libgo: update to go1.9

[official-gcc.git] / libgo / go / html / template / escape.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 template

import (
        "bytes"
        "fmt"
        "html"
        "io"
        "text/template"
        "text/template/parse"
)

// escapeTemplate rewrites the named template, which must be
// associated with t, to guarantee that the output of any of the named
// templates is properly escaped. If no error is returned, then the named templates have
// been modified. Otherwise the named templates have been rendered
// unusable.
func escapeTemplate(tmpl *Template, node parse.Node, name string) error {
        c, _ := tmpl.esc.escapeTree(context{}, node, name, 0)
        var err error
        if c.err != nil {
                err, c.err.Name = c.err, name
        } else if c.state != stateText {
                err = &Error{ErrEndContext, nil, name, 0, fmt.Sprintf("ends in a non-text context: %v", c)}
        }
        if err != nil {
                // Prevent execution of unsafe templates.
                if t := tmpl.set[name]; t != nil {
                        t.escapeErr = err
                        t.text.Tree = nil
                        t.Tree = nil
                }
                return err
        }
        tmpl.esc.commit()
        if t := tmpl.set[name]; t != nil {
                t.escapeErr = escapeOK
                t.Tree = t.text.Tree
        }
        return nil
}

// evalArgs formats the list of arguments into a string. It is equivalent to
// fmt.Sprint(args...), except that it deferences all pointers.
func evalArgs(args ...interface{}) string {
        // Optimization for simple common case of a single string argument.
        if len(args) == 1 {
                if s, ok := args[0].(string); ok {
                        return s
                }
        }
        for i, arg := range args {
                args[i] = indirectToStringerOrError(arg)
        }
        return fmt.Sprint(args...)
}

// funcMap maps command names to functions that render their inputs safe.
var funcMap = template.FuncMap{
        "_html_template_attrescaper":     attrEscaper,
        "_html_template_commentescaper":  commentEscaper,
        "_html_template_cssescaper":      cssEscaper,
        "_html_template_cssvaluefilter":  cssValueFilter,
        "_html_template_htmlnamefilter":  htmlNameFilter,
        "_html_template_htmlescaper":     htmlEscaper,
        "_html_template_jsregexpescaper": jsRegexpEscaper,
        "_html_template_jsstrescaper":    jsStrEscaper,
        "_html_template_jsvalescaper":    jsValEscaper,
        "_html_template_nospaceescaper":  htmlNospaceEscaper,
        "_html_template_rcdataescaper":   rcdataEscaper,
        "_html_template_urlescaper":      urlEscaper,
        "_html_template_urlfilter":       urlFilter,
        "_html_template_urlnormalizer":   urlNormalizer,
        "_eval_args_":                    evalArgs,
}

// escaper collects type inferences about templates and changes needed to make
// templates injection safe.
type escaper struct {
        // ns is the nameSpace that this escaper is associated with.
        ns *nameSpace
        // output[templateName] is the output context for a templateName that
        // has been mangled to include its input context.
        output map[string]context
        // derived[c.mangle(name)] maps to a template derived from the template
        // named name templateName for the start context c.
        derived map[string]*template.Template
        // called[templateName] is a set of called mangled template names.
        called map[string]bool
        // xxxNodeEdits are the accumulated edits to apply during commit.
        // Such edits are not applied immediately in case a template set
        // executes a given template in different escaping contexts.
        actionNodeEdits   map[*parse.ActionNode][]string
        templateNodeEdits map[*parse.TemplateNode]string
        textNodeEdits     map[*parse.TextNode][]byte
}

// makeEscaper creates a blank escaper for the given set.
func makeEscaper(n *nameSpace) escaper {
        return escaper{
                n,
                map[string]context{},
                map[string]*template.Template{},
                map[string]bool{},
                map[*parse.ActionNode][]string{},
                map[*parse.TemplateNode]string{},
                map[*parse.TextNode][]byte{},
        }
}

