Source file src/mime/type.go

Documentation: mime

     1  // Copyright 2010 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 mime implements parts of the MIME spec.
     6  package mime
     7  
     8  import (
     9  	"fmt"
    10  	"strings"
    11  	"sync"
    12  )
    13  
    14  var (
    15  	mimeTypes      sync.Map // map[string]string; ".Z" => "application/x-compress"
    16  	mimeTypesLower sync.Map // map[string]string; ".z" => "application/x-compress"
    17  
    18  	// extensions maps from MIME type to list of lowercase file
    19  	// extensions: "image/jpeg" => [".jpg", ".jpeg"]
    20  	extensionsMu sync.Mutex // Guards stores (but not loads) on extensions.
    21  	extensions   sync.Map   // map[string][]string; slice values are append-only.
    22  )
    23  
    24  func clearSyncMap(m *sync.Map) {
    25  	m.Range(func(k, _ interface{}) bool {
    26  		m.Delete(k)
    27  		return true
    28  	})
    29  }
    30  
    31  // setMimeTypes is used by initMime's non-test path, and by tests.
    32  func setMimeTypes(lowerExt, mixExt map[string]string) {
    33  	clearSyncMap(&mimeTypes)
    34  	clearSyncMap(&mimeTypesLower)
    35  	clearSyncMap(&extensions)
    36  
    37  	for k, v := range lowerExt {
    38  		mimeTypesLower.Store(k, v)
    39  	}
    40  	for k, v := range mixExt {
    41  		mimeTypes.Store(k, v)
    42  	}
    43  
    44  	extensionsMu.Lock()
    45  	defer extensionsMu.Unlock()
    46  	for k, v := range lowerExt {
    47  		justType, _, err := ParseMediaType(v)
    48  		if err != nil {
    49  			panic(err)
    50  		}
    51  		var exts []string
    52  		if ei, ok := extensions.Load(k); ok {
    53  			exts = ei.([]string)
    54  		}
    55  		extensions.Store(justType, append(exts, k))
    56  	}
    57  }
    58  
    59  var builtinTypesLower = map[string]string{
    60  	".css":  "text/css; charset=utf-8",
    61  	".gif":  "image/gif",
    62  	".htm":  "text/html; charset=utf-8",
    63  	".html": "text/html; charset=utf-8",
    64  	".jpeg": "image/jpeg",
    65  	".jpg":  "image/jpeg",
    66  	".js":   "application/javascript",
    67  	".mjs":  "application/javascript",
    68  	".pdf":  "application/pdf",
    69  	".png":  "image/png",
    70  	".svg":  "image/svg+xml",
    71  	".wasm": "application/wasm",
    72  	".webp": "image/webp",
    73  	".xml":  "text/xml; charset=utf-8",
    74  }
    75  
    76  var once sync.Once // guards initMime
    77  
    78  var testInitMime, osInitMime func()
    79  
    80  func initMime() {
    81  	if fn := testInitMime; fn != nil {
    82  		fn()
    83  	} else {
    84  		setMimeTypes(builtinTypesLower, builtinTypesLower)
    85  		osInitMime()
    86  	}
    87  }
    88  
    89  // TypeByExtension returns the MIME type associated with the file extension ext.
    90  // The extension ext should begin with a leading dot, as in ".html".
    91  // When ext has no associated type, TypeByExtension returns "".
    92  //
    93  // Extensions are looked up first case-sensitively, then case-insensitively.
    94  //
    95  // The built-in table is small but on unix it is augmented by the local
    96  // system's mime.types file(s) if available under one or more of these
    97  // names:
    98  //
    99  //   /etc/mime.types
   100  //   /etc/apache2/mime.types
   101  //   /etc/apache/mime.types
   102  //
   103  // On Windows, MIME types are extracted from the registry.
   104  //
   105  // Text types have the charset parameter set to "utf-8" by default.
   106  func TypeByExtension(ext string) string {
   107  	once.Do(initMime)
   108  
   109  	// Case-sensitive lookup.
   110  	if v, ok := mimeTypes.Load(ext); ok {
   111  		return v.(string)
   112  	}
   113  
   114  	// Case-insensitive lookup.
   115  	// Optimistically assume a short ASCII extension and be
   116  	// allocation-free in that case.
   117  	var buf [10]byte
   118  	lower := buf[:0]
   119  	const utf8RuneSelf = 0x80 // from utf8 package, but not importing it.
   120  	for i := 0; i < len(ext); i++ {
   121  		c := ext[i]
   122  		if c >= utf8RuneSelf {
   123  			// Slow path.
   124  			si, _ := mimeTypesLower.Load(strings.ToLower(ext))
   125  			s, _ := si.(string)
   126  			return s
   127  		}
   128  		if 'A' <= c && c <= 'Z' {
   129  			lower = append(lower, c+('a'-'A'))
   130  		} else {
   131  			lower = append(lower, c)
   132  		}
   133  	}
   134  	si, _ := mimeTypesLower.Load(string(lower))
   135  	s, _ := si.(string)
   136  	return s
   137  }
   138  
   139  // ExtensionsByType returns the extensions known to be associated with the MIME
   140  // type typ. The returned extensions will each begin with a leading dot, as in
   141  // ".html". When typ has no associated extensions, ExtensionsByType returns an
   142  // nil slice.
   143  func ExtensionsByType(typ string) ([]string, error) {
   144  	justType, _, err := ParseMediaType(typ)
   145  	if err != nil {
   146  		return nil, err
   147  	}
   148  
   149  	once.Do(initMime)
   150  	s, ok := extensions.Load(justType)
   151  	if !ok {
   152  		return nil, nil
   153  	}
   154  	return append([]string{}, s.([]string)...), nil
   155  }
   156  
   157  // AddExtensionType sets the MIME type associated with
   158  // the extension ext to typ. The extension should begin with
   159  // a leading dot, as in ".html".
   160  func AddExtensionType(ext, typ string) error {
   161  	if !strings.HasPrefix(ext, ".") {
   162  		return fmt.Errorf("mime: extension %q missing leading dot", ext)
   163  	}
   164  	once.Do(initMime)
   165  	return setExtensionType(ext, typ)
   166  }
   167  
   168  func setExtensionType(extension, mimeType string) error {
   169  	justType, param, err := ParseMediaType(mimeType)
   170  	if err != nil {
   171  		return err
   172  	}
   173  	if strings.HasPrefix(mimeType, "text/") && param["charset"] == "" {
   174  		param["charset"] = "utf-8"
   175  		mimeType = FormatMediaType(mimeType, param)
   176  	}
   177  	extLower := strings.ToLower(extension)
   178  
   179  	mimeTypes.Store(extension, mimeType)
   180  	mimeTypesLower.Store(extLower, mimeType)
   181  
   182  	extensionsMu.Lock()
   183  	defer extensionsMu.Unlock()
   184  	var exts []string
   185  	if ei, ok := extensions.Load(justType); ok {
   186  		exts = ei.([]string)
   187  	}
   188  	for _, v := range exts {
   189  		if v == extLower {
   190  			return nil
   191  		}
   192  	}
   193  	extensions.Store(justType, append(exts, extLower))
   194  	return nil
   195  }
   196  

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