)
        if err != nil {
                return nil, err
        }
        tx.stmts.Lock()
        tx.stmts.v = append(tx.stmts.v, stmt)
        tx.stmts.Unlock()
        return stmt, nil
}

// Prepare creates a prepared statement for use within a transaction.
//
// The returned statement operates within the transaction and can no longer
// be used once the transaction has been committed or rolled back.
//
// To use an existing prepared statement on this transaction, see Tx.Stmt.
func (tx *Tx) Prepare(query string) (*Stmt, error) {
        return tx.PrepareContext(context.Background(), query)
}

// StmtContext returns a transaction-specific prepared statement from
// an existing statement.
//
// Example:
//  updateMoney, err := db.Prepare("UPDATE balance SET money=money+? WHERE id=?")
//  ...
//  tx, err := db.Begin()
//  ...
//  res, err := tx.StmtContext(ctx, updateMoney).Exec(123.45, 98293203)
//
// The returned statement operates within the transaction and will be closed
// when the transaction has been committed or rolled back.
func (tx *Tx) StmtContext(ctx context.Context, stmt *Stmt) *Stmt {
        dc, release, err := tx.grabConn(ctx)
        if err != nil {
                return &Stmt{stickyErr: err}
        }
        defer release(nil)

        if tx.db != stmt.db {
                return &Stmt{stickyErr: errors.New("sql: Tx.Stmt: statement from different database used")}
        }
        var si driver.Stmt
        var parentStmt *Stmt
        stmt.mu.Lock()
        if stmt.closed || stmt.cg != nil {
                // If the statement has been closed or already belongs to a
                // transaction, we can't reuse it in this connection.
                // Since tx.StmtContext should never need to be called with a
                // Stmt already belonging to tx, we ignore this edge case and
                // re-prepare the statement in this case. No need to add
                // code-complexity for this.
                stmt.mu.Unlock()
                withLock(dc, func() {
                        si, err = ctxDriverPrepare(ctx, dc.ci, stmt.query)
                })
                if err != nil {
                        return &Stmt{stickyErr: err}
                }
        } else {
                stmt.removeClosedStmtLocked()
                // See if the statement has already been prepared on this connection,
                // and reuse it if possible.
                for _, v := range stmt.css {
                        if v.dc == dc {
                                si = v.ds.si
                                break
                        }
                }

                stmt.mu.Unlock()

                if si == nil {
                        cs, err := stmt.prepareOnConnLocked(ctx, dc)
                        if err != nil {
                                return &Stmt{stickyErr: err}
                        }
                        si = cs.si
                }
                parentStmt = stmt
        }

        txs := &Stmt{
                db: tx.db,
                cg: tx,
                cgds: &driverStmt{
                        Locker: dc,
                        si:     si,
                },
                parentStmt: parentStmt,
                query:      stmt.query,
        }
        if parentStmt != nil {
                tx.db.addDep(parentStmt, txs)
        }
        tx.stmts.Lock()
        tx.stmts.v = append(tx.stmts.v, txs)
        tx.stmts.Unlock()
        return txs
}

// Stmt returns a transaction-specific prepared statement from
// an existing statement.
//
// Example:
//  updateMoney, err := db.Prepare("UPDATE balance SET money=money+? WHERE id=?")
//  ...
//  tx, err := db.Begin()
//  ...
//  res, err := tx.Stmt(updateMoney).Exec(123.45, 98293203)
//
// The returned statement operates within the transaction and will be closed
// when the transaction has been committed or rolled back.
func (tx *Tx) Stmt(stmt *Stmt) *Stmt {
        return tx.StmtContext(context.Background(), stmt)
}

// ExecContext executes a query that doesn't return rows.
// For example: an INSERT and UPDATE.
func (tx *Tx) ExecContext(ctx context.Context, query string, args ...interface{}) (Result, error) {
        dc, release, err := tx.grabConn(ctx)
        if err != nil {
                return nil, err
        }
        return tx.db.execDC(ctx, dc, release, query, args)
}

