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Source file src/html/template/escape.go

Documentation: html/template

     1  // Copyright 2011 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package template
     6  
     7  import (
     8  	"bytes"
     9  	"fmt"
    10  	"html"
    11  	"io"
    12  	"text/template"
    13  	"text/template/parse"
    14  )
    15  
    16  // escapeTemplate rewrites the named template, which must be
    17  // associated with t, to guarantee that the output of any of the named
    18  // templates is properly escaped. If no error is returned, then the named templates have
    19  // been modified. Otherwise the named templates have been rendered
    20  // unusable.
    21  func escapeTemplate(tmpl *Template, node parse.Node, name string) error {
    22  	c, _ := tmpl.esc.escapeTree(context{}, node, name, 0)
    23  	var err error
    24  	if c.err != nil {
    25  		err, c.err.Name = c.err, name
    26  	} else if c.state != stateText {
    27  		err = &Error{ErrEndContext, nil, name, 0, fmt.Sprintf("ends in a non-text context: %v", c)}
    28  	}
    29  	if err != nil {
    30  		// Prevent execution of unsafe templates.
    31  		if t := tmpl.set[name]; t != nil {
    32  			t.escapeErr = err
    33  			t.text.Tree = nil
    34  			t.Tree = nil
    35  		}
    36  		return err
    37  	}
    38  	tmpl.esc.commit()
    39  	if t := tmpl.set[name]; t != nil {
    40  		t.escapeErr = escapeOK
    41  		t.Tree = t.text.Tree
    42  	}
    43  	return nil
    44  }
    45  
    46  // evalArgs formats the list of arguments into a string. It is equivalent to
    47  // fmt.Sprint(args...), except that it deferences all pointers.
    48  func evalArgs(args ...interface{}) string {
    49  	// Optimization for simple common case of a single string argument.
    50  	if len(args) == 1 {
    51  		if s, ok := args[0].(string); ok {
    52  			return s
    53  		}
    54  	}
    55  	for i, arg := range args {
    56  		args[i] = indirectToStringerOrError(arg)
    57  	}
    58  	return fmt.Sprint(args...)
    59  }
    60  
    61  // funcMap maps command names to functions that render their inputs safe.
    62  var funcMap = template.FuncMap{
    63  	"_html_template_attrescaper":     attrEscaper,
    64  	"_html_template_commentescaper":  commentEscaper,
    65  	"_html_template_cssescaper":      cssEscaper,
    66  	"_html_template_cssvaluefilter":  cssValueFilter,
    67  	"_html_template_htmlnamefilter":  htmlNameFilter,
    68  	"_html_template_htmlescaper":     htmlEscaper,
    69  	"_html_template_jsregexpescaper": jsRegexpEscaper,
    70  	"_html_template_jsstrescaper":    jsStrEscaper,
    71  	"_html_template_jsvalescaper":    jsValEscaper,
    72  	"_html_template_nospaceescaper":  htmlNospaceEscaper,
    73  	"_html_template_rcdataescaper":   rcdataEscaper,
    74  	"_html_template_srcsetescaper":   srcsetFilterAndEscaper,
    75  	"_html_template_urlescaper":      urlEscaper,
    76  	"_html_template_urlfilter":       urlFilter,
    77  	"_html_template_urlnormalizer":   urlNormalizer,
    78  	"_eval_args_":                    evalArgs,
    79  }
    80  
    81  // escaper collects type inferences about templates and changes needed to make
    82  // templates injection safe.
    83  type escaper struct {
    84  	// ns is the nameSpace that this escaper is associated with.
    85  	ns *nameSpace
    86  	// output[templateName] is the output context for a templateName that
    87  	// has been mangled to include its input context.
    88  	output map[string]context
    89  	// derived[c.mangle(name)] maps to a template derived from the template
    90  	// named name templateName for the start context c.
    91  	derived map[string]*template.Template
    92  	// called[templateName] is a set of called mangled template names.
    93  	called map[string]bool
    94  	// xxxNodeEdits are the accumulated edits to apply during commit.
    95  	// Such edits are not applied immediately in case a template set
    96  	// executes a given template in different escaping contexts.
    97  	actionNodeEdits   map[*parse.ActionNode][]string
    98  	templateNodeEdits map[*parse.TemplateNode]string
    99  	textNodeEdits     map[*parse.TextNode][]byte
   100  }
   101  
   102  // makeEscaper creates a blank escaper for the given set.
   103  func makeEscaper(n *nameSpace) escaper {
   104  	return escaper{
   105  		n,
   106  		map[string]context{},
   107  		map[string]*template.Template{},
   108  		map[string]bool{},
   109  		map[*parse.ActionNode][]string{},
   110  		map[*parse.TemplateNode]string{},
   111  		map[*parse.TextNode][]byte{},
   112  	}
   113  }
   114  
   115  // filterFailsafe is an innocuous word that is emitted in place of unsafe values
   116  // by sanitizer functions. It is not a keyword in any programming language,
   117  // contains no special characters, is not empty, and when it appears in output
   118  // it is distinct enough that a developer can find the source of the problem
   119  // via a search engine.
