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Source file src/html/template/error.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  	"fmt"
     9  	"text/template/parse"
    10  )
    11  
    12  // Error describes a problem encountered during template Escaping.
    13  type Error struct {
    14  	// ErrorCode describes the kind of error.
    15  	ErrorCode ErrorCode
    16  	// Node is the node that caused the problem, if known.
    17  	// If not nil, it overrides Name and Line.
    18  	Node parse.Node
    19  	// Name is the name of the template in which the error was encountered.
    20  	Name string
    21  	// Line is the line number of the error in the template source or 0.
    22  	Line int
    23  	// Description is a human-readable description of the problem.
    24  	Description string
    25  }
    26  
    27  // ErrorCode is a code for a kind of error.
    28  type ErrorCode int
    29  
    30  // We define codes for each error that manifests while escaping templates, but
    31  // escaped templates may also fail at runtime.
    32  //
    33  // Output: "ZgotmplZ"
    34  // Example:
    35  //   <img src="{{.X}}">
    36  //   where {{.X}} evaluates to `javascript:...`
    37  // Discussion:
    38  //   "ZgotmplZ" is a special value that indicates that unsafe content reached a
    39  //   CSS or URL context at runtime. The output of the example will be
    40  //     <img src="#ZgotmplZ">
    41  //   If the data comes from a trusted source, use content types to exempt it
    42  //   from filtering: URL(`javascript:...`).
    43  const (
    44  	// OK indicates the lack of an error.
    45  	OK ErrorCode = iota
    46  
    47  	// ErrAmbigContext: "... appears in an ambiguous context within a URL"
    48  	// Example:
    49  	//   <a href="
    50  	//      {{if .C}}
    51  	//        /path/
    52  	//      {{else}}
    53  	//        /search?q=
    54  	//      {{end}}
    55  	//      {{.X}}
    56  	//   ">
    57  	// Discussion:
    58  	//   {{.X}} is in an ambiguous URL context since, depending on {{.C}},
    59  	//  it may be either a URL suffix or a query parameter.
    60  	//   Moving {{.X}} into the condition removes the ambiguity:
    61  	//   <a href="{{if .C}}/path/{{.X}}{{else}}/search?q={{.X}}">
    62  	ErrAmbigContext
    63  
    64  	// ErrBadHTML: "expected space, attr name, or end of tag, but got ...",
    65  	//   "... in unquoted attr", "... in attribute name"
    66  	// Example:
    67  	//   <a href = /search?q=foo>
    68  	//   <href=foo>
    69  	//   <form na<e=...>
    70  	//   <option selected<
    71  	// Discussion:
    72  	//   This is often due to a typo in an HTML element, but some runes
    73  	//   are banned in tag names, attribute names, and unquoted attribute
    74  	//   values because they can tickle parser ambiguities.
    75  	//   Quoting all attributes is the best policy.
    76  	ErrBadHTML
    77  
    78  	// ErrBranchEnd: "{{if}} branches end in different contexts"
    79  	// Example:
    80  	//   {{if .C}}<a href="{{end}}{{.X}}
    81  	// Discussion:
    82  	//   Package html/template statically examines each path through an
    83  	//   {{if}}, {{range}}, or {{with}} to escape any following pipelines.
    84  	//   The example is ambiguous since {{.X}} might be an HTML text node,
    85  	//   or a URL prefix in an HTML attribute. The context of {{.X}} is
    86  	//   used to figure out how to escape it, but that context depends on
    87  	//   the run-time value of {{.C}} which is not statically known.
    88  	//
    89  	//   The problem is usually something like missing quotes or angle
    90  	//   brackets, or can be avoided by refactoring to put the two contexts
    91  	//   into different branches of an if, range or with. If the problem
    92  	//   is in a {{range}} over a collection that should never be empty,
    93  	//   adding a dummy {{else}} can help.
    94  	ErrBranchEnd
    95  
    96  	// ErrEndContext: "... ends in a non-text context: ..."
    97  	// Examples:
    98  	//   <div
    99  	//   <div title="no close quote>
   100  	//   <script>f()
   101  	// Discussion:
   102  	//   Executed templates should produce a DocumentFragment of HTML.
   103  	//   Templates that end without closing tags will trigger this error.
   104  	//   Templates that should not be used in an HTML context or that
   105  	//   produce incomplete Fragments should not be executed directly.
   106  	//
   107  	//   {{define "main"}} <script>{{template "helper"}}</script> {{end}}
   108  	//   {{define "helper"}} document.write(' <div title=" ') {{end}}
   109  	//
   110  	//   "helper" does not produce a valid document fragment, so should
   111  	//   not be Executed directly.
   112  	ErrEndContext
   113  
   114  	// ErrNoSuchTemplate: "no such template ..."
   115  	// Examples:
   116  	//   {{define "main"}}<div {{template "attrs"}}>{{end}}
   117  	//   {{define "attrs"}}href="{{.URL}}"{{end}}
   118  	// Discussion:
   119  	//   Package html/template looks through template calls to compute the
   120  	//   context.
   121  	//   Here the {{.URL}} in "attrs" must be treated as a URL when called
   122  	//   from "main", but you will get this error if "attrs" is not defined
   123  	//   when "main" is parsed.
   124  	ErrNoSuchTemplate
   125  
   126  	// ErrOutputContext: "cannot compute output context for template ..."