// filterFailsafe is an innocuous word that is emitted in place of unsafe values
// by sanitizer functions. It is not a keyword in any programming language,
// contains no special characters, is not empty, and when it appears in output
// it is distinct enough that a developer can find the source of the problem
// via a search engine.
const filterFailsafe = "ZgotmplZ"

// escape escapes a template node.
func (e *escaper) escape(c context, n parse.Node) context {
        switch n := n.(type) {
        case *parse.ActionNode:
                return e.escapeAction(c, n)
        case *parse.IfNode:
                return e.escapeBranch(c, &n.BranchNode, "if")
        case *parse.ListNode:
                return e.escapeList(c, n)
        case *parse.RangeNode:
                return e.escapeBranch(c, &n.BranchNode, "range")
        case *parse.TemplateNode:
                return e.escapeTemplate(c, n)
        case *parse.TextNode:
                return e.escapeText(c, n)
        case *parse.WithNode:
                return e.escapeBranch(c, &n.BranchNode, "with")
        }
        panic("escaping " + n.String() + " is unimplemented")
}

// escapeAction escapes an action template node.
func (e *escaper) escapeAction(c context, n *parse.ActionNode) context {
        if len(n.Pipe.Decl) != 0 {
                // A local variable assignment, not an interpolation.
                return c
        }
        c = nudge(c)
        // Check for disallowed use of predefined escapers in the pipeline.
        for pos, idNode := range n.Pipe.Cmds {
                node, ok := idNode.Args[0].(*parse.IdentifierNode)
                if !ok {
                        // A predefined escaper "esc" will never be found as an identifier in a
                        // Chain or Field node, since:
                        // - "esc.x ..." is invalid, since predefined escapers return strings, and
                        //   strings do not have methods, keys or fields.
                        // - "... .esc" is invalid, since predefined escapers are global functions,
                        //   not methods or fields of any types.
                        // Therefore, it is safe to ignore these two node types.
                        continue
                }
                ident := node.Ident
                if _, ok := predefinedEscapers[ident]; ok {
                        if pos < len(n.Pipe.Cmds)-1 ||
                                c.state == stateAttr && c.delim == delimSpaceOrTagEnd && ident == "html" {
                                return context{
                                        state: stateError,
                                        err:   errorf(ErrPredefinedEscaper, n, n.Line, "predefined escaper %q disallowed in template", ident),
                                }
                        }
                }
        }
        s := make([]string, 0, 3)
        switch c.state {
        case stateError:
                return c
        case stateURL, stateCSSDqStr, stateCSSSqStr, stateCSSDqURL, stateCSSSqURL, stateCSSURL:
                switch c.urlPart {
                case urlPartNone:
                        s = append(s, "_html_template_urlfilter")
                        fallthrough
                case urlPartPreQuery:
                        switch c.state {
                        case stateCSSDqStr, stateCSSSqStr:
                                s = append(s, "_html_template_cssescaper")
                        default:
                                s = append(s, "_html_template_urlnormalizer")
                        }
                case urlPartQueryOrFrag:
                        s = append(s, "_html_template_urlescaper")
                case urlPartUnknown:
                        return context{
                                state: stateError,
                                err:   errorf(ErrAmbigContext, n, n.Line, "%s appears in an ambiguous context within a URL", n),
                        }
                default:
                        panic(c.urlPart.String())
                }
        case stateJS:
                s = append(s, "_html_template_jsvalescaper")
                // A slash after a value starts a div operator.
                c.jsCtx = jsCtxDivOp
        case stateJSDqStr, stateJSSqStr:
                s = append(s, "_html_template_jsstrescaper")
        case stateJSRegexp:
                s = append(s, "_html_template_jsregexpescaper")
        case stateCSS:
                s = append(s, "_html_template_cssvaluefilter")
        case stateText:
                s = append(s, "_html_template_htmlescaper")
        case stateRCDATA:
                s = append(s, "_html_template_rcdataescaper")
        case stateAttr:
                // Handled below in delim check.
        case stateAttrName, stateTag:
                c.state = stateAttrName
                s = append(s, "_html_template_htmlnamefilter")
        default:
                if isComment(c.state) {
                        s = append(s, "_html_template_commentescaper")
                } else {
                        panic("unexpected state " + c.state.String())
                }
        }
        switch c.delim {
        case delimNone:
                // No extra-escaping needed for raw text content.
        case delimSpaceOrTagEnd:
                s = append(s, "_html_template_nospaceescaper")
        default:
                s = append(s, "_html_template_attrescaper")
        }
        e.editActionNode(n, s)
        return c
}