// Exec executes a query that doesn't return rows.
// For example: an INSERT and UPDATE.
func (tx *Tx) Exec(query string, args ...interface{}) (Result, error) {
        return tx.ExecContext(context.Background(), query, args...)
}

// QueryContext executes a query that returns rows, typically a SELECT.
func (tx *Tx) QueryContext(ctx context.Context, query string, args ...interface{}) (*Rows, error) {
        dc, release, err := tx.grabConn(ctx)
        if err != nil {
                return nil, err
        }

        return tx.db.queryDC(ctx, tx.ctx, dc, release, query, args)
}

// Query executes a query that returns rows, typically a SELECT.
func (tx *Tx) Query(query string, args ...interface{}) (*Rows, error) {
        return tx.QueryContext(context.Background(), query, args...)
}

// QueryRowContext executes a query that is expected to return at most one row.
// QueryRowContext always returns a non-nil value. Errors are deferred until
// Row's Scan method is called.
// If the query selects no rows, the *Row's Scan will return ErrNoRows.
// Otherwise, the *Row's Scan scans the first selected row and discards
// the rest.
func (tx *Tx) QueryRowContext(ctx context.Context, query string, args ...interface{}) *Row {
        rows, err := tx.QueryContext(ctx, query, args...)
        return &Row{rows: rows, err: err}
}

// QueryRow executes a query that is expected to return at most one row.
// QueryRow always returns a non-nil value. Errors are deferred until
// Row's Scan method is called.
// If the query selects no rows, the *Row's Scan will return ErrNoRows.
// Otherwise, the *Row's Scan scans the first selected row and discards
// the rest.
func (tx *Tx) QueryRow(query string, args ...interface{}) *Row {
        return tx.QueryRowContext(context.Background(), query, args...)
}

// connStmt is a prepared statement on a particular connection.
type connStmt struct {
        dc *driverConn
        ds *driverStmt
}

// stmtConnGrabber represents a Tx or Conn that will return the underlying
// driverConn and release function.
type stmtConnGrabber interface {
        // grabConn returns the driverConn and the associated release function
        // that must be called when the operation completes.
        grabConn(context.Context) (*driverConn, releaseConn, error)

        // txCtx returns the transaction context if available.
        // The returned context should be selected on along with
        // any query context when awaiting a cancel.
        txCtx() context.Context
}

var (
        _ stmtConnGrabber = &Tx{}
        _ stmtConnGrabber = &Conn{}
)

// Stmt is a prepared statement.
// A Stmt is safe for concurrent use by multiple goroutines.
type Stmt struct {
        // Immutable:
        db        *DB    // where we came from
        query     string // that created the Stmt
        stickyErr error  // if non-nil, this error is returned for all operations

        closemu sync.RWMutex // held exclusively during close, for read otherwise.

        // If Stmt is prepared on a Tx or Conn then cg is present and will
        // only ever grab a connection from cg.
        // If cg is nil then the Stmt must grab an arbitrary connection
        // from db and determine if it must prepare the stmt again by
        // inspecting css.
        cg   stmtConnGrabber
        cgds *driverStmt

        // parentStmt is set when a transaction-specific statement
        // is requested from an identical statement prepared on the same
        // conn. parentStmt is used to track the dependency of this statement
        // on its originating ("parent") statement so that parentStmt may
        // be closed by the user without them having to know whether or not
        // any transactions are still using it.
        parentStmt *Stmt

        mu     sync.Mutex // protects the rest of the fields
        closed bool

        // css is a list of underlying driver statement interfaces
        // that are valid on particular connections. This is only
        // used if cg == nil and one is found that has idle
        // connections. If cg != nil, cgds is always used.
        css []connStmt

        // lastNumClosed is copied from db.numClosed when Stmt is created
        // without tx and closed connections in css are removed.
        lastNumClosed uint64
}

