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[official-gcc.git] / main / 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 http://golang.org/s/sqldrivers for a list of drivers.
//
// For more usage examples, see the wiki page at
// http://golang.org/s/sqlwiki.
package sql

import (
        "container/list"
        "database/sql/driver"
        "errors"
        "fmt"
        "io"
        "runtime"
        "sync"
)

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

// 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) {
        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
}

// 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 restricted
        // set of types:
        //
        //    int64
        //    float64
        //    bool
        //    []byte
        //    string
        //    time.Time
        //    nil - for NULL values
        //
        // An error should be returned if the value can not be stored
        // without loss of information.
        Scan(src interface{}) error
}

// 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. 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

        mu           sync.Mutex // protects following fields
        freeConn     *list.List // of *driverConn
        connRequests *list.List // of connRequest
        numOpen      int
        pendingOpens int
        // 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
}

// 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

        sync.Mutex  // guards following
        ci          driver.Conn
        closed      bool
        finalClosed bool // ci.Close has been called
        openStmt    map[driver.Stmt]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 connIfFree
        // This is the Element returned by db.freeConn.PushFront(conn).
        // It's used by connIfFree to remove the conn from the freeConn list.
        listElem *list.Element
}

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

func (dc *driverConn) removeOpenStmt(si driver.Stmt) {
        dc.Lock()
        defer dc.Unlock()
        delete(dc.openStmt, si)
}

func (dc *driverConn) prepareLocked(query string) (driver.Stmt, error) {
        si, err := dc.ci.Prepare(query)
        if err == nil {
                // Track each driverConn's open statements, so we can close them
                // before closing the conn.
                //
                // TODO(bradfitz): let drivers opt out of caring about
                // stmt closes if the conn is about to close anyway? For now
                // do the safe thing, in case stmts need to be closed.
                //
                // TODO(bradfitz): after Go 1.1, closing driver.Stmts
                // should be moved to driverStmt, using unique
                // *driverStmts everywhere (including from
                // *Stmt.connStmt, instead of returning a
                // driver.Stmt), using driverStmt as a pointer
                // everywhere, and making it a finalCloser.
                if dc.openStmt == nil {
                        dc.openStmt = make(map[driver.Stmt]bool)
                }
                dc.openStmt[si] = true
        }
        return si, err
}

// 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 {
        dc.Lock()

        for si := range dc.openStmt {
                si.Close()
        }
        dc.openStmt = nil

        err := dc.ci.Close()
        dc.ci = nil
        dc.finalClosed = true
        dc.Unlock()

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

        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
}

func (ds *driverStmt) Close() error {
        ds.Lock()
        defer ds.Unlock()
        return ds.si.Close()
}

// 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 (dn.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 satify 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 http://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.
func Open(driverName, dataSourceName string) (*DB, error) {
        driveri, ok := drivers[driverName]
        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),
        }
        db.freeConn = list.New()
        db.connRequests = list.New()
        go db.connectionOpener()
        return db, nil
}

// Ping verifies a connection to the database is still alive,
// establishing a connection if necessary.
func (db *DB) Ping() error {
        // TODO(bradfitz): give drivers an optional hook to implement
        // this in a more efficient or more reliable way, if they
        // have one.
        dc, err := db.conn()
        if err != nil {
                return err
        }
        db.putConn(dc, nil)
        return nil
}

// Close closes the database, releasing any open resources.
func (db *DB) Close() error {
        db.mu.Lock()
        if db.closed { // Make DB.Close idempotent
                db.mu.Unlock()
                return nil
        }
        close(db.openerCh)
        var err error
        fns := make([]func() error, 0, db.freeConn.Len())
        for db.freeConn.Front() != nil {
                dc := db.freeConn.Front().Value.(*driverConn)
                dc.listElem = nil
                fns = append(fns, dc.closeDBLocked())
                db.freeConn.Remove(db.freeConn.Front())
        }
        db.closed = true
        for db.connRequests.Front() != nil {
                req := db.connRequests.Front().Value.(connRequest)
                db.connRequests.Remove(db.connRequests.Front())
                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
        for db.freeConn.Len() > db.maxIdleConnsLocked() {
                dc := db.freeConn.Back().Value.(*driverConn)
                dc.listElem = nil
                db.freeConn.Remove(db.freeConn.Back())
                closing = append(closing, dc)
        }
        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)
        }
}