// ensurePipelineContains ensures that the pipeline ends with the commands with
// the identifiers in s in order. If the pipeline ends with a predefined escaper
// (i.e. "html" or "urlquery"), merge it with the identifiers in s.
func ensurePipelineContains(p *parse.PipeNode, s []string) {
        if len(s) == 0 {
                // Do not rewrite pipeline if we have no escapers to insert.
                return
        }
        // Precondition: p.Cmds contains at most one predefined escaper and the
        // escaper will be present at p.Cmds[len(p.Cmds)-1]. This precondition is
        // always true because of the checks in escapeAction.
        pipelineLen := len(p.Cmds)
        if pipelineLen > 0 {
                lastCmd := p.Cmds[pipelineLen-1]
                if idNode, ok := lastCmd.Args[0].(*parse.IdentifierNode); ok {
                        if esc := idNode.Ident; predefinedEscapers[esc] {
                                // Pipeline ends with a predefined escaper.
                                if len(p.Cmds) == 1 && len(lastCmd.Args) > 1 {
                                        // Special case: pipeline is of the form {{ esc arg1 arg2 ... argN }},
                                        // where esc is the predefined escaper, and arg1...argN are its arguments.
                                        // Convert this into the equivalent form
                                        // {{ _eval_args_ arg1 arg2 ... argN | esc }}, so that esc can be easily
                                        // merged with the escapers in s.
                                        lastCmd.Args[0] = parse.NewIdentifier("_eval_args_").SetTree(nil).SetPos(lastCmd.Args[0].Position())
                                        p.Cmds = appendCmd(p.Cmds, newIdentCmd(esc, p.Position()))
                                        pipelineLen++
                                }
                                // If any of the commands in s that we are about to insert is equivalent
                                // to the predefined escaper, use the predefined escaper instead.
                                dup := false
                                for i, escaper := range s {
                                        if escFnsEq(esc, escaper) {
                                                s[i] = idNode.Ident
                                                dup = true
                                        }
                                }
                                if dup {
                                        // The predefined escaper will already be inserted along with the
                                        // escapers in s, so do not copy it to the rewritten pipeline.
                                        pipelineLen--
                                }
                        }
                }
        }
        // Rewrite the pipeline, creating the escapers in s at the end of the pipeline.
        newCmds := make([]*parse.CommandNode, pipelineLen, pipelineLen+len(s))
        copy(newCmds, p.Cmds)
        for _, name := range s {
                newCmds = appendCmd(newCmds, newIdentCmd(name, p.Position()))
        }
        p.Cmds = newCmds
}

// predefinedEscapers contains template predefined escapers that are equivalent
// to some contextual escapers. Keep in sync with equivEscapers.
var predefinedEscapers = map[string]bool{
        "html":     true,
        "urlquery": true,
}

// equivEscapers matches contextual escapers to equivalent predefined
// template escapers.
var equivEscapers = map[string]string{
        // The following pairs of HTML escapers provide equivalent security
        // guarantees, since they all escape '\000', '\'', '"', '&', '<', and '>'.
        "_html_template_attrescaper":   "html",
        "_html_template_htmlescaper":   "html",
        "_html_template_rcdataescaper": "html",
        // These two URL escapers produce URLs safe for embedding in a URL query by
        // percent-encoding all the reserved characters specified in RFC 3986 Section
        // 2.2
        "_html_template_urlescaper": "urlquery",
        // These two functions are not actually equivalent; urlquery is stricter as it
        // escapes reserved characters (e.g. '#'), while _html_template_urlnormalizer
        // does not. It is therefore only safe to replace _html_template_urlnormalizer
        // with urlquery (this happens in ensurePipelineContains), but not the otherI've
        // way around. We keep this entry around to preserve the behavior of templates
        // written before Go 1.9, which might depend on this substitution taking place.
        "_html_template_urlnormalizer": "urlquery",
}