   120  const filterFailsafe = "ZgotmplZ"
   121  
   122  // escape escapes a template node.
   123  func (e *escaper) escape(c context, n parse.Node) context {
   124  	switch n := n.(type) {
   125  	case *parse.ActionNode:
   126  		return e.escapeAction(c, n)
   127  	case *parse.IfNode:
   128  		return e.escapeBranch(c, &n.BranchNode, "if")
   129  	case *parse.ListNode:
   130  		return e.escapeList(c, n)
   131  	case *parse.RangeNode:
   132  		return e.escapeBranch(c, &n.BranchNode, "range")
   133  	case *parse.TemplateNode:
   134  		return e.escapeTemplate(c, n)
   135  	case *parse.TextNode:
   136  		return e.escapeText(c, n)
   137  	case *parse.WithNode:
   138  		return e.escapeBranch(c, &n.BranchNode, "with")
   139  	}
   140  	panic("escaping " + n.String() + " is unimplemented")
   141  }
   142  
   143  // escapeAction escapes an action template node.
   144  func (e *escaper) escapeAction(c context, n *parse.ActionNode) context {
   145  	if len(n.Pipe.Decl) != 0 {
   146  		// A local variable assignment, not an interpolation.
   147  		return c
   148  	}
   149  	c = nudge(c)
   150  	// Check for disallowed use of predefined escapers in the pipeline.
   151  	for pos, idNode := range n.Pipe.Cmds {
   152  		node, ok := idNode.Args[0].(*parse.IdentifierNode)
   153  		if !ok {
   154  			// A predefined escaper "esc" will never be found as an identifier in a
   155  			// Chain or Field node, since:
   156  			// - "esc.x ..." is invalid, since predefined escapers return strings, and
   157  			//   strings do not have methods, keys or fields.
   158  			// - "... .esc" is invalid, since predefined escapers are global functions,
   159  			//   not methods or fields of any types.
   160  			// Therefore, it is safe to ignore these two node types.
   161  			continue
   162  		}
   163  		ident := node.Ident
   164  		if _, ok := predefinedEscapers[ident]; ok {
   165  			if pos < len(n.Pipe.Cmds)-1 ||
   166  				c.state == stateAttr && c.delim == delimSpaceOrTagEnd && ident == "html" {
   167  				return context{
   168  					state: stateError,
   169  					err:   errorf(ErrPredefinedEscaper, n, n.Line, "predefined escaper %q disallowed in template", ident),
   170  				}
   171  			}
   172  		}
   173  	}
   174  	s := make([]string, 0, 3)
   175  	switch c.state {
   176  	case stateError:
   177  		return c
   178  	case stateURL, stateCSSDqStr, stateCSSSqStr, stateCSSDqURL, stateCSSSqURL, stateCSSURL:
   179  		switch c.urlPart {
   180  		case urlPartNone:
   181  			s = append(s, "_html_template_urlfilter")
   182  			fallthrough
   183  		case urlPartPreQuery:
   184  			switch c.state {
   185  			case stateCSSDqStr, stateCSSSqStr:
   186  				s = append(s, "_html_template_cssescaper")
   187  			default:
   188  				s = append(s, "_html_template_urlnormalizer")
   189  			}
   190  		case urlPartQueryOrFrag:
   191  			s = append(s, "_html_template_urlescaper")
   192  		case urlPartUnknown:
   193  			return context{
   194  				state: stateError,
   195  				err:   errorf(ErrAmbigContext, n, n.Line, "%s appears in an ambiguous context within a URL", n),
   196  			}
   197  		default:
   198  			panic(c.urlPart.String())
   199  		}
   200  	case stateJS:
   201  		s = append(s, "_html_template_jsvalescaper")
   202  		// A slash after a value starts a div operator.
   203  		c.jsCtx = jsCtxDivOp
   204  	case stateJSDqStr, stateJSSqStr:
   205  		s = append(s, "_html_template_jsstrescaper")
   206  	case stateJSRegexp:
   207  		s = append(s, "_html_template_jsregexpescaper")
   208  	case stateCSS:
   209  		s = append(s, "_html_template_cssvaluefilter")
   210  	case stateText:
   211  		s = append(s, "_html_template_htmlescaper")
   212  	case stateRCDATA:
   213  		s = append(s, "_html_template_rcdataescaper")
   214  	case stateAttr:
   215  		// Handled below in delim check.