// 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 := db.connRequests.Len() - db.pendingOpens
        if db.maxOpen > 0 {
                numCanOpen := db.maxOpen - (db.numOpen + db.pendingOpens)
                if numRequests > numCanOpen {
                        numRequests = numCanOpen
                }
        }
        for numRequests > 0 {
                db.pendingOpens++
                numRequests--
                db.openerCh <- struct{}{}
        }
}

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

// Open one new connection
func (db *DB) openNewConnection() {
        ci, err := db.driver.Open(db.dsn)
        db.mu.Lock()
        defer db.mu.Unlock()
        if db.closed {
                if err == nil {
                        ci.Close()
                }
                return
        }
        db.pendingOpens--
        if err != nil {
                db.putConnDBLocked(nil, err)
                return
        }
        dc := &driverConn{
                db: db,
                ci: ci,
        }
        if db.putConnDBLocked(dc, err) {
                db.addDepLocked(dc, dc)
                db.numOpen++
        } else {
                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 chan<- interface{} // takes either a *driverConn or an error

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

// conn returns a newly-opened or cached *driverConn
func (db *DB) conn() (*driverConn, error) {
        db.mu.Lock()
        if db.closed {
                db.mu.Unlock()
                return nil, errDBClosed
        }

        // If db.maxOpen > 0 and the number of open connections is over the limit
        // and there are no free connection, make a request and wait.
        if db.maxOpen > 0 && db.numOpen >= db.maxOpen && db.freeConn.Len() == 0 {
                // Make the connRequest channel. It's buffered so that the
                // connectionOpener doesn't block while waiting for the req to be read.
                ch := make(chan interface{}, 1)
                req := connRequest(ch)
                db.connRequests.PushBack(req)
                db.maybeOpenNewConnections()
                db.mu.Unlock()
                ret, ok := <-ch
                if !ok {
                        return nil, errDBClosed
                }
                switch ret.(type) {
                case *driverConn:
                        return ret.(*driverConn), nil
                case error:
                        return nil, ret.(error)
                default:
                        panic("sql: Unexpected type passed through connRequest.ch")
                }
        }

        if f := db.freeConn.Front(); f != nil {
                conn := f.Value.(*driverConn)
                conn.listElem = nil
                db.freeConn.Remove(f)
                conn.inUse = true
                db.mu.Unlock()
                return conn, nil
        }

        db.mu.Unlock()
        ci, err := db.driver.Open(db.dsn)
        if err != nil {
                return nil, err
        }
        db.mu.Lock()
        db.numOpen++
        dc := &driverConn{
                db: db,
                ci: ci,
        }
        db.addDepLocked(dc, dc)
        dc.inUse = true
        db.mu.Unlock()
        return dc, nil
}

var (
        errConnClosed = errors.New("database/sql: internal sentinel error: conn is closed")
        errConnBusy   = errors.New("database/sql: internal sentinel error: conn is busy")
)

// connIfFree returns (wanted, nil) if wanted is still a valid conn and
// isn't in use.
//
// The error is errConnClosed if the connection if the requested connection
// is invalid because it's been closed.
//
// The error is errConnBusy if the connection is in use.
func (db *DB) connIfFree(wanted *driverConn) (*driverConn, error) {
        db.mu.Lock()
        defer db.mu.Unlock()
        if wanted.dbmuClosed {
                return nil, errConnClosed
        }
        if wanted.inUse {
                return nil, errConnBusy
        }
        if wanted.listElem != nil {
                db.freeConn.Remove(wanted.listElem)
                wanted.listElem = nil
                wanted.inUse = true
                return wanted, nil
        }
        // TODO(bradfitz): shouldn't get here. After Go 1.1, change this to:
        // panic("connIfFree call requested a non-closed, non-busy, non-free conn")
        // Which passes all the tests, but I'm too paranoid to include this
        // late in Go 1.1.
        // Instead, treat it like a busy connection:
        return nil, errConnBusy
}