// escFnsEq reports whether the two escaping functions are equivalent.
func escFnsEq(a, b string) bool {
        if e := equivEscapers[a]; e != "" {
                a = e
        }
        if e := equivEscapers[b]; e != "" {
                b = e
        }
        return a == b
}

// redundantFuncs[a][b] implies that funcMap[b](funcMap[a](x)) == funcMap[a](x)
// for all x.
var redundantFuncs = map[string]map[string]bool{
        "_html_template_commentescaper": {
                "_html_template_attrescaper":    true,
                "_html_template_nospaceescaper": true,
                "_html_template_htmlescaper":    true,
        },
        "_html_template_cssescaper": {
                "_html_template_attrescaper": true,
        },
        "_html_template_jsregexpescaper": {
                "_html_template_attrescaper": true,
        },
        "_html_template_jsstrescaper": {
                "_html_template_attrescaper": true,
        },
        "_html_template_urlescaper": {
                "_html_template_urlnormalizer": true,
        },
}

// appendCmd appends the given command to the end of the command pipeline
// unless it is redundant with the last command.
func appendCmd(cmds []*parse.CommandNode, cmd *parse.CommandNode) []*parse.CommandNode {
        if n := len(cmds); n != 0 {
                last, okLast := cmds[n-1].Args[0].(*parse.IdentifierNode)
                next, okNext := cmd.Args[0].(*parse.IdentifierNode)
                if okLast && okNext && redundantFuncs[last.Ident][next.Ident] {
                        return cmds
                }
        }
        return append(cmds, cmd)
}

// indexOfStr is the first i such that eq(s, strs[i]) or -1 if s was not found.
func indexOfStr(s string, strs []string, eq func(a, b string) bool) int {
        for i, t := range strs {
                if eq(s, t) {
                        return i
                }
        }
        return -1
}

// newIdentCmd produces a command containing a single identifier node.
func newIdentCmd(identifier string, pos parse.Pos) *parse.CommandNode {
        return &parse.CommandNode{
                NodeType: parse.NodeCommand,
                Args:     []parse.Node{parse.NewIdentifier(identifier).SetTree(nil).SetPos(pos)}, // TODO: SetTree.
        }
}

// nudge returns the context that would result from following empty string
// transitions from the input context.
// For example, parsing:
//     `<a href=`
// will end in context{stateBeforeValue, attrURL}, but parsing one extra rune:
//     `<a href=x`
// will end in context{stateURL, delimSpaceOrTagEnd, ...}.
// There are two transitions that happen when the 'x' is seen:
// (1) Transition from a before-value state to a start-of-value state without
//     consuming any character.
// (2) Consume 'x' and transition past the first value character.
// In this case, nudging produces the context after (1) happens.
func nudge(c context) context {
        switch c.state {
        case stateTag:
                // In `<foo {{.}}`, the action should emit an attribute.
                c.state = stateAttrName
        case stateBeforeValue:
                // In `<foo bar={{.}}`, the action is an undelimited value.
                c.state, c.delim, c.attr = attrStartStates[c.attr], delimSpaceOrTagEnd, attrNone
        case stateAfterName:
                // In `<foo bar {{.}}`, the action is an attribute name.
                c.state, c.attr = stateAttrName, attrNone
        }
        return c
}

// join joins the two contexts of a branch template node. The result is an
// error context if either of the input contexts are error contexts, or if the
// the input contexts differ.
func join(a, b context, node parse.Node, nodeName string) context {
        if a.state == stateError {
                return a
        }
        if b.state == stateError {
                return b
        }
        if a.eq(b) {
                return a
        }

        c := a
        c.urlPart = b.urlPart
        if c.eq(b) {
                // The contexts differ only by urlPart.
                c.urlPart = urlPartUnknown
                return c
        }

        c = a
        c.jsCtx = b.jsCtx
        if c.eq(b) {
                // The contexts differ only by jsCtx.
                c.jsCtx = jsCtxUnknown
                return c
        }

