type.go

Documentation: mime

  // Copyright 2010 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 mime implements parts of the MIME spec.
  package mime
  
  import (
  	"fmt"
  	"strings"
  	"sync"
  )
  
  var (
  	mimeTypes      sync.Map // map[string]string; ".Z" => "application/x-compress"
  	mimeTypesLower sync.Map // map[string]string; ".z" => "application/x-compress"
  
  	// extensions maps from MIME type to list of lowercase file
  	// extensions: "image/jpeg" => [".jpg", ".jpeg"]
  	extensionsMu sync.Mutex // Guards stores (but not loads) on extensions.
  	extensions   sync.Map   // map[string][]string; slice values are append-only.
  )
  
  func clearSyncMap(m *sync.Map) {
  	m.Range(func(k, _ interface{}) bool {
  		m.Delete(k)
  		return true
  	})
  }
  
  // setMimeTypes is used by initMime's non-test path, and by tests.
  func setMimeTypes(lowerExt, mixExt map[string]string) {
  	clearSyncMap(&mimeTypes)
  	clearSyncMap(&mimeTypesLower)
  	clearSyncMap(&extensions)
  
  	for k, v := range lowerExt {
  		mimeTypesLower.Store(k, v)
  	}
  	for k, v := range mixExt {
  		mimeTypes.Store(k, v)
  	}
  
  	extensionsMu.Lock()
  	defer extensionsMu.Unlock()
  	for k, v := range lowerExt {
  		justType, _, err := ParseMediaType(v)
  		if err != nil {
  			panic(err)
  		}
  		var exts []string
  		if ei, ok := extensions.Load(k); ok {
  			exts = ei.([]string)
  		}
  		extensions.Store(justType, append(exts, k))
  	}
  }
  
  var builtinTypesLower = map[string]string{
  	".css":  "text/css; charset=utf-8",
  	".gif":  "image/gif",
  	".htm":  "text/html; charset=utf-8",
  	".html": "text/html; charset=utf-8",
  	".jpg":  "image/jpeg",
  	".js":   "application/x-javascript",
  	".pdf":  "application/pdf",
  	".png":  "image/png",
  	".svg":  "image/svg+xml",
  	".xml":  "text/xml; charset=utf-8",
  }
  
  var once sync.Once // guards initMime
  
  var testInitMime, osInitMime func()
  
  func initMime() {
  	if fn := testInitMime; fn != nil {
  		fn()
  	} else {
  		setMimeTypes(builtinTypesLower, builtinTypesLower)
  		osInitMime()
  	}
  }
  
  // TypeByExtension returns the MIME type associated with the file extension ext.
  // The extension ext should begin with a leading dot, as in ".html".
  // When ext has no associated type, TypeByExtension returns "".
  //
  // Extensions are looked up first case-sensitively, then case-insensitively.
  //
  // The built-in table is small but on unix it is augmented by the local
  // system's mime.types file(s) if available under one or more of these
  // names:
  //
  //   /etc/mime.types
  //   /etc/apache2/mime.types
  //   /etc/apache/mime.types
  //
  // On Windows, MIME types are extracted from the registry.
  //
  // Text types have the charset parameter set to "utf-8" by default.
  func TypeByExtension(ext string) string {
  	once.Do(initMime)
  
  	// Case-sensitive lookup.
  	if v, ok := mimeTypes.Load(ext); ok {
  		return v.(string)
  	}
  
  	// Case-insensitive lookup.
  	// Optimistically assume a short ASCII extension and be
  	// allocation-free in that case.
  	var buf [10]byte
  	lower := buf[:0]
  	const utf8RuneSelf = 0x80 // from utf8 package, but not importing it.
  	for i := 0; i < len(ext); i++ {
  		c := ext[i]
  		if c >= utf8RuneSelf {
  			// Slow path.
  			si, _ := mimeTypesLower.Load(strings.ToLower(ext))
  			s, _ := si.(string)
  			return s
  		}
  		if 'A' <= c && c <= 'Z' {
  			lower = append(lower, c+('a'-'A'))
  		} else {
  			lower = append(lower, c)
  		}
  	}
  	si, _ := mimeTypesLower.Load(string(lower))
  	s, _ := si.(string)
  	return s
  }
  
  // ExtensionsByType returns the extensions known to be associated with the MIME
  // type typ. The returned extensions will each begin with a leading dot, as in
  // ".html". When typ has no associated extensions, ExtensionsByType returns an
  // nil slice.
  func ExtensionsByType(typ string) ([]string, error) {
  	justType, _, err := ParseMediaType(typ)
  	if err != nil {
  		return nil, err
  	}
  
  	once.Do(initMime)
  	s, ok := extensions.Load(justType)
  	if !ok {
  		return nil, nil
  	}
  	return append([]string{}, s.([]string)...), nil
  }
  
  // AddExtensionType sets the MIME type associated with
  // the extension ext to typ. The extension should begin with
  // a leading dot, as in ".html".
  func AddExtensionType(ext, typ string) error {
  	if !strings.HasPrefix(ext, ".") {
  		return fmt.Errorf("mime: extension %q missing leading dot", ext)
  	}
  	once.Do(initMime)
  	return setExtensionType(ext, typ)
  }
  
  func setExtensionType(extension, mimeType string) error {
  	justType, param, err := ParseMediaType(mimeType)
  	if err != nil {
  		return err
  	}
  	if strings.HasPrefix(mimeType, "text/") && param["charset"] == "" {
  		param["charset"] = "utf-8"
  		mimeType = FormatMediaType(mimeType, param)
  	}
  	extLower := strings.ToLower(extension)
  
  	mimeTypes.Store(extension, mimeType)
  	mimeTypesLower.Store(extLower, mimeType)
  
  	extensionsMu.Lock()
  	defer extensionsMu.Unlock()
  	var exts []string
  	if ei, ok := extensions.Load(justType); ok {
  		exts = ei.([]string)
  	}
  	for _, v := range exts {
  		if v == extLower {
  			return nil
  		}
  	}
  	extensions.Store(justType, append(exts, extLower))
  	return nil
  }
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