// ExecContext executes a prepared statement with the given arguments and
// returns a Result summarizing the effect of the statement.
func (s *Stmt) ExecContext(ctx context.Context, args ...interface{}) (Result, error) {
        s.closemu.RLock()
        defer s.closemu.RUnlock()

        var res Result
        strategy := cachedOrNewConn
        for i := 0; i < maxBadConnRetries+1; i++ {
                if i == maxBadConnRetries {
                        strategy = alwaysNewConn
                }
                dc, releaseConn, ds, err := s.connStmt(ctx, strategy)
                if err != nil {
                        if err == driver.ErrBadConn {
                                continue
                        }
                        return nil, err
                }

                res, err = resultFromStatement(ctx, dc.ci, ds, args...)
                releaseConn(err)
                if err != driver.ErrBadConn {
                        return res, err
                }
        }
        return nil, driver.ErrBadConn
}

// Exec executes a prepared statement with the given arguments and
// returns a Result summarizing the effect of the statement.
func (s *Stmt) Exec(args ...interface{}) (Result, error) {
        return s.ExecContext(context.Background(), args...)
}

func resultFromStatement(ctx context.Context, ci driver.Conn, ds *driverStmt, args ...interface{}) (Result, error) {
        dargs, err := driverArgs(ci, ds, args)
        if err != nil {
                return nil, err
        }

        ds.Lock()
        defer ds.Unlock()

        resi, err := ctxDriverStmtExec(ctx, ds.si, dargs)
        if err != nil {
                return nil, err
        }
        return driverResult{ds.Locker, resi}, nil
}

// removeClosedStmtLocked removes closed conns in s.css.
//
// To avoid lock contention on DB.mu, we do it only when
// s.db.numClosed - s.lastNum is large enough.
func (s *Stmt) removeClosedStmtLocked() {
        t := len(s.css)/2 + 1
        if t > 10 {
                t = 10
        }
        dbClosed := atomic.LoadUint64(&s.db.numClosed)
        if dbClosed-s.lastNumClosed < uint64(t) {
                return
        }

        s.db.mu.Lock()
        for i := 0; i < len(s.css); i++ {
                if s.css[i].dc.dbmuClosed {
                        s.css[i] = s.css[len(s.css)-1]
                        s.css = s.css[:len(s.css)-1]
                        i--
                }
        }
        s.db.mu.Unlock()
        s.lastNumClosed = dbClosed
}

// connStmt returns a free driver connection on which to execute the
// statement, a function to call to release the connection, and a
// statement bound to that connection.
func (s *Stmt) connStmt(ctx context.Context, strategy connReuseStrategy) (dc *driverConn, releaseConn func(error), ds *driverStmt, err error) {
        if err = s.stickyErr; err != nil {
                return
        }
        s.mu.Lock()
        if s.closed {
                s.mu.Unlock()
                err = errors.New("sql: statement is closed")
                return
        }

        // In a transaction or connection, we always use the connection that the
        // the stmt was created on.
        if s.cg != nil {
                s.mu.Unlock()
                dc, releaseConn, err = s.cg.grabConn(ctx) // blocks, waiting for the connection.
                if err != nil {
                        return
                }
                return dc, releaseConn, s.cgds, nil
        }

        s.removeClosedStmtLocked()
        s.mu.Unlock()

        dc, err = s.db.conn(ctx, strategy)
        if err != nil {
                return nil, nil, nil, err
        }

        s.mu.Lock()
        for _, v := range s.css {
                if v.dc == dc {
                        s.mu.Unlock()
                        return dc, dc.releaseConn, v.ds, nil
                }
        }
        s.mu.Unlock()

        // No luck; we need to prepare the statement on this connection
        withLock(dc, func() {
                ds, err = s.prepareOnConnLocked(ctx, dc)
        })
        if err != nil {
                dc.releaseConn(err)
                return nil, nil, nil, err
        }

        return dc, dc.releaseConn, ds, nil
}

// prepareOnConnLocked prepares the query in Stmt s on dc and adds it to the list of
// open connStmt on the statement. It assumes the caller is holding the lock on dc.
func (s *Stmt) prepareOnConnLocked(ctx context.Context, dc *driverConn) (*driverStmt, error) {
        si, err := dc.prepareLocked(ctx, s.cg, s.query)
        if err != nil {
                return nil, err
        }
        cs := connStmt{dc, si}
        s.mu.Lock()
        s.css = append(s.css, cs)
        s.mu.Unlock()
        return cs.ds, nil
}