        // Allow a nudged context to join with an unnudged one.
        // This means that
        //   <p title={{if .C}}{{.}}{{end}}
        // ends in an unquoted value state even though the else branch
        // ends in stateBeforeValue.
        if c, d := nudge(a), nudge(b); !(c.eq(a) && d.eq(b)) {
                if e := join(c, d, node, nodeName); e.state != stateError {
                        return e
                }
        }

        return context{
                state: stateError,
                err:   errorf(ErrBranchEnd, node, 0, "{{%s}} branches end in different contexts: %v, %v", nodeName, a, b),
        }
}

// escapeBranch escapes a branch template node: "if", "range" and "with".
func (e *escaper) escapeBranch(c context, n *parse.BranchNode, nodeName string) context {
        c0 := e.escapeList(c, n.List)
        if nodeName == "range" && c0.state != stateError {
                // The "true" branch of a "range" node can execute multiple times.
                // We check that executing n.List once results in the same context
                // as executing n.List twice.
                c1, _ := e.escapeListConditionally(c0, n.List, nil)
                c0 = join(c0, c1, n, nodeName)
                if c0.state == stateError {
                        // Make clear that this is a problem on loop re-entry
                        // since developers tend to overlook that branch when
                        // debugging templates.
                        c0.err.Line = n.Line
                        c0.err.Description = "on range loop re-entry: " + c0.err.Description
                        return c0
                }
        }
        c1 := e.escapeList(c, n.ElseList)
        return join(c0, c1, n, nodeName)
}

// escapeList escapes a list template node.
func (e *escaper) escapeList(c context, n *parse.ListNode) context {
        if n == nil {
                return c
        }
        for _, m := range n.Nodes {
                c = e.escape(c, m)
        }
        return c
}

// escapeListConditionally escapes a list node but only preserves edits and
// inferences in e if the inferences and output context satisfy filter.
// It returns the best guess at an output context, and the result of the filter
// which is the same as whether e was updated.
func (e *escaper) escapeListConditionally(c context, n *parse.ListNode, filter func(*escaper, context) bool) (context, bool) {
        e1 := makeEscaper(e.ns)
        // Make type inferences available to f.
        for k, v := range e.output {
                e1.output[k] = v
        }
        c = e1.escapeList(c, n)
        ok := filter != nil && filter(&e1, c)
        if ok {
                // Copy inferences and edits from e1 back into e.
                for k, v := range e1.output {
                        e.output[k] = v
                }
                for k, v := range e1.derived {
                        e.derived[k] = v
                }
                for k, v := range e1.called {
                        e.called[k] = v
                }
                for k, v := range e1.actionNodeEdits {
                        e.editActionNode(k, v)
                }
                for k, v := range e1.templateNodeEdits {
                        e.editTemplateNode(k, v)
                }
                for k, v := range e1.textNodeEdits {
                        e.editTextNode(k, v)
                }
        }
        return c, ok
}

// escapeTemplate escapes a {{template}} call node.
func (e *escaper) escapeTemplate(c context, n *parse.TemplateNode) context {
        c, name := e.escapeTree(c, n, n.Name, n.Line)
        if name != n.Name {
                e.editTemplateNode(n, name)
        }
        return c
}