   216  	case stateAttrName, stateTag:
   217  		c.state = stateAttrName
   218  		s = append(s, "_html_template_htmlnamefilter")
   219  	case stateSrcset:
   220  		s = append(s, "_html_template_srcsetescaper")
   221  	default:
   222  		if isComment(c.state) {
   223  			s = append(s, "_html_template_commentescaper")
   224  		} else {
   225  			panic("unexpected state " + c.state.String())
   226  		}
   227  	}
   228  	switch c.delim {
   229  	case delimNone:
   230  		// No extra-escaping needed for raw text content.
   231  	case delimSpaceOrTagEnd:
   232  		s = append(s, "_html_template_nospaceescaper")
   233  	default:
   234  		s = append(s, "_html_template_attrescaper")
   235  	}
   236  	e.editActionNode(n, s)
   237  	return c
   238  }
   239  
   240  // ensurePipelineContains ensures that the pipeline ends with the commands with
   241  // the identifiers in s in order. If the pipeline ends with a predefined escaper
   242  // (i.e. "html" or "urlquery"), merge it with the identifiers in s.
   243  func ensurePipelineContains(p *parse.PipeNode, s []string) {
   244  	if len(s) == 0 {
   245  		// Do not rewrite pipeline if we have no escapers to insert.
   246  		return
   247  	}
   248  	// Precondition: p.Cmds contains at most one predefined escaper and the
   249  	// escaper will be present at p.Cmds[len(p.Cmds)-1]. This precondition is
   250  	// always true because of the checks in escapeAction.
   251  	pipelineLen := len(p.Cmds)
   252  	if pipelineLen > 0 {
   253  		lastCmd := p.Cmds[pipelineLen-1]
   254  		if idNode, ok := lastCmd.Args[0].(*parse.IdentifierNode); ok {
   255  			if esc := idNode.Ident; predefinedEscapers[esc] {
   256  				// Pipeline ends with a predefined escaper.
   257  				if len(p.Cmds) == 1 && len(lastCmd.Args) > 1 {
   258  					// Special case: pipeline is of the form {{ esc arg1 arg2 ... argN }},
   259  					// where esc is the predefined escaper, and arg1...argN are its arguments.
   260  					// Convert this into the equivalent form
   261  					// {{ _eval_args_ arg1 arg2 ... argN | esc }}, so that esc can be easily
   262  					// merged with the escapers in s.
   263  					lastCmd.Args[0] = parse.NewIdentifier("_eval_args_").SetTree(nil).SetPos(lastCmd.Args[0].Position())
   264  					p.Cmds = appendCmd(p.Cmds, newIdentCmd(esc, p.Position()))
   265  					pipelineLen++
   266  				}
   267  				// If any of the commands in s that we are about to insert is equivalent
   268  				// to the predefined escaper, use the predefined escaper instead.
   269  				dup := false
   270  				for i, escaper := range s {
   271  					if escFnsEq(esc, escaper) {
   272  						s[i] = idNode.Ident
   273  						dup = true
   274  					}
   275  				}
   276  				if dup {
   277  					// The predefined escaper will already be inserted along with the
   278  					// escapers in s, so do not copy it to the rewritten pipeline.
   279  					pipelineLen--
   280  				}
   281  			}
   282  		}
   283  	}
   284  	// Rewrite the pipeline, creating the escapers in s at the end of the pipeline.
   285  	newCmds := make([]*parse.CommandNode, pipelineLen, pipelineLen+len(s))
   286  	insertedIdents := make(map[string]bool)
   287  	for i := 0; i < pipelineLen; i++ {
   288  		cmd := p.Cmds[i]
   289  		newCmds[i] = cmd
   290  		if idNode, ok := cmd.Args[0].(*parse.IdentifierNode); ok {
   291  			insertedIdents[normalizeEscFn(idNode.Ident)] = true
   292  		}
   293  	}
   294  	for _, name := range s {
   295  		if !insertedIdents[normalizeEscFn(name)] {
   296  			// When two templates share an underlying parse tree via the use of
   297  			// AddParseTree and one template is executed after the other, this check
   298  			// ensures that escapers that were already inserted into the pipeline on
   299  			// the first escaping pass do not get inserted again.
   300  			newCmds = appendCmd(newCmds, newIdentCmd(name, p.Position()))
   301  		}
   302  	}
   303  	p.Cmds = newCmds
   304  }
   305  
   306  // predefinedEscapers contains template predefined escapers that are equivalent
   307  // to some contextual escapers. Keep in sync with equivEscapers.
   308  var predefinedEscapers = map[string]bool{
   309  	"html":     true,
   310  	"urlquery": true,
   311  }
   312  
   313  // equivEscapers matches contextual escapers to equivalent predefined
   314  // template escapers.
   315  var equivEscapers = map[string]string{
   316  	// The following pairs of HTML escapers provide equivalent security
   317  	// guarantees, since they all escape '\000', '\'', '"', '&', '<', and '>'.
