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Source file src/html/template/error.go

Documentation: html/template

  // 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 (
  // Error describes a problem encountered during template Escaping.
  type Error struct {
  	// ErrorCode describes the kind of error.
  	ErrorCode ErrorCode
  	// Node is the node that caused the problem, if known.
  	// If not nil, it overrides Name and Line.
  	Node parse.Node
  	// Name is the name of the template in which the error was encountered.
  	Name string
  	// Line is the line number of the error in the template source or 0.
  	Line int
  	// Description is a human-readable description of the problem.
  	Description string
  // ErrorCode is a code for a kind of error.
  type ErrorCode int
  // We define codes for each error that manifests while escaping templates, but
  // escaped templates may also fail at runtime.
  // Output: "ZgotmplZ"
  // Example:
  //   <img src="{{.X}}">
  //   where {{.X}} evaluates to `javascript:...`
  // Discussion:
  //   "ZgotmplZ" is a special value that indicates that unsafe content reached a
  //   CSS or URL context at runtime. The output of the example will be
  //     <img src="#ZgotmplZ">
  //   If the data comes from a trusted source, use content types to exempt it
  //   from filtering: URL(`javascript:...`).
  const (
  	// OK indicates the lack of an error.
  	OK ErrorCode = iota
  	// ErrAmbigContext: "... appears in an ambiguous context within a URL"
  	// Example:
  	//   <a href="
  	//      {{if .C}}
  	//        /path/
  	//      {{else}}
  	//        /search?q=
  	//      {{end}}
  	//      {{.X}}
  	//   ">
  	// Discussion:
  	//   {{.X}} is in an ambiguous URL context since, depending on {{.C}},
  	//  it may be either a URL suffix or a query parameter.
  	//   Moving {{.X}} into the condition removes the ambiguity:
  	//   <a href="{{if .C}}/path/{{.X}}{{else}}/search?q={{.X}}">
  	// ErrBadHTML: "expected space, attr name, or end of tag, but got ...",
  	//   "... in unquoted attr", "... in attribute name"
  	// Example:
  	//   <a href = /search?q=foo>
  	//   <href=foo>
  	//   <form na<e=...>
  	//   <option selected<
  	// Discussion:
  	//   This is often due to a typo in an HTML element, but some runes
  	//   are banned in tag names, attribute names, and unquoted attribute
  	//   values because they can tickle parser ambiguities.
  	//   Quoting all attributes is the best policy.
  	// ErrBranchEnd: "{{if}} branches end in different contexts"
  	// Example:
  	//   {{if .C}}<a href="{{end}}{{.X}}
  	// Discussion:
  	//   Package html/template statically examines each path through an
  	//   {{if}}, {{range}}, or {{with}} to escape any following pipelines.
  	//   The example is ambiguous since {{.X}} might be an HTML text node,
  	//   or a URL prefix in an HTML attribute. The context of {{.X}} is
  	//   used to figure out how to escape it, but that context depends on
  	//   the run-time value of {{.C}} which is not statically known.
  	//   The problem is usually something like missing quotes or angle
  	//   brackets, or can be avoided by refactoring to put the two contexts
  	//   into different branches of an if, range or with. If the problem
  	//   is in a {{range}} over a collection that should never be empty,
  	//   adding a dummy {{else}} can help.
  	// ErrEndContext: "... ends in a non-text context: ..."
  	// Examples:
  	//   <div
  	//   <div title="no close quote>
  	//   <script>f()
  	// Discussion:
  	//   Executed templates should produce a DocumentFragment of HTML.
  	//   Templates that end without closing tags will trigger this error.
  	//   Templates that should not be used in an HTML context or that
  	//   produce incomplete Fragments should not be executed directly.
  	//   {{define "main"}} <script>{{template "helper"}}</script> {{end}}
  	//   {{define "helper"}} document.write(' <div title=" ') {{end}}
  	//   "helper" does not produce a valid document fragment, so should
  	//   not be Executed directly.
  	// ErrNoSuchTemplate: "no such template ..."
  	// Examples:
  	//   {{define "main"}}<div {{template "attrs"}}>{{end}}
  	//   {{define "attrs"}}href="{{.URL}}"{{end}}
  	// Discussion:
  	//   Package html/template looks through template calls to compute the
  	//   context.
  	//   Here the {{.URL}} in "attrs" must be treated as a URL when called
  	//   from "main", but you will get this error if "attrs" is not defined
  	//   when "main" is parsed.
  	// ErrOutputContext: "cannot compute output context for template ..."
  	// Examples:
  	//   {{define "t"}}{{if .T}}{{template "t" .T}}{{end}}{{.H}}",{{end}}
  	// Discussion:
  	//   A recursive template does not end in the same context in which it
  	//   starts, and a reliable output context cannot be computed.
  	//   Look for typos in the named template.
  	//   If the template should not be called in the named start context,
  	//   look for calls to that template in unexpected contexts.
  	//   Maybe refactor recursive templates to not be recursive.
  	// ErrPartialCharset: "unfinished JS regexp charset in ..."
  	// Example:
  	//     <script>var pattern = /foo[{{.Chars}}]/</script>
  	// Discussion:
  	//   Package html/template does not support interpolation into regular
  	//   expression literal character sets.
  	// ErrPartialEscape: "unfinished escape sequence in ..."
  	// Example:
  	//   <script>alert("\{{.X}}")</script>
  	// Discussion:
  	//   Package html/template does not support actions following a
  	//   backslash.
  	//   This is usually an error and there are better solutions; for
  	//   example
  	//     <script>alert("{{.X}}")</script>
  	//   should work, and if {{.X}} is a partial escape sequence such as
  	//   "xA0", mark the whole sequence as safe content: JSStr(`\xA0`)
  	// ErrRangeLoopReentry: "on range loop re-entry: ..."
  	// Example:
  	//   <script>var x = [{{range .}}'{{.}},{{end}}]</script>
  	// Discussion:
  	//   If an iteration through a range would cause it to end in a
  	//   different context than an earlier pass, there is no single context.
  	//   In the example, there is missing a quote, so it is not clear
  	//   whether {{.}} is meant to be inside a JS string or in a JS value
  	//   context. The second iteration would produce something like
  	//     <script>var x = ['firstValue,'secondValue]</script>
  	// ErrSlashAmbig: '/' could start a division or regexp.
  	// Example:
  	//   <script>
  	//     {{if .C}}var x = 1{{end}}
  	//     /-{{.N}}/i.test(x) ? doThis : doThat();
  	//   </script>
  	// Discussion:
  	//   The example above could produce `var x = 1/-2/i.test(s)...`
  	//   in which the first '/' is a mathematical division operator or it
  	//   could produce `/-2/i.test(s)` in which the first '/' starts a
  	//   regexp literal.
  	//   Look for missing semicolons inside branches, and maybe add
  	//   parentheses to make it clear which interpretation you intend.
  	// ErrPredefinedEscaper: "predefined escaper ... disallowed in template"
  	// Example:
  	//   <div class={{. | html}}>Hello<div>
  	// Discussion:
  	//   Package html/template already contextually escapes all pipelines to
  	//   produce HTML output safe against code injection. Manually escaping
  	//   pipeline output using the predefined escapers "html" or "urlquery" is
  	//   unnecessary, and may affect the correctness or safety of the escaped
  	//   pipeline output in Go 1.8 and earlier.
  	//   In most cases, such as the given example, this error can be resolved by
  	//   simply removing the predefined escaper from the pipeline and letting the
  	//   contextual autoescaper handle the escaping of the pipeline. In other
  	//   instances, where the predefined escaper occurs in the middle of a
  	//   pipeline where subsequent commands expect escaped input, e.g.
  	//     {{.X | html | makeALink}}
  	//   where makeALink does
  	//     return `<a href="`+input+`">link</a>`
  	//   consider refactoring the surrounding template to make use of the
  	//   contextual autoescaper, i.e.
  	//     <a href="{{.X}}">link</a>
  	//   To ease migration to Go 1.9 and beyond, "html" and "urlquery" will
  	//   continue to be allowed as the last command in a pipeline. However, if the
  	//   pipeline occurs in an unquoted attribute value context, "html" is
  	//   disallowed. Avoid using "html" and "urlquery" entirely in new templates.
  func (e *Error) Error() string {
  	switch {
  	case e.Node != nil:
  		loc, _ := (*parse.Tree)(nil).ErrorContext(e.Node)
  		return fmt.Sprintf("html/template:%s: %s", loc, e.Description)
  	case e.Line != 0:
  		return fmt.Sprintf("html/template:%s:%d: %s", e.Name, e.Line, e.Description)
  	case e.Name != "":
  		return fmt.Sprintf("html/template:%s: %s", e.Name, e.Description)
  	return "html/template: " + e.Description
  // errorf creates an error given a format string f and args.
  // The template Name still needs to be supplied.
  func errorf(k ErrorCode, node parse.Node, line int, f string, args ...interface{}) *Error {
  	return &Error{k, node, "", line, fmt.Sprintf(f, args...)}

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