// escapeTree escapes the named template starting in the given context as
// necessary and returns its output context.
func (e *escaper) escapeTree(c context, node parse.Node, name string, line int) (context, string) {
        // Mangle the template name with the input context to produce a reliable
        // identifier.
        dname := c.mangle(name)
        e.called[dname] = true
        if out, ok := e.output[dname]; ok {
                // Already escaped.
                return out, dname
        }
        t := e.template(name)
        if t == nil {
                // Two cases: The template exists but is empty, or has never been mentioned at
                // all. Distinguish the cases in the error messages.
                if e.ns.set[name] != nil {
                        return context{
                                state: stateError,
                                err:   errorf(ErrNoSuchTemplate, node, line, "%q is an incomplete or empty template", name),
                        }, dname
                }
                return context{
                        state: stateError,
                        err:   errorf(ErrNoSuchTemplate, node, line, "no such template %q", name),
                }, dname
        }
        if dname != name {
                // Use any template derived during an earlier call to escapeTemplate
                // with different top level templates, or clone if necessary.
                dt := e.template(dname)
                if dt == nil {
                        dt = template.New(dname)
                        dt.Tree = &parse.Tree{Name: dname, Root: t.Root.CopyList()}
                        e.derived[dname] = dt
                }
                t = dt
        }
        return e.computeOutCtx(c, t), dname
}

// computeOutCtx takes a template and its start context and computes the output
// context while storing any inferences in e.
func (e *escaper) computeOutCtx(c context, t *template.Template) context {
        // Propagate context over the body.
        c1, ok := e.escapeTemplateBody(c, t)
        if !ok {
                // Look for a fixed point by assuming c1 as the output context.
                if c2, ok2 := e.escapeTemplateBody(c1, t); ok2 {
                        c1, ok = c2, true
                }
                // Use c1 as the error context if neither assumption worked.
        }
        if !ok && c1.state != stateError {
                return context{
                        state: stateError,
                        err:   errorf(ErrOutputContext, t.Tree.Root, 0, "cannot compute output context for template %s", t.Name()),
                }
        }
        return c1
}

// escapeTemplateBody escapes the given template assuming the given output
// context, and returns the best guess at the output context and whether the
// assumption was correct.
func (e *escaper) escapeTemplateBody(c context, t *template.Template) (context, bool) {
        filter := func(e1 *escaper, c1 context) bool {
                if c1.state == stateError {
                        // Do not update the input escaper, e.
                        return false
                }
                if !e1.called[t.Name()] {
                        // If t is not recursively called, then c1 is an
                        // accurate output context.
                        return true
                }
                // c1 is accurate if it matches our assumed output context.
                return c.eq(c1)
        }
        // We need to assume an output context so that recursive template calls
        // take the fast path out of escapeTree instead of infinitely recursing.
        // Naively assuming that the input context is the same as the output
        // works >90% of the time.
        e.output[t.Name()] = c
        return e.escapeListConditionally(c, t.Tree.Root, filter)
}

// delimEnds maps each delim to a string of characters that terminate it.
var delimEnds = [...]string{
        delimDoubleQuote: `"`,
        delimSingleQuote: "'",
        // Determined empirically by running the below in various browsers.
        // var div = document.createElement("DIV");
        // for (var i = 0; i < 0x10000; ++i) {
        //   div.innerHTML = "<span title=x" + String.fromCharCode(i) + "-bar>";
        //   if (div.getElementsByTagName("SPAN")[0].title.indexOf("bar") < 0)
        //     document.write("<p>U+" + i.toString(16));
        // }
        delimSpaceOrTagEnd: " \t\n\f\r>",
}

var doctypeBytes = []byte("<!DOCTYPE")