// QueryContext executes a prepared query statement with the given arguments
// and returns the query results as a *Rows.
func (s *Stmt) QueryContext(ctx context.Context, args ...interface{}) (*Rows, error) {
        s.closemu.RLock()
        defer s.closemu.RUnlock()

        var rowsi driver.Rows
        strategy := cachedOrNewConn
        for i := 0; i < maxBadConnRetries+1; i++ {
                if i == maxBadConnRetries {
                        strategy = alwaysNewConn
                }
                dc, releaseConn, ds, err := s.connStmt(ctx, strategy)
                if err != nil {
                        if err == driver.ErrBadConn {
                                continue
                        }
                        return nil, err
                }

                rowsi, err = rowsiFromStatement(ctx, dc.ci, ds, args...)
                if err == nil {
                        // Note: ownership of ci passes to the *Rows, to be freed
                        // with releaseConn.
                        rows := &Rows{
                                dc:    dc,
                                rowsi: rowsi,
                                // releaseConn set below
                        }
                        // addDep must be added before initContextClose or it could attempt
                        // to removeDep before it has been added.
                        s.db.addDep(s, rows)

                        // releaseConn must be set before initContextClose or it could
                        // release the connection before it is set.
                        rows.releaseConn = func(err error) {
                                releaseConn(err)
                                s.db.removeDep(s, rows)
                        }
                        var txctx context.Context
                        if s.cg != nil {
                                txctx = s.cg.txCtx()
                        }
                        rows.initContextClose(ctx, txctx)
                        return rows, nil
                }

                releaseConn(err)
                if err != driver.ErrBadConn {
                        return nil, err
                }
        }
        return nil, driver.ErrBadConn
}

// Query executes a prepared query statement with the given arguments
// and returns the query results as a *Rows.
func (s *Stmt) Query(args ...interface{}) (*Rows, error) {
        return s.QueryContext(context.Background(), args...)
}

func rowsiFromStatement(ctx context.Context, ci driver.Conn, ds *driverStmt, args ...interface{}) (driver.Rows, error) {
        var want int
        withLock(ds, func() {
                want = ds.si.NumInput()
        })

        // -1 means the driver doesn't know how to count the number of
        // placeholders, so we won't sanity check input here and instead let the
        // driver deal with errors.
        if want != -1 && len(args) != want {
                return nil, fmt.Errorf("sql: statement expects %d inputs; got %d", want, len(args))
        }

        dargs, err := driverArgs(ci, ds, args)
        if err != nil {
                return nil, err
        }

        ds.Lock()
        defer ds.Unlock()

        rowsi, err := ctxDriverStmtQuery(ctx, ds.si, dargs)
        if err != nil {
                return nil, err
        }
        return rowsi, nil
}

// QueryRowContext executes a prepared query statement with the given arguments.
// If an error occurs during the execution of the statement, that error will
// be returned by a call to Scan on the returned *Row, which is always non-nil.
// If the query selects no rows, the *Row's Scan will return ErrNoRows.
// Otherwise, the *Row's Scan scans the first selected row and discards
// the rest.
//
// Example usage:
//
//  var name string
//  err := nameByUseridStmt.QueryRowContext(ctx, id).Scan(&name)
func (s *Stmt) QueryRowContext(ctx context.Context, args ...interface{}) *Row {
        rows, err := s.QueryContext(ctx, args...)
        if err != nil {
                return &Row{err: err}
        }
        return &Row{rows: rows}
}

// QueryRow executes a prepared query statement with the given arguments.
// If an error occurs during the execution of the statement, that error will
// be returned by a call to Scan on the returned *Row, which is always non-nil.
// If the query selects no rows, the *Row's Scan will return ErrNoRows.
// Otherwise, the *Row's Scan scans the first selected row and discards
// the rest.
//
// Example usage:
//
//  var name string
//  err := nameByUseridStmt.QueryRow(id).Scan(&name)
func (s *Stmt) QueryRow(args ...interface{}) *Row {
        return s.QueryRowContext(context.Background(), args...)
}