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

// noteUnusedDriverStatement notes that si 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, si driver.Stmt) {
        db.mu.Lock()
        defer db.mu.Unlock()
        if c.inUse {
                c.onPut = append(c.onPut, func() {
                        si.Close()
                })
        } else {
                c.Lock()
                defer c.Unlock()
                if !c.finalClosed {
                        si.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 would not be reached.
// If err != nil, the value of dc is ignored.
// If err == nil, then dc must not equal nil.
// If a connRequest was fullfilled 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.connRequests.Len() > 0 {
                req := db.connRequests.Front().Value.(connRequest)
                db.connRequests.Remove(db.connRequests.Front())
                if err != nil {
                        req <- err
                } else {
                        dc.inUse = true
                        req <- dc
                }
                return true
        } else if err == nil && !db.closed && db.maxIdleConnsLocked() > 0 && db.maxIdleConnsLocked() > db.freeConn.Len() {
                dc.listElem = db.freeConn.PushFront(dc)
                return true
        }
        return false
}

// Prepare creates a prepared statement for later queries or executions.
// Multiple queries or executions may be run concurrently from the
// returned statement.
func (db *DB) Prepare(query string) (*Stmt, error) {
        var stmt *Stmt
        var err error
        for i := 0; i < 10; i++ {
                stmt, err = db.prepare(query)
                if err != driver.ErrBadConn {
                        break
                }
        }
        return stmt, err
}

func (db *DB) prepare(query string) (*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()
        if err != nil {
                return nil, err
        }
        dc.Lock()
        si, err := dc.prepareLocked(query)
        dc.Unlock()
        if err != nil {
                db.putConn(dc, err)
                return nil, err
        }
        stmt := &Stmt{
                db:    db,
                query: query,
                css:   []connStmt{{dc, si}},
        }
        db.addDep(stmt, stmt)
        db.putConn(dc, nil)
        return stmt, nil
}

// 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) {
        var res Result
        var err error
        for i := 0; i < 10; i++ {
                res, err = db.exec(query, args)
                if err != driver.ErrBadConn {
                        break
                }
        }
        return res, err
}

func (db *DB) exec(query string, args []interface{}) (res Result, err error) {
        dc, err := db.conn()
        if err != nil {
                return nil, err
        }
        defer func() {
                db.putConn(dc, err)
        }()

        if execer, ok := dc.ci.(driver.Execer); ok {
                dargs, err := driverArgs(nil, args)
                if err != nil {
                        return nil, err
                }
                dc.Lock()
                resi, err := execer.Exec(query, dargs)
                dc.Unlock()
                if err != driver.ErrSkip {
                        if err != nil {
                                return nil, err
                        }
                        return driverResult{dc, resi}, nil
                }
        }

        dc.Lock()
        si, err := dc.ci.Prepare(query)
        dc.Unlock()
        if err != nil {
                return nil, err
        }
        defer withLock(dc, func() { si.Close() })
        return resultFromStatement(driverStmt{dc, si}, args...)
}

// 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) {
        var rows *Rows
        var err error
        for i := 0; i < 10; i++ {
                rows, err = db.query(query, args)
                if err != driver.ErrBadConn {
                        break
                }
        }
        return rows, err
}

func (db *DB) query(query string, args []interface{}) (*Rows, error) {
        ci, err := db.conn()
        if err != nil {
                return nil, err
        }