// escapeText escapes a text template node.
func (e *escaper) escapeText(c context, n *parse.TextNode) context {
        s, written, i, b := n.Text, 0, 0, new(bytes.Buffer)
        for i != len(s) {
                c1, nread := contextAfterText(c, s[i:])
                i1 := i + nread
                if c.state == stateText || c.state == stateRCDATA {
                        end := i1
                        if c1.state != c.state {
                                for j := end - 1; j >= i; j-- {
                                        if s[j] == '<' {
                                                end = j
                                                break
                                        }
                                }
                        }
                        for j := i; j < end; j++ {
                                if s[j] == '<' && !bytes.HasPrefix(bytes.ToUpper(s[j:]), doctypeBytes) {
                                        b.Write(s[written:j])
                                        b.WriteString("&lt;")
                                        written = j + 1
                                }
                        }
                } else if isComment(c.state) && c.delim == delimNone {
                        switch c.state {
                        case stateJSBlockCmt:
                                // http://es5.github.com/#x7.4:
                                // "Comments behave like white space and are
                                // discarded except that, if a MultiLineComment
                                // contains a line terminator character, then
                                // the entire comment is considered to be a
                                // LineTerminator for purposes of parsing by
                                // the syntactic grammar."
                                if bytes.ContainsAny(s[written:i1], "\n\r\u2028\u2029") {
                                        b.WriteByte('\n')
                                } else {
                                        b.WriteByte(' ')
                                }
                        case stateCSSBlockCmt:
                                b.WriteByte(' ')
                        }
                        written = i1
                }
                if c.state != c1.state && isComment(c1.state) && c1.delim == delimNone {
                        // Preserve the portion between written and the comment start.
                        cs := i1 - 2
                        if c1.state == stateHTMLCmt {
                                // "<!--" instead of "/*" or "//"
                                cs -= 2
                        }
                        b.Write(s[written:cs])
                        written = i1
                }
                if i == i1 && c.state == c1.state {
                        panic(fmt.Sprintf("infinite loop from %v to %v on %q..%q", c, c1, s[:i], s[i:]))
                }
                c, i = c1, i1
        }

        if written != 0 && c.state != stateError {
                if !isComment(c.state) || c.delim != delimNone {
                        b.Write(n.Text[written:])
                }
                e.editTextNode(n, b.Bytes())
        }
        return c
}

// contextAfterText starts in context c, consumes some tokens from the front of
// s, then returns the context after those tokens and the unprocessed suffix.
func contextAfterText(c context, s []byte) (context, int) {
        if c.delim == delimNone {
                c1, i := tSpecialTagEnd(c, s)
                if i == 0 {
                        // A special end tag (`</script>`) has been seen and
                        // all content preceding it has been consumed.
                        return c1, 0
                }
                // Consider all content up to any end tag.
                return transitionFunc[c.state](c, s[:i])
        }

        // We are at the beginning of an attribute value.

        i := bytes.IndexAny(s, delimEnds[c.delim])
        if i == -1 {
                i = len(s)
        }
        if c.delim == delimSpaceOrTagEnd {
                // http://www.w3.org/TR/html5/syntax.html#attribute-value-(unquoted)-state
                // lists the runes below as error characters.
                // Error out because HTML parsers may differ on whether
                // "<a id= onclick=f("     ends inside id's or onclick's value,
                // "<a class=`foo "        ends inside a value,
                // "<a style=font:'Arial'" needs open-quote fixup.
                // IE treats '`' as a quotation character.
                if j := bytes.IndexAny(s[:i], "\"'<=`"); j >= 0 {
                        return context{
                                state: stateError,
                                err:   errorf(ErrBadHTML, nil, 0, "%q in unquoted attr: %q", s[j:j+1], s[:i]),
                        }, len(s)
                }
        }
        if i == len(s) {
                // Remain inside the attribute.
                // Decode the value so non-HTML rules can easily handle
                //     <button onclick="alert(&quot;Hi!&quot;)">
                // without having to entity decode token boundaries.
                for u := []byte(html.UnescapeString(string(s))); len(u) != 0; {
                        c1, i1 := transitionFunc[c.state](c, u)
                        c, u = c1, u[i1:]
                }
                return c, len(s)
        }

        element := c.element

        // If this is a non-JS "type" attribute inside "script" tag, do not treat the contents as JS.
        if c.state == stateAttr && c.element == elementScript && c.attr == attrScriptType && !isJSType(string(s[:i])) {
                element = elementNone
        }

        if c.delim != delimSpaceOrTagEnd {
                // Consume any quote.
                i++
        }
        // On exiting an attribute, we discard all state information
        // except the state and element.
        return context{state: stateTag, element: element}, i
}