// Close closes the statement.
func (s *Stmt) Close() error {
        s.closemu.Lock()
        defer s.closemu.Unlock()

        if s.stickyErr != nil {
                return s.stickyErr
        }
        s.mu.Lock()
        if s.closed {
                s.mu.Unlock()
                return nil
        }
        s.closed = true
        txds := s.cgds
        s.cgds = nil

        s.mu.Unlock()

        if s.cg == nil {
                return s.db.removeDep(s, s)
        }

        if s.parentStmt != nil {
                // If parentStmt is set, we must not close s.txds since it's stored
                // in the css array of the parentStmt.
                return s.db.removeDep(s.parentStmt, s)
        }
        return txds.Close()
}

func (s *Stmt) finalClose() error {
        s.mu.Lock()
        defer s.mu.Unlock()
        if s.css != nil {
                for _, v := range s.css {
                        s.db.noteUnusedDriverStatement(v.dc, v.ds)
                        v.dc.removeOpenStmt(v.ds)
                }
                s.css = nil
        }
        return nil
}

// Rows is the result of a query. Its cursor starts before the first row
// of the result set. Use Next to advance through the rows:
//
//     rows, err := db.Query("SELECT ...")
//     ...
//     defer rows.Close()
//     for rows.Next() {
//         var id int
//         var name string
//         err = rows.Scan(&id, &name)
//         ...
//     }
//     err = rows.Err() // get any error encountered during iteration
//     ...
type Rows struct {
        dc          *driverConn // owned; must call releaseConn when closed to release
        releaseConn func(error)
        rowsi       driver.Rows
        cancel      func()      // called when Rows is closed, may be nil.
        closeStmt   *driverStmt // if non-nil, statement to Close on close

        // closemu prevents Rows from closing while there
        // is an active streaming result. It is held for read during non-close operations
        // and exclusively during close.
        //
        // closemu guards lasterr and closed.
        closemu sync.RWMutex
        closed  bool
        lasterr error // non-nil only if closed is true

        // lastcols is only used in Scan, Next, and NextResultSet which are expected
        // not to be called concurrently.
        lastcols []driver.Value
}

func (rs *Rows) initContextClose(ctx, txctx context.Context) {
        ctx, rs.cancel = context.WithCancel(ctx)
        go rs.awaitDone(ctx, txctx)
}

// awaitDone blocks until either ctx or txctx is canceled. The ctx is provided
// from the query context and is canceled when the query Rows is closed.
// If the query was issued in a transaction, the transaction's context
// is also provided in txctx to ensure Rows is closed if the Tx is closed.
func (rs *Rows) awaitDone(ctx, txctx context.Context) {
        var txctxDone <-chan struct{}
        if txctx != nil {
                txctxDone = txctx.Done()
        }
        select {
        case <-ctx.Done():
        case <-txctxDone:
        }
        rs.close(ctx.Err())
}

// Next prepares the next result row for reading with the Scan method. It
// returns true on success, or false if there is no next result row or an error
// happened while preparing it. Err should be consulted to distinguish between
// the two cases.
//
// Every call to Scan, even the first one, must be preceded by a call to Next.
func (rs *Rows) Next() bool {
        var doClose, ok bool
        withLock(rs.closemu.RLocker(), func() {
                doClose, ok = rs.nextLocked()
        })
        if doClose {
                rs.Close()
        }
        return ok
}

func (rs *Rows) nextLocked() (doClose, ok bool) {
        if rs.closed {
                return false, false
        }
        if rs.lastcols == nil {
                rs.lastcols = make([]driver.Value, len(rs.rowsi.Columns()))
        }
        rs.lasterr = rs.rowsi.Next(rs.lastcols)
        if rs.lasterr != nil {
                // Close the connection if there is a driver error.
                if rs.lasterr != io.EOF {
                        return true, false
                }
                nextResultSet, ok := rs.rowsi.(driver.RowsNextResultSet)
                if !ok {
                        return true, false
                }
                // The driver is at the end of the current result set.
                // Test to see if there is another result set after the current one.
                // Only close Rows if there is no further result sets to read.
                if !nextResultSet.HasNextResultSet() {
                        doClose = true
                }
                return doClose, false
        }
        return false, true
}