   318  	"_html_template_attrescaper":   "html",
   319  	"_html_template_htmlescaper":   "html",
   320  	"_html_template_rcdataescaper": "html",
   321  	// These two URL escapers produce URLs safe for embedding in a URL query by
   322  	// percent-encoding all the reserved characters specified in RFC 3986 Section
   323  	// 2.2
   324  	"_html_template_urlescaper": "urlquery",
   325  	// These two functions are not actually equivalent; urlquery is stricter as it
   326  	// escapes reserved characters (e.g. '#'), while _html_template_urlnormalizer
   327  	// does not. It is therefore only safe to replace _html_template_urlnormalizer
   328  	// with urlquery (this happens in ensurePipelineContains), but not the otherI've
   329  	// way around. We keep this entry around to preserve the behavior of templates
   330  	// written before Go 1.9, which might depend on this substitution taking place.
   331  	"_html_template_urlnormalizer": "urlquery",
   332  }
   333  
   334  // escFnsEq reports whether the two escaping functions are equivalent.
   335  func escFnsEq(a, b string) bool {
   336  	return normalizeEscFn(a) == normalizeEscFn(b)
   337  }
   338  
   339  // normalizeEscFn(a) is equal to normalizeEscFn(b) for any pair of names of
   340  // escaper functions a and b that are equivalent.
   341  func normalizeEscFn(e string) string {
   342  	if norm := equivEscapers[e]; norm != "" {
   343  		return norm
   344  	}
   345  	return e
   346  }
   347  
   348  // redundantFuncs[a][b] implies that funcMap[b](funcMap[a](x)) == funcMap[a](x)
   349  // for all x.
   350  var redundantFuncs = map[string]map[string]bool{
   351  	"_html_template_commentescaper": {
   352  		"_html_template_attrescaper":    true,
   353  		"_html_template_nospaceescaper": true,
   354  		"_html_template_htmlescaper":    true,
   355  	},
   356  	"_html_template_cssescaper": {
   357  		"_html_template_attrescaper": true,
   358  	},
   359  	"_html_template_jsregexpescaper": {
   360  		"_html_template_attrescaper": true,
   361  	},
   362  	"_html_template_jsstrescaper": {
   363  		"_html_template_attrescaper": true,
   364  	},
   365  	"_html_template_urlescaper": {
   366  		"_html_template_urlnormalizer": true,
   367  	},
   368  }
   369  
   370  // appendCmd appends the given command to the end of the command pipeline
   371  // unless it is redundant with the last command.
   372  func appendCmd(cmds []*parse.CommandNode, cmd *parse.CommandNode) []*parse.CommandNode {
   373  	if n := len(cmds); n != 0 {
   374  		last, okLast := cmds[n-1].Args[0].(*parse.IdentifierNode)
   375  		next, okNext := cmd.Args[0].(*parse.IdentifierNode)
   376  		if okLast && okNext && redundantFuncs[last.Ident][next.Ident] {
   377  			return cmds
   378  		}
   379  	}
   380  	return append(cmds, cmd)
   381  }
   382  
   383  // newIdentCmd produces a command containing a single identifier node.
   384  func newIdentCmd(identifier string, pos parse.Pos) *parse.CommandNode {
   385  	return &parse.CommandNode{
   386  		NodeType: parse.NodeCommand,
   387  		Args:     []parse.Node{parse.NewIdentifier(identifier).SetTree(nil).SetPos(pos)}, // TODO: SetTree.
   388  	}
   389  }
   390  
   391  // nudge returns the context that would result from following empty string
   392  // transitions from the input context.
   393  // For example, parsing:
   394  //     `<a href=`
   395  // will end in context{stateBeforeValue, attrURL}, but parsing one extra rune:
   396  //     `<a href=x`
   397  // will end in context{stateURL, delimSpaceOrTagEnd, ...}.
   398  // There are two transitions that happen when the 'x' is seen:
   399  // (1) Transition from a before-value state to a start-of-value state without
   400  //     consuming any character.
   401  // (2) Consume 'x' and transition past the first value character.
   402  // In this case, nudging produces the context after (1) happens.
   403  func nudge(c context) context {
   404  	switch c.state {
   405  	case stateTag:
   406  		// In `<foo {{.}}`, the action should emit an attribute.
   407  		c.state = stateAttrName
   408  	case stateBeforeValue:
   409  		// In `<foo bar={{.}}`, the action is an undelimited value.
   410  		c.state, c.delim, c.attr = attrStartStates[c.attr], delimSpaceOrTagEnd, attrNone
   411  	case stateAfterName:
   412  		// In `<foo bar {{.}}`, the action is an attribute name.
   413  		c.state, c.attr = stateAttrName, attrNone
   414  	}
   415  	return c
   416  }
   417  
   418  // join joins the two contexts of a branch template node. The result is an
   419  // error context if either of the input contexts are error contexts, or if the
   420  // input contexts differ.