// editActionNode records a change to an action pipeline for later commit.
func (e *escaper) editActionNode(n *parse.ActionNode, cmds []string) {
        if _, ok := e.actionNodeEdits[n]; ok {
                panic(fmt.Sprintf("node %s shared between templates", n))
        }
        e.actionNodeEdits[n] = cmds
}

// editTemplateNode records a change to a {{template}} callee for later commit.
func (e *escaper) editTemplateNode(n *parse.TemplateNode, callee string) {
        if _, ok := e.templateNodeEdits[n]; ok {
                panic(fmt.Sprintf("node %s shared between templates", n))
        }
        e.templateNodeEdits[n] = callee
}

// editTextNode records a change to a text node for later commit.
func (e *escaper) editTextNode(n *parse.TextNode, text []byte) {
        if _, ok := e.textNodeEdits[n]; ok {
                panic(fmt.Sprintf("node %s shared between templates", n))
        }
        e.textNodeEdits[n] = text
}

// commit applies changes to actions and template calls needed to contextually
// autoescape content and adds any derived templates to the set.
func (e *escaper) commit() {
        for name := range e.output {
                e.template(name).Funcs(funcMap)
        }
        // Any template from the name space associated with this escaper can be used
        // to add derived templates to the underlying text/template name space.
        tmpl := e.arbitraryTemplate()
        for _, t := range e.derived {
                if _, err := tmpl.text.AddParseTree(t.Name(), t.Tree); err != nil {
                        panic("error adding derived template")
                }
        }
        for n, s := range e.actionNodeEdits {
                ensurePipelineContains(n.Pipe, s)
        }
        for n, name := range e.templateNodeEdits {
                n.Name = name
        }
        for n, s := range e.textNodeEdits {
                n.Text = s
        }
        // Reset state that is specific to this commit so that the same changes are
        // not re-applied to the template on subsequent calls to commit.
        e.called = make(map[string]bool)
        e.actionNodeEdits = make(map[*parse.ActionNode][]string)
        e.templateNodeEdits = make(map[*parse.TemplateNode]string)
        e.textNodeEdits = make(map[*parse.TextNode][]byte)
}

// template returns the named template given a mangled template name.
func (e *escaper) template(name string) *template.Template {
        // Any template from the name space associated with this escaper can be used
        // to look up templates in the underlying text/template name space.
        t := e.arbitraryTemplate().text.Lookup(name)
        if t == nil {
                t = e.derived[name]
        }
        return t
}

// arbitraryTemplate returns an arbitrary template from the name space
// associated with e and panics if no templates are found.
func (e *escaper) arbitraryTemplate() *Template {
        for _, t := range e.ns.set {
                return t
        }
        panic("no templates in name space")
}

// Forwarding functions so that clients need only import this package
// to reach the general escaping functions of text/template.

// HTMLEscape writes to w the escaped HTML equivalent of the plain text data b.
func HTMLEscape(w io.Writer, b []byte) {
        template.HTMLEscape(w, b)
}

// HTMLEscapeString returns the escaped HTML equivalent of the plain text data s.
func HTMLEscapeString(s string) string {
        return template.HTMLEscapeString(s)
}

// HTMLEscaper returns the escaped HTML equivalent of the textual
// representation of its arguments.
func HTMLEscaper(args ...interface{}) string {
        return template.HTMLEscaper(args...)
}

// JSEscape writes to w the escaped JavaScript equivalent of the plain text data b.
func JSEscape(w io.Writer, b []byte) {
        template.JSEscape(w, b)
}

// JSEscapeString returns the escaped JavaScript equivalent of the plain text data s.
func JSEscapeString(s string) string {
        return template.JSEscapeString(s)
}

// JSEscaper returns the escaped JavaScript equivalent of the textual
// representation of its arguments.
func JSEscaper(args ...interface{}) string {
        return template.JSEscaper(args...)
}

// URLQueryEscaper returns the escaped value of the textual representation of
// its arguments in a form suitable for embedding in a URL query.
func URLQueryEscaper(args ...interface{}) string {
        return template.URLQueryEscaper(args...)
}