// NextResultSet prepares the next result set for reading. It returns true if
// there is further result sets, or false if there is no further result set
// or if there is an error advancing to it. The Err method should be consulted
// to distinguish between the two cases.
//
// After calling NextResultSet, the Next method should always be called before
// scanning. If there are further result sets they may not have rows in the result
// set.
func (rs *Rows) NextResultSet() bool {
        var doClose bool
        defer func() {
                if doClose {
                        rs.Close()
                }
        }()
        rs.closemu.RLock()
        defer rs.closemu.RUnlock()

        if rs.closed {
                return false
        }

        rs.lastcols = nil
        nextResultSet, ok := rs.rowsi.(driver.RowsNextResultSet)
        if !ok {
                doClose = true
                return false
        }
        rs.lasterr = nextResultSet.NextResultSet()
        if rs.lasterr != nil {
                doClose = true
                return false
        }
        return true
}

// Err returns the error, if any, that was encountered during iteration.
// Err may be called after an explicit or implicit Close.
func (rs *Rows) Err() error {
        rs.closemu.RLock()
        defer rs.closemu.RUnlock()
        if rs.lasterr == io.EOF {
                return nil
        }
        return rs.lasterr
}

// Columns returns the column names.
// Columns returns an error if the rows are closed, or if the rows
// are from QueryRow and there was a deferred error.
func (rs *Rows) Columns() ([]string, error) {
        rs.closemu.RLock()
        defer rs.closemu.RUnlock()
        if rs.closed {
                return nil, errors.New("sql: Rows are closed")
        }
        if rs.rowsi == nil {
                return nil, errors.New("sql: no Rows available")
        }
        return rs.rowsi.Columns(), nil
}

// ColumnTypes returns column information such as column type, length,
// and nullable. Some information may not be available from some drivers.
func (rs *Rows) ColumnTypes() ([]*ColumnType, error) {
        rs.closemu.RLock()
        defer rs.closemu.RUnlock()
        if rs.closed {
                return nil, errors.New("sql: Rows are closed")
        }
        if rs.rowsi == nil {
                return nil, errors.New("sql: no Rows available")
        }
        return rowsColumnInfoSetup(rs.rowsi), nil
}

// ColumnType contains the name and type of a column.
type ColumnType struct {
        name string

        hasNullable       bool
        hasLength         bool
        hasPrecisionScale bool

        nullable     bool
        length       int64
        databaseType string
        precision    int64
        scale        int64
        scanType     reflect.Type
}

// Name returns the name or alias of the column.
func (ci *ColumnType) Name() string {
        return ci.name
}

// Length returns the column type length for variable length column types such
// as text and binary field types. If the type length is unbounded the value will
// be math.MaxInt64 (any database limits will still apply).
// If the column type is not variable length, such as an int, or if not supported
// by the driver ok is false.
func (ci *ColumnType) Length() (length int64, ok bool) {
        return ci.length, ci.hasLength
}

// DecimalSize returns the scale and precision of a decimal type.
// If not applicable or if not supported ok is false.
func (ci *ColumnType) DecimalSize() (precision, scale int64, ok bool) {
        return ci.precision, ci.scale, ci.hasPrecisionScale
}

// ScanType returns a Go type suitable for scanning into using Rows.Scan.
// If a driver does not support this property ScanType will return
// the type of an empty interface.
func (ci *ColumnType) ScanType() reflect.Type {
        return ci.scanType
}

// Nullable returns whether the column may be null.
// If a driver does not support this property ok will be false.
func (ci *ColumnType) Nullable() (nullable, ok bool) {
        return ci.nullable, ci.hasNullable
}