        return db.queryConn(ci, ci.releaseConn, query, args)
}

// queryConn executes a query on the given connection.
// The connection gets released by the releaseConn function.
func (db *DB) queryConn(dc *driverConn, releaseConn func(error), query string, args []interface{}) (*Rows, error) {
        if queryer, ok := dc.ci.(driver.Queryer); ok {
                dargs, err := driverArgs(nil, args)
                if err != nil {
                        releaseConn(err)
                        return nil, err
                }
                dc.Lock()
                rowsi, err := queryer.Query(query, dargs)
                dc.Unlock()
                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,
                        }
                        return rows, nil
                }
        }

        dc.Lock()
        si, err := dc.ci.Prepare(query)
        dc.Unlock()
        if err != nil {
                releaseConn(err)
                return nil, err
        }

        ds := driverStmt{dc, si}
        rowsi, err := rowsiFromStatement(ds, args...)
        if err != nil {
                dc.Lock()
                si.Close()
                dc.Unlock()
                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:   si,
        }
        return rows, nil
}

// QueryRow executes a query that is expected to return at most one row.
// QueryRow always return a non-nil value. Errors are deferred until
// Row's Scan method is called.
func (db *DB) QueryRow(query string, args ...interface{}) *Row {
        rows, err := db.Query(query, args...)
        return &Row{rows: rows, err: err}
}

// Begin starts a transaction. The isolation level is dependent on
// the driver.
func (db *DB) Begin() (*Tx, error) {
        var tx *Tx
        var err error
        for i := 0; i < 10; i++ {
                tx, err = db.begin()
                if err != driver.ErrBadConn {
                        break
                }
        }
        return tx, err
}

func (db *DB) begin() (tx *Tx, err error) {
        dc, err := db.conn()
        if err != nil {
                return nil, err
        }
        dc.Lock()
        txi, err := dc.ci.Begin()
        dc.Unlock()
        if err != nil {
                db.putConn(dc, err)
                return nil, err
        }
        return &Tx{
                db:  db,
                dc:  dc,
                txi: txi,
        }, nil
}

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

// 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.
type Tx struct {
        db *DB

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

        // done transitions from false to true exactly once, on Commit
        // or Rollback. once done, all operations fail with
        // ErrTxDone.
        done bool
}

var ErrTxDone = errors.New("sql: Transaction has already been committed or rolled back")

func (tx *Tx) close() {
        if tx.done {
                panic("double close") // internal error
        }
        tx.done = true
        tx.db.putConn(tx.dc, nil)
        tx.dc = nil
        tx.txi = nil
}

func (tx *Tx) grabConn() (*driverConn, error) {
        if tx.done {
                return nil, ErrTxDone
        }
        return tx.dc, nil
}

// Commit commits the transaction.
func (tx *Tx) Commit() error {
        if tx.done {
                return ErrTxDone
        }
        defer tx.close()
        tx.dc.Lock()
        defer tx.dc.Unlock()
        return tx.txi.Commit()
}

// Rollback aborts the transaction.
func (tx *Tx) Rollback() error {
        if tx.done {
                return ErrTxDone
        }
        defer tx.close()
        tx.dc.Lock()
        defer tx.dc.Unlock()
        return tx.txi.Rollback()
}

// 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) {
        // TODO(bradfitz): We could be more efficient here and either
        // provide a method to take an existing Stmt (created on
        // perhaps a different Conn), and re-create it on this Conn if
        // necessary. Or, better: keep a map in DB of query string to
        // Stmts, and have Stmt.Execute do the right thing and
        // re-prepare if the Conn in use doesn't have that prepared
        // statement.  But we'll want to avoid caching the statement
        // in the case where we only call conn.Prepare implicitly
        // (such as in db.Exec or tx.Exec), but the caller package
        // can't be holding a reference to the returned statement.
        // Perhaps just looking at the reference count (by noting
        // Stmt.Close) would be enough. We might also want a finalizer
        // on Stmt to drop the reference count.
        dc, err := tx.grabConn()
        if err != nil {
                return nil, err
        }

        dc.Lock()
        si, err := dc.ci.Prepare(query)
        dc.Unlock()
        if err != nil {
                return nil, err
        }

        stmt := &Stmt{
                db: tx.db,
                tx: tx,
                txsi: &driverStmt{
                        Locker: dc,
                        si:     si,
                },
                query: query,
        }
        return stmt, nil
}