   421  func join(a, b context, node parse.Node, nodeName string) context {
   422  	if a.state == stateError {
   423  		return a
   424  	}
   425  	if b.state == stateError {
   426  		return b
   427  	}
   428  	if a.eq(b) {
   429  		return a
   430  	}
   431  
   432  	c := a
   433  	c.urlPart = b.urlPart
   434  	if c.eq(b) {
   435  		// The contexts differ only by urlPart.
   436  		c.urlPart = urlPartUnknown
   437  		return c
   438  	}
   439  
   440  	c = a
   441  	c.jsCtx = b.jsCtx
   442  	if c.eq(b) {
   443  		// The contexts differ only by jsCtx.
   444  		c.jsCtx = jsCtxUnknown
   445  		return c
   446  	}
   447  
   448  	// Allow a nudged context to join with an unnudged one.
   449  	// This means that
   450  	//   <p title={{if .C}}{{.}}{{end}}
   451  	// ends in an unquoted value state even though the else branch
   452  	// ends in stateBeforeValue.
   453  	if c, d := nudge(a), nudge(b); !(c.eq(a) && d.eq(b)) {
   454  		if e := join(c, d, node, nodeName); e.state != stateError {
   455  			return e
   456  		}
   457  	}
   458  
   459  	return context{
   460  		state: stateError,
   461  		err:   errorf(ErrBranchEnd, node, 0, "{{%s}} branches end in different contexts: %v, %v", nodeName, a, b),
   462  	}
   463  }
   464  
   465  // escapeBranch escapes a branch template node: "if", "range" and "with".
   466  func (e *escaper) escapeBranch(c context, n *parse.BranchNode, nodeName string) context {
   467  	c0 := e.escapeList(c, n.List)
   468  	if nodeName == "range" && c0.state != stateError {
   469  		// The "true" branch of a "range" node can execute multiple times.
   470  		// We check that executing n.List once results in the same context
   471  		// as executing n.List twice.
   472  		c1, _ := e.escapeListConditionally(c0, n.List, nil)
   473  		c0 = join(c0, c1, n, nodeName)
   474  		if c0.state == stateError {
   475  			// Make clear that this is a problem on loop re-entry
   476  			// since developers tend to overlook that branch when
   477  			// debugging templates.
   478  			c0.err.Line = n.Line
   479  			c0.err.Description = "on range loop re-entry: " + c0.err.Description
   480  			return c0
   481  		}
   482  	}
   483  	c1 := e.escapeList(c, n.ElseList)
   484  	return join(c0, c1, n, nodeName)
   485  }
   486  
   487  // escapeList escapes a list template node.
   488  func (e *escaper) escapeList(c context, n *parse.ListNode) context {
   489  	if n == nil {
   490  		return c
   491  	}
   492  	for _, m := range n.Nodes {
   493  		c = e.escape(c, m)
   494  	}
   495  	return c
   496  }
   497  
   498  // escapeListConditionally escapes a list node but only preserves edits and
   499  // inferences in e if the inferences and output context satisfy filter.
   500  // It returns the best guess at an output context, and the result of the filter
   501  // which is the same as whether e was updated.
   502  func (e *escaper) escapeListConditionally(c context, n *parse.ListNode, filter func(*escaper, context) bool) (context, bool) {
   503  	e1 := makeEscaper(e.ns)
   504  	// Make type inferences available to f.
   505  	for k, v := range e.output {
   506  		e1.output[k] = v
   507  	}
   508  	c = e1.escapeList(c, n)
   509  	ok := filter != nil && filter(&e1, c)
   510  	if ok {
   511  		// Copy inferences and edits from e1 back into e.
   512  		for k, v := range e1.output {
   513  			e.output[k] = v
   514  		}
   515  		for k, v := range e1.derived {
   516  			e.derived[k] = v
   517  		}
   518  		for k, v := range e1.called {
   519  			e.called[k] = v
   520  		}
   521  		for k, v := range e1.actionNodeEdits {
   522  			e.editActionNode(k, v)
   523  		}
   524  		for k, v := range e1.templateNodeEdits {
   525  			e.editTemplateNode(k, v)
   526  		}
   527  		for k, v := range e1.textNodeEdits {
   528  			e.editTextNode(k, v)
   529  		}
   530  	}
   531  	return c, ok
   532  }
   533  
   534  // escapeTemplate escapes a {{template}} call node.
   535  func (e *escaper) escapeTemplate(c context, n *parse.TemplateNode) context {
   536  	c, name := e.escapeTree(c, n, n.Name, n.Line)
   537  	if name != n.Name {
   538  		e.editTemplateNode(n, name)
   539  	}
   540  	return c
   541  }
   542  
   543  // escapeTree escapes the named template starting in the given context as
   544  // necessary and returns its output context.
   545  func (e *escaper) escapeTree(c context, node parse.Node, name string, line int) (context, string) {
   546  	// Mangle the template name with the input context to produce a reliable
   547  	// identifier.