// DatabaseTypeName returns the database system name of the column type. If an empty
// string is returned the driver type name is not supported.
// Consult your driver documentation for a list of driver data types. Length specifiers
// are not included.
// Common type include "VARCHAR", "TEXT", "NVARCHAR", "DECIMAL", "BOOL", "INT", "BIGINT".
func (ci *ColumnType) DatabaseTypeName() string {
        return ci.databaseType
}

func rowsColumnInfoSetup(rowsi driver.Rows) []*ColumnType {
        names := rowsi.Columns()

        list := make([]*ColumnType, len(names))
        for i := range list {
                ci := &ColumnType{
                        name: names[i],
                }
                list[i] = ci

                if prop, ok := rowsi.(driver.RowsColumnTypeScanType); ok {
                        ci.scanType = prop.ColumnTypeScanType(i)
                } else {
                        ci.scanType = reflect.TypeOf(new(interface{})).Elem()
                }
                if prop, ok := rowsi.(driver.RowsColumnTypeDatabaseTypeName); ok {
                        ci.databaseType = prop.ColumnTypeDatabaseTypeName(i)
                }
                if prop, ok := rowsi.(driver.RowsColumnTypeLength); ok {
                        ci.length, ci.hasLength = prop.ColumnTypeLength(i)
                }
                if prop, ok := rowsi.(driver.RowsColumnTypeNullable); ok {
                        ci.nullable, ci.hasNullable = prop.ColumnTypeNullable(i)
                }
                if prop, ok := rowsi.(driver.RowsColumnTypePrecisionScale); ok {
                        ci.precision, ci.scale, ci.hasPrecisionScale = prop.ColumnTypePrecisionScale(i)
                }
        }
        return list
}

// Scan copies the columns in the current row into the values pointed
// at by dest. The number of values in dest must be the same as the
// number of columns in Rows.
//
// Scan converts columns read from the database into the following
// common Go types and special types provided by the sql package:
//
//    *string
//    *[]byte
//    *int, *int8, *int16, *int32, *int64
//    *uint, *uint8, *uint16, *uint32, *uint64
//    *bool
//    *float32, *float64
//    *interface{}
//    *RawBytes
//    any type implementing Scanner (see Scanner docs)
//
// In the most simple case, if the type of the value from the source
// column is an integer, bool or string type T and dest is of type *T,
// Scan simply assigns the value through the pointer.
//
// Scan also converts between string and numeric types, as long as no
// information would be lost. While Scan stringifies all numbers
// scanned from numeric database columns into *string, scans into
// numeric types are checked for overflow. For example, a float64 with
// value 300 or a string with value "300" can scan into a uint16, but
// not into a uint8, though float64(255) or "255" can scan into a
// uint8. One exception is that scans of some float64 numbers to
// strings may lose information when stringifying. In general, scan
// floating point columns into *float64.
//
// If a dest argument has type *[]byte, Scan saves in that argument a
// copy of the corresponding data. The copy is owned by the caller and
// can be modified and held indefinitely. The copy can be avoided by
// using an argument of type *RawBytes instead; see the documentation
// for RawBytes for restrictions on its use.
//
// If an argument has type *interface{}, Scan copies the value
// provided by the underlying driver without conversion. When scanning
// from a source value of type []byte to *interface{}, a copy of the
// slice is made and the caller owns the result.
//
// Source values of type time.Time may be scanned into values of type
// *time.Time, *interface{}, *string, or *[]byte. When converting to
// the latter two, time.Format3339Nano is used.
//
// Source values of type bool may be scanned into types *bool,
// *interface{}, *string, *[]byte, or *RawBytes.
//
// For scanning into *bool, the source may be true, false, 1, 0, or
// string inputs parseable by strconv.ParseBool.
func (rs *Rows) Scan(dest ...interface{}) error {
        rs.closemu.RLock()
        if rs.closed {
                rs.closemu.RUnlock()
                return errors.New("sql: Rows are closed")
        }
        rs.closemu.RUnlock()

        if rs.lastcols == nil {
                return errors.New("sql: Scan called without calling Next")
        }
        if len(dest) != len(rs.lastcols) {
                return fmt.Errorf("sql: expected %d destination arguments in Scan, not %d", len(rs.lastcols), len(dest))
        }
        for i, sv := range rs.lastcols {
                err := convertAssign(dest[i], sv)
                if err != nil {
                        return fmt.Errorf("sql: Scan error on column index %d: %v", i, err)
                }
        }
        return nil
}