// 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)
func (tx *Tx) Stmt(stmt *Stmt) *Stmt {
        // TODO(bradfitz): optimize this. Currently this re-prepares
        // each time.  This is fine for now to illustrate the API but
        // we should really cache already-prepared statements
        // per-Conn. See also the big comment in Tx.Prepare.

        if tx.db != stmt.db {
                return &Stmt{stickyErr: errors.New("sql: Tx.Stmt: statement from different database used")}
        }
        dc, err := tx.grabConn()
        if err != nil {
                return &Stmt{stickyErr: err}
        }
        dc.Lock()
        si, err := dc.ci.Prepare(stmt.query)
        dc.Unlock()
        return &Stmt{
                db: tx.db,
                tx: tx,
                txsi: &driverStmt{
                        Locker: dc,
                        si:     si,
                },
                query:     stmt.query,
                stickyErr: err,
        }
}

// 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) {
        dc, err := tx.grabConn()
        if err != nil {
                return nil, err
        }

        if execer, ok := dc.ci.(driver.Execer); ok {
                dargs, err := driverArgs(nil, args)
                if err != nil {
                        return nil, err
                }
                dc.Lock()
                resi, err := execer.Exec(query, dargs)
                dc.Unlock()
                if err == nil {
                        return driverResult{dc, resi}, nil
                }
                if err != driver.ErrSkip {
                        return nil, err
                }
        }

        dc.Lock()
        si, err := dc.ci.Prepare(query)
        dc.Unlock()
        if err != nil {
                return nil, err
        }
        defer withLock(dc, func() { si.Close() })

        return resultFromStatement(driverStmt{dc, si}, args...)
}

// Query executes a query that returns rows, typically a SELECT.
func (tx *Tx) Query(query string, args ...interface{}) (*Rows, error) {
        dc, err := tx.grabConn()
        if err != nil {
                return nil, err
        }
        releaseConn := func(error) {}
        return tx.db.queryConn(dc, releaseConn, query, args)
}

// QueryRow executes a query that is expected to return at most one row.
// QueryRow always return a non-nil value. Errors are deferred until
// Row's Scan method is called.
func (tx *Tx) QueryRow(query string, args ...interface{}) *Row {
        rows, err := tx.Query(query, args...)
        return &Row{rows: rows, err: err}
}

// connStmt is a prepared statement on a particular connection.
type connStmt struct {
        dc *driverConn
        si driver.Stmt
}

// Stmt is a prepared statement. 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 in a transaction, else both nil:
        tx   *Tx
        txsi *driverStmt

        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 tx == nil and one is found that has idle
        // connections.  If tx != nil, txsi is always used.
        css []connStmt
}

// 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) {
        s.closemu.RLock()
        defer s.closemu.RUnlock()
        dc, releaseConn, si, err := s.connStmt()
        if err != nil {
                return nil, err
        }
        defer releaseConn(nil)

        return resultFromStatement(driverStmt{dc, si}, args...)
}

func resultFromStatement(ds driverStmt, args ...interface{}) (Result, error) {
        ds.Lock()
        want := ds.si.NumInput()
        ds.Unlock()

        // -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: expected %d arguments, got %d", want, len(args))
        }

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

        ds.Lock()
        resi, err := ds.si.Exec(dargs)
        ds.Unlock()
        if err != nil {
                return nil, err
        }
        return driverResult{ds.Locker, resi}, nil
}