   548  	dname := c.mangle(name)
   549  	e.called[dname] = true
   550  	if out, ok := e.output[dname]; ok {
   551  		// Already escaped.
   552  		return out, dname
   553  	}
   554  	t := e.template(name)
   555  	if t == nil {
   556  		// Two cases: The template exists but is empty, or has never been mentioned at
   557  		// all. Distinguish the cases in the error messages.
   558  		if e.ns.set[name] != nil {
   559  			return context{
   560  				state: stateError,
   561  				err:   errorf(ErrNoSuchTemplate, node, line, "%q is an incomplete or empty template", name),
   562  			}, dname
   563  		}
   564  		return context{
   565  			state: stateError,
   566  			err:   errorf(ErrNoSuchTemplate, node, line, "no such template %q", name),
   567  		}, dname
   568  	}
   569  	if dname != name {
   570  		// Use any template derived during an earlier call to escapeTemplate
   571  		// with different top level templates, or clone if necessary.
   572  		dt := e.template(dname)
   573  		if dt == nil {
   574  			dt = template.New(dname)
   575  			dt.Tree = &parse.Tree{Name: dname, Root: t.Root.CopyList()}
   576  			e.derived[dname] = dt
   577  		}
   578  		t = dt
   579  	}
   580  	return e.computeOutCtx(c, t), dname
   581  }
   582  
   583  // computeOutCtx takes a template and its start context and computes the output
   584  // context while storing any inferences in e.
   585  func (e *escaper) computeOutCtx(c context, t *template.Template) context {
   586  	// Propagate context over the body.
   587  	c1, ok := e.escapeTemplateBody(c, t)
   588  	if !ok {
   589  		// Look for a fixed point by assuming c1 as the output context.
   590  		if c2, ok2 := e.escapeTemplateBody(c1, t); ok2 {
   591  			c1, ok = c2, true
   592  		}
   593  		// Use c1 as the error context if neither assumption worked.
   594  	}
   595  	if !ok && c1.state != stateError {
   596  		return context{
   597  			state: stateError,
   598  			err:   errorf(ErrOutputContext, t.Tree.Root, 0, "cannot compute output context for template %s", t.Name()),
   599  		}
   600  	}
   601  	return c1
   602  }
   603  
   604  // escapeTemplateBody escapes the given template assuming the given output
   605  // context, and returns the best guess at the output context and whether the
   606  // assumption was correct.
   607  func (e *escaper) escapeTemplateBody(c context, t *template.Template) (context, bool) {
   608  	filter := func(e1 *escaper, c1 context) bool {
   609  		if c1.state == stateError {
   610  			// Do not update the input escaper, e.
   611  			return false
   612  		}
   613  		if !e1.called[t.Name()] {
   614  			// If t is not recursively called, then c1 is an
   615  			// accurate output context.
   616  			return true
   617  		}
   618  		// c1 is accurate if it matches our assumed output context.
   619  		return c.eq(c1)
   620  	}
   621  	// We need to assume an output context so that recursive template calls
   622  	// take the fast path out of escapeTree instead of infinitely recursing.
   623  	// Naively assuming that the input context is the same as the output
   624  	// works >90% of the time.
   625  	e.output[t.Name()] = c
   626  	return e.escapeListConditionally(c, t.Tree.Root, filter)
   627  }
   628  
   629  // delimEnds maps each delim to a string of characters that terminate it.
   630  var delimEnds = [...]string{
   631  	delimDoubleQuote: `"`,
   632  	delimSingleQuote: "'",
   633  	// Determined empirically by running the below in various browsers.
   634  	// var div = document.createElement("DIV");
   635  	// for (var i = 0; i < 0x10000; ++i) {
   636  	//   div.innerHTML = "<span title=x" + String.fromCharCode(i) + "-bar>";
   637  	//   if (div.getElementsByTagName("SPAN")[0].title.indexOf("bar") < 0)
   638  	//     document.write("<p>U+" + i.toString(16));
   639  	// }
   640  	delimSpaceOrTagEnd: " \t\n\f\r>",
   641  }
   642  
   643  var doctypeBytes = []byte("<!DOCTYPE")
   644  
   645  // escapeText escapes a text template node.
   646  func (e *escaper) escapeText(c context, n *parse.TextNode) context {
   647  	s, written, i, b := n.Text, 0, 0, new(bytes.Buffer)
   648  	for i != len(s) {
   649  		c1, nread := contextAfterText(c, s[i:])
   650  		i1 := i + nread
   651  		if c.state == stateText || c.state == stateRCDATA {
   652  			end := i1
   653  			if c1.state != c.state {
   654  				for j := end - 1; j >= i; j-- {
   655  					if s[j] == '<' {
   656  						end = j
   657  						break
   658  					}
   659  				}
   660  			}
   661  			for j := i; j < end; j++ {
   662  				if s[j] == '<' && !bytes.HasPrefix(bytes.ToUpper(s[j:]), doctypeBytes) {
   663  					b.Write(s[written:j])
   664  					b.WriteString("&lt;")
   665  					written = j + 1
   666  				}
   667  			}
   668  		} else if isComment(c.state) && c.delim == delimNone {
   669  			switch c.state {
   670  			case stateJSBlockCmt:
   671  				// https://es5.github.com/#x7.4:
   672  				// "Comments behave like white space and are
   673  				// discarded except that, if a MultiLineComment
   674  				// contains a line terminator character, then
   675  				// the entire comment is considered to be a
   676  				// LineTerminator for purposes of parsing by
   677  				// the syntactic grammar."