// rowsCloseHook returns a function so tests may install the
// hook through a test only mutex.
var rowsCloseHook = func() func(*Rows, *error) { return nil }

// Close closes the Rows, preventing further enumeration. If Next is called
// and returns false and there are no further result sets,
// the Rows are closed automatically and it will suffice to check the
// result of Err. Close is idempotent and does not affect the result of Err.
func (rs *Rows) Close() error {
        return rs.close(nil)
}

func (rs *Rows) close(err error) error {
        rs.closemu.Lock()
        defer rs.closemu.Unlock()

        if rs.closed {
                return nil
        }
        rs.closed = true

        if rs.lasterr == nil {
                rs.lasterr = err
        }

        withLock(rs.dc, func() {
                err = rs.rowsi.Close()
        })
        if fn := rowsCloseHook(); fn != nil {
                fn(rs, &err)
        }
        if rs.cancel != nil {
                rs.cancel()
        }

        if rs.closeStmt != nil {
                rs.closeStmt.Close()
        }
        rs.releaseConn(err)
        return err
}

// Row is the result of calling QueryRow to select a single row.
type Row struct {
        // One of these two will be non-nil:
        err  error // deferred error for easy chaining
        rows *Rows
}

// Scan copies the columns from the matched row into the values
// pointed at by dest. See the documentation on Rows.Scan for details.
// If more than one row matches the query,
// Scan uses the first row and discards the rest. If no row matches
// the query, Scan returns ErrNoRows.
func (r *Row) Scan(dest ...interface{}) error {
        if r.err != nil {
                return r.err
        }

        // TODO(bradfitz): for now we need to defensively clone all
        // []byte that the driver returned (not permitting
        // *RawBytes in Rows.Scan), since we're about to close
        // the Rows in our defer, when we return from this function.
        // the contract with the driver.Next(...) interface is that it
        // can return slices into read-only temporary memory that's
        // only valid until the next Scan/Close. But the TODO is that
        // for a lot of drivers, this copy will be unnecessary. We
        // should provide an optional interface for drivers to
        // implement to say, "don't worry, the []bytes that I return
        // from Next will not be modified again." (for instance, if
        // they were obtained from the network anyway) But for now we
        // don't care.
        defer r.rows.Close()
        for _, dp := range dest {
                if _, ok := dp.(*RawBytes); ok {
                        return errors.New("sql: RawBytes isn't allowed on Row.Scan")
                }
        }

        if !r.rows.Next() {
                if err := r.rows.Err(); err != nil {
                        return err
                }
                return ErrNoRows
        }
        err := r.rows.Scan(dest...)
        if err != nil {
                return err
        }
        // Make sure the query can be processed to completion with no errors.
        if err := r.rows.Close(); err != nil {
                return err
        }

        return nil
}

// A Result summarizes an executed SQL command.
type Result interface {
        // LastInsertId returns the integer generated by the database
        // in response to a command. Typically this will be from an
        // "auto increment" column when inserting a new row. Not all
        // databases support this feature, and the syntax of such
        // statements varies.
        LastInsertId() (int64, error)

        // RowsAffected returns the number of rows affected by an
        // update, insert, or delete. Not every database or database
        // driver may support this.
        RowsAffected() (int64, error)
}

type driverResult struct {
        sync.Locker // the *driverConn
        resi        driver.Result
}

func (dr driverResult) LastInsertId() (int64, error) {
        dr.Lock()
        defer dr.Unlock()
        return dr.resi.LastInsertId()
}

func (dr driverResult) RowsAffected() (int64, error) {
        dr.Lock()
        defer dr.Unlock()
        return dr.resi.RowsAffected()
}

func stack() string {
        var buf [2 << 10]byte
        return string(buf[:runtime.Stack(buf[:], false)])
}

// withLock runs while holding lk.
func withLock(lk sync.Locker, fn func()) {
        lk.Lock()
        defer lk.Unlock() // in case fn panics
        fn()
}