// 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() (ci *driverConn, releaseConn func(error), si driver.Stmt, 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, we always use the connection that the
        // transaction was created on.
        if s.tx != nil {
                s.mu.Unlock()
                ci, err = s.tx.grabConn() // blocks, waiting for the connection.
                if err != nil {
                        return
                }
                releaseConn = func(error) {}
                return ci, releaseConn, s.txsi.si, nil
        }

        var cs connStmt
        match := false
        for i := 0; i < len(s.css); i++ {
                v := s.css[i]
                _, err := s.db.connIfFree(v.dc)
                if err == nil {
                        match = true
                        cs = v
                        break
                }
                if err == errConnClosed {
                        // Lazily remove dead conn from our freelist.
                        s.css[i] = s.css[len(s.css)-1]
                        s.css = s.css[:len(s.css)-1]
                        i--
                }

        }
        s.mu.Unlock()

        // Make a new conn if all are busy.
        // TODO(bradfitz): or wait for one? make configurable later?
        if !match {
                for i := 0; ; i++ {
                        dc, err := s.db.conn()
                        if err != nil {
                                return nil, nil, nil, err
                        }
                        dc.Lock()
                        si, err := dc.prepareLocked(s.query)
                        dc.Unlock()
                        if err == driver.ErrBadConn && i < 10 {
                                continue
                        }
                        if err != nil {
                                return nil, nil, nil, err
                        }
                        s.mu.Lock()
                        cs = connStmt{dc, si}
                        s.css = append(s.css, cs)
                        s.mu.Unlock()
                        break
                }
        }

        conn := cs.dc
        return conn, conn.releaseConn, cs.si, nil
}

// 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) {
        s.closemu.RLock()
        defer s.closemu.RUnlock()

        dc, releaseConn, si, err := s.connStmt()
        if err != nil {
                return nil, err
        }

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

        // Note: ownership of ci passes to the *Rows, to be freed
        // with releaseConn.
        rows := &Rows{
                dc:    dc,
                rowsi: rowsi,
                // releaseConn set below
        }
        s.db.addDep(s, rows)
        rows.releaseConn = func(err error) {
                releaseConn(err)
                s.db.removeDep(s, rows)
        }
        return rows, nil
}

func rowsiFromStatement(ds driverStmt, args ...interface{}) (driver.Rows, error) {
        ds.Lock()
        want := ds.si.NumInput()
        ds.Unlock()

        // -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(&ds, args)
        if err != nil {
                return nil, err
        }

        ds.Lock()
        rowsi, err := ds.si.Query(dargs)
        ds.Unlock()
        if err != nil {
                return nil, err
        }
        return rowsi, nil
}

// 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 {
        rows, err := s.Query(args...)
        if err != nil {
                return &Row{err: err}
        }
        return &Row{rows: rows}
}

// 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

        if s.tx != nil {
                s.txsi.Close()
                s.mu.Unlock()
                return nil
        }
        s.mu.Unlock()

        return s.db.removeDep(s, s)
}

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.si)
                        v.dc.removeOpenStmt(v.si)
                }
                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 ...")
//     ...
//     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

        closed    bool
        lastcols  []driver.Value
        lasterr   error       // non-nil only if closed is true
        closeStmt driver.Stmt // if non-nil, statement to Close on close
}

// Next prepares the next result row for reading with the Scan method.
// It returns true on success, false if there is no next result row.
// Every call to Scan, even the first one, must be preceded by a call
// to Next.
func (rs *Rows) Next() bool {
        if rs.closed {
                return 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 {
                rs.Close()
                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 {
        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) {
        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
}

// Scan copies the columns in the current row into the values pointed
// at by dest.
//
// If an 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. If the value
// is of type []byte, a copy is made and the caller owns the result.
func (rs *Rows) Scan(dest ...interface{}) error {
        if rs.closed {
                return errors.New("sql: Rows are closed")
        }
        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
}

var rowsCloseHook func(*Rows, *error)

// Close closes the Rows, preventing further enumeration. If Next returns
// false, 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 {
        if rs.closed {
                return nil
        }
        rs.closed = true
        err := rs.rowsi.Close()
        if fn := rowsCloseHook; fn != nil {
                fn(rs, &err)
        }
        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.  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() {
                return ErrNoRows
        }
        err := r.rows.Scan(dest...)
        if 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()
        fn()
        lk.Unlock()
}