   678  				if bytes.ContainsAny(s[written:i1], "\n\r\u2028\u2029") {
   679  					b.WriteByte('\n')
   680  				} else {
   681  					b.WriteByte(' ')
   682  				}
   683  			case stateCSSBlockCmt:
   684  				b.WriteByte(' ')
   685  			}
   686  			written = i1
   687  		}
   688  		if c.state != c1.state && isComment(c1.state) && c1.delim == delimNone {
   689  			// Preserve the portion between written and the comment start.
   690  			cs := i1 - 2
   691  			if c1.state == stateHTMLCmt {
   692  				// "<!--" instead of "/*" or "//"
   693  				cs -= 2
   694  			}
   695  			b.Write(s[written:cs])
   696  			written = i1
   697  		}
   698  		if i == i1 && c.state == c1.state {
   699  			panic(fmt.Sprintf("infinite loop from %v to %v on %q..%q", c, c1, s[:i], s[i:]))
   700  		}
   701  		c, i = c1, i1
   702  	}
   703  
   704  	if written != 0 && c.state != stateError {
   705  		if !isComment(c.state) || c.delim != delimNone {
   706  			b.Write(n.Text[written:])
   707  		}
   708  		e.editTextNode(n, b.Bytes())
   709  	}
   710  	return c
   711  }
   712  
   713  // contextAfterText starts in context c, consumes some tokens from the front of
   714  // s, then returns the context after those tokens and the unprocessed suffix.
   715  func contextAfterText(c context, s []byte) (context, int) {
   716  	if c.delim == delimNone {
   717  		c1, i := tSpecialTagEnd(c, s)
   718  		if i == 0 {
   719  			// A special end tag (`</script>`) has been seen and
   720  			// all content preceding it has been consumed.
   721  			return c1, 0
   722  		}
   723  		// Consider all content up to any end tag.
   724  		return transitionFunc[c.state](c, s[:i])
   725  	}
   726  
   727  	// We are at the beginning of an attribute value.
   728  
   729  	i := bytes.IndexAny(s, delimEnds[c.delim])
   730  	if i == -1 {
   731  		i = len(s)
   732  	}
   733  	if c.delim == delimSpaceOrTagEnd {
   734  		// https://www.w3.org/TR/html5/syntax.html#attribute-value-(unquoted)-state
   735  		// lists the runes below as error characters.
   736  		// Error out because HTML parsers may differ on whether
   737  		// "<a id= onclick=f("     ends inside id's or onclick's value,
   738  		// "<a class=`foo "        ends inside a value,
   739  		// "<a style=font:'Arial'" needs open-quote fixup.
   740  		// IE treats '`' as a quotation character.
   741  		if j := bytes.IndexAny(s[:i], "\"'<=`"); j >= 0 {
   742  			return context{
   743  				state: stateError,
   744  				err:   errorf(ErrBadHTML, nil, 0, "%q in unquoted attr: %q", s[j:j+1], s[:i]),
   745  			}, len(s)
   746  		}
   747  	}
   748  	if i == len(s) {
   749  		// Remain inside the attribute.
   750  		// Decode the value so non-HTML rules can easily handle
   751  		//     <button onclick="alert(&quot;Hi!&quot;)">
   752  		// without having to entity decode token boundaries.
   753  		for u := []byte(html.UnescapeString(string(s))); len(u) != 0; {
   754  			c1, i1 := transitionFunc[c.state](c, u)
   755  			c, u = c1, u[i1:]
   756  		}
   757  		return c, len(s)
   758  	}
   759  
   760  	element := c.element
   761  
   762  	// If this is a non-JS "type" attribute inside "script" tag, do not treat the contents as JS.
   763  	if c.state == stateAttr && c.element == elementScript && c.attr == attrScriptType && !isJSType(string(s[:i])) {
   764  		element = elementNone
   765  	}
   766  
   767  	if c.delim != delimSpaceOrTagEnd {
   768  		// Consume any quote.
   769  		i++
   770  	}
   771  	// On exiting an attribute, we discard all state information
   772  	// except the state and element.
   773  	return context{state: stateTag, element: element}, i
   774  }
   775  
   776  // editActionNode records a change to an action pipeline for later commit.
