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Source file src/embed/embed.go

Documentation: embed

     1  // Copyright 2020 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 embed provides access to files embedded in the running Go program.
     6  //
     7  // Go source files that import "embed" can use the //go:embed directive
     8  // to initialize a variable of type string, []byte, or FS with the contents of
     9  // files read from the package directory or subdirectories at compile time.
    10  //
    11  // For example, here are three ways to embed a file named hello.txt
    12  // and then print its contents at run time.
    13  //
    14  // Embedding one file into a string:
    15  //
    16  //	import _ "embed"
    17  //
    18  //	//go:embed hello.txt
    19  //	var s string
    20  //	print(s)
    21  //
    22  // Embedding one file into a slice of bytes:
    23  //
    24  //	import _ "embed"
    25  //
    26  //	//go:embed hello.txt
    27  //	var b []byte
    28  //	print(string(b))
    29  //
    30  // Embedded one or more files into a file system:
    31  //
    32  //	import "embed"
    33  //
    34  //	//go:embed hello.txt
    35  //	var f embed.FS
    36  //	data, _ := f.ReadFile("hello.txt")
    37  //	print(string(data))
    38  //
    39  // Directives
    40  //
    41  // A //go:embed directive above a variable declaration specifies which files to embed,
    42  // using one or more path.Match patterns.
    43  //
    44  // The directive must immediately precede a line containing the declaration of a single variable.
    45  // Only blank lines and ‘//’ line comments are permitted between the directive and the declaration.
    46  //
    47  // The type of the variable must be a string type, or a slice of a byte type,
    48  // or FS (or an alias of FS).
    49  //
    50  // For example:
    51  //
    52  //	package server
    53  //
    54  //	import "embed"
    55  //
    56  //	// content holds our static web server content.
    57  //	//go:embed image/* template/*
    58  //	//go:embed html/index.html
    59  //	var content embed.FS
    60  //
    61  // The Go build system will recognize the directives and arrange for the declared variable
    62  // (in the example above, content) to be populated with the matching files from the file system.
    63  //
    64  // The //go:embed directive accepts multiple space-separated patterns for
    65  // brevity, but it can also be repeated, to avoid very long lines when there are
    66  // many patterns. The patterns are interpreted relative to the package directory
    67  // containing the source file. The path separator is a forward slash, even on
    68  // Windows systems. Patterns may not contain ‘.’ or ‘..’ or empty path elements,
    69  // nor may they begin or end with a slash. To match everything in the current
    70  // directory, use ‘*’ instead of ‘.’. To allow for naming files with spaces in
    71  // their names, patterns can be written as Go double-quoted or back-quoted
    72  // string literals.
    73  //
    74  // If a pattern names a directory, all files in the subtree rooted at that directory are
    75  // embedded (recursively), except that files with names beginning with ‘.’ or ‘_’
    76  // are excluded. So the variable in the above example is almost equivalent to:
    77  //
    78  //	// content is our static web server content.
    79  //	//go:embed image template html/index.html
    80  //	var content embed.FS
    81  //
    82  // The difference is that ‘image/*’ embeds ‘image/.tempfile’ while ‘image’ does not.
    83  //
    84  // The //go:embed directive can be used with both exported and unexported variables,
    85  // depending on whether the package wants to make the data available to other packages.
    86  // It can only be used with global variables at package scope,
    87  // not with local variables.
    88  //
    89  // Patterns must not match files outside the package's module, such as ‘.git/*’ or symbolic links.
    90  // Matches for empty directories are ignored. After that, each pattern in a //go:embed line
    91  // must match at least one file or non-empty directory.
    92  //
    93  // If any patterns are invalid or have invalid matches, the build will fail.
    94  //
    95  // Strings and Bytes
    96  //
    97  // The //go:embed line for a variable of type string or []byte can have only a single pattern,
    98  // and that pattern can match only a single file. The string or []byte is initialized with
    99  // the contents of that file.
   100  //
   101  // The //go:embed directive requires importing "embed", even when using a string or []byte.
   102  // In source files that don't refer to embed.FS, use a blank import (import _ "embed").
   103  //
   104  // File Systems
   105  