   127  	// Examples:
   128  	//   {{define "t"}}{{if .T}}{{template "t" .T}}{{end}}{{.H}}",{{end}}
   129  	// Discussion:
   130  	//   A recursive template does not end in the same context in which it
   131  	//   starts, and a reliable output context cannot be computed.
   132  	//   Look for typos in the named template.
   133  	//   If the template should not be called in the named start context,
   134  	//   look for calls to that template in unexpected contexts.
   135  	//   Maybe refactor recursive templates to not be recursive.
   136  	ErrOutputContext
   137  
   138  	// ErrPartialCharset: "unfinished JS regexp charset in ..."
   139  	// Example:
   140  	//     <script>var pattern = /foo[{{.Chars}}]/</script>
   141  	// Discussion:
   142  	//   Package html/template does not support interpolation into regular
   143  	//   expression literal character sets.
   144  	ErrPartialCharset
   145  
   146  	// ErrPartialEscape: "unfinished escape sequence in ..."
   147  	// Example:
   148  	//   <script>alert("\{{.X}}")</script>
   149  	// Discussion:
   150  	//   Package html/template does not support actions following a
   151  	//   backslash.
   152  	//   This is usually an error and there are better solutions; for
   153  	//   example
   154  	//     <script>alert("{{.X}}")</script>
   155  	//   should work, and if {{.X}} is a partial escape sequence such as
   156  	//   "xA0", mark the whole sequence as safe content: JSStr(`\xA0`)
   157  	ErrPartialEscape
   158  
   159  	// ErrRangeLoopReentry: "on range loop re-entry: ..."
   160  	// Example:
   161  	//   <script>var x = [{{range .}}'{{.}},{{end}}]</script>
   162  	// Discussion:
   163  	//   If an iteration through a range would cause it to end in a
   164  	//   different context than an earlier pass, there is no single context.
   165  	//   In the example, there is missing a quote, so it is not clear
   166  	//   whether {{.}} is meant to be inside a JS string or in a JS value
   167  	//   context. The second iteration would produce something like
   168  	//
   169  	//     <script>var x = ['firstValue,'secondValue]</script>
   170  	ErrRangeLoopReentry
   171  
   172  	// ErrSlashAmbig: '/' could start a division or regexp.
   173  	// Example:
   174  	//   <script>
   175  	//     {{if .C}}var x = 1{{end}}
   176  	//     /-{{.N}}/i.test(x) ? doThis : doThat();
   177  	//   </script>
   178  	// Discussion:
   179  	//   The example above could produce `var x = 1/-2/i.test(s)...`
   180  	//   in which the first '/' is a mathematical division operator or it
   181  	//   could produce `/-2/i.test(s)` in which the first '/' starts a
   182  	//   regexp literal.
   183  	//   Look for missing semicolons inside branches, and maybe add
   184  	//   parentheses to make it clear which interpretation you intend.
   185  	ErrSlashAmbig
   186  
   187  	// ErrPredefinedEscaper: "predefined escaper ... disallowed in template"
   188  	// Example:
   189  	//   <div class={{. | html}}>Hello<div>
   190  	// Discussion:
   191  	//   Package html/template already contextually escapes all pipelines to
   192  	//   produce HTML output safe against code injection. Manually escaping
   193  	//   pipeline output using the predefined escapers "html" or "urlquery" is
   194  	//   unnecessary, and may affect the correctness or safety of the escaped
   195  	//   pipeline output in Go 1.8 and earlier.
   196  	//
   197  	//   In most cases, such as the given example, this error can be resolved by
   198  	//   simply removing the predefined escaper from the pipeline and letting the
   199  	//   contextual autoescaper handle the escaping of the pipeline. In other
   200  	//   instances, where the predefined escaper occurs in the middle of a
   201  	//   pipeline where subsequent commands expect escaped input, e.g.
   202  	//     {{.X | html | makeALink}}
   203  	//   where makeALink does
   204  	//     return `<a href="`+input+`">link</a>`
   205  	//   consider refactoring the surrounding template to make use of the
   206  	//   contextual autoescaper, i.e.
   207  	//     <a href="{{.X}}">link</a>
   208  	//
   209  	//   To ease migration to Go 1.9 and beyond, "html" and "urlquery" will
   210  	//   continue to be allowed as the last command in a pipeline. However, if the
   211  	//   pipeline occurs in an unquoted attribute value context, "html" is
   212  	//   disallowed. Avoid using "html" and "urlquery" entirely in new templates.
   213  	ErrPredefinedEscaper
   214  )
   215  
   216  func (e *Error) Error() string {
   217  	switch {
   218  	case e.Node != nil:
   219  		loc, _ := (*parse.Tree)(nil).ErrorContext(e.Node)
   220  		return fmt.Sprintf("html/template:%s: %s", loc, e.Description)
   221  	case e.Line != 0:
   222  		return fmt.Sprintf("html/template:%s:%d: %s", e.Name, e.Line, e.Description)
   223  	case e.Name != "":
   224  		return fmt.Sprintf("html/template:%s: %s", e.Name, e.Description)
   225  	}
   226  	return "html/template: " + e.Description
   227  }
   228  
   229  // errorf creates an error given a format string f and args.
   230  // The template Name still needs to be supplied.
   231  func errorf(k ErrorCode, node parse.Node, line int, f string, args ...interface{}) *Error {
   232  	return &Error{k, node, "", line, fmt.Sprintf(f, args...)}
   233  }
   234  

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