   777  func (e *escaper) editActionNode(n *parse.ActionNode, cmds []string) {
   778  	if _, ok := e.actionNodeEdits[n]; ok {
   779  		panic(fmt.Sprintf("node %s shared between templates", n))
   780  	}
   781  	e.actionNodeEdits[n] = cmds
   782  }
   783  
   784  // editTemplateNode records a change to a {{template}} callee for later commit.
   785  func (e *escaper) editTemplateNode(n *parse.TemplateNode, callee string) {
   786  	if _, ok := e.templateNodeEdits[n]; ok {
   787  		panic(fmt.Sprintf("node %s shared between templates", n))
   788  	}
   789  	e.templateNodeEdits[n] = callee
   790  }
   791  
   792  // editTextNode records a change to a text node for later commit.
   793  func (e *escaper) editTextNode(n *parse.TextNode, text []byte) {
   794  	if _, ok := e.textNodeEdits[n]; ok {
   795  		panic(fmt.Sprintf("node %s shared between templates", n))
   796  	}
   797  	e.textNodeEdits[n] = text
   798  }
   799  
   800  // commit applies changes to actions and template calls needed to contextually
   801  // autoescape content and adds any derived templates to the set.
   802  func (e *escaper) commit() {
   803  	for name := range e.output {
   804  		e.template(name).Funcs(funcMap)
   805  	}
   806  	// Any template from the name space associated with this escaper can be used
   807  	// to add derived templates to the underlying text/template name space.
   808  	tmpl := e.arbitraryTemplate()
   809  	for _, t := range e.derived {
   810  		if _, err := tmpl.text.AddParseTree(t.Name(), t.Tree); err != nil {
   811  			panic("error adding derived template")
   812  		}
   813  	}
   814  	for n, s := range e.actionNodeEdits {
   815  		ensurePipelineContains(n.Pipe, s)
   816  	}
   817  	for n, name := range e.templateNodeEdits {
   818  		n.Name = name
   819  	}
   820  	for n, s := range e.textNodeEdits {
   821  		n.Text = s
   822  	}
   823  	// Reset state that is specific to this commit so that the same changes are
   824  	// not re-applied to the template on subsequent calls to commit.
   825  	e.called = make(map[string]bool)
   826  	e.actionNodeEdits = make(map[*parse.ActionNode][]string)
   827  	e.templateNodeEdits = make(map[*parse.TemplateNode]string)
   828  	e.textNodeEdits = make(map[*parse.TextNode][]byte)
   829  }
   830  
   831  // template returns the named template given a mangled template name.
   832  func (e *escaper) template(name string) *template.Template {
   833  	// Any template from the name space associated with this escaper can be used
   834  	// to look up templates in the underlying text/template name space.
   835  	t := e.arbitraryTemplate().text.Lookup(name)
   836  	if t == nil {
   837  		t = e.derived[name]
   838  	}
   839  	return t
   840  }
   841  
   842  // arbitraryTemplate returns an arbitrary template from the name space
   843  // associated with e and panics if no templates are found.
   844  func (e *escaper) arbitraryTemplate() *Template {
   845  	for _, t := range e.ns.set {
   846  		return t
   847  	}
   848  	panic("no templates in name space")
   849  }
   850  
   851  // Forwarding functions so that clients need only import this package
   852  // to reach the general escaping functions of text/template.
   853  
   854  // HTMLEscape writes to w the escaped HTML equivalent of the plain text data b.
   855  func HTMLEscape(w io.Writer, b []byte) {
   856  	template.HTMLEscape(w, b)
   857  }
   858  
   859  // HTMLEscapeString returns the escaped HTML equivalent of the plain text data s.
   860  func HTMLEscapeString(s string) string {
   861  	return template.HTMLEscapeString(s)
   862  }
   863  
   864  // HTMLEscaper returns the escaped HTML equivalent of the textual
   865  // representation of its arguments.
   866  func HTMLEscaper(args ...interface{}) string {
   867  	return template.HTMLEscaper(args...)
   868  }
   869  
   870  // JSEscape writes to w the escaped JavaScript equivalent of the plain text data b.
   871  func JSEscape(w io.Writer, b []byte) {
   872  	template.JSEscape(w, b)
   873  }
   874  
   875  // JSEscapeString returns the escaped JavaScript equivalent of the plain text data s.
   876  func JSEscapeString(s string) string {
   877  	return template.JSEscapeString(s)
   878  }
   879  
   880  // JSEscaper returns the escaped JavaScript equivalent of the textual
   881  // representation of its arguments.
   882  func JSEscaper(args ...interface{}) string {
   883  	return template.JSEscaper(args...)
   884  }
   885  
   886  // URLQueryEscaper returns the escaped value of the textual representation of
   887  // its arguments in a form suitable for embedding in a URL query.
   888  func URLQueryEscaper(args ...interface{}) string {
   889  	return template.URLQueryEscaper(args...)
   890  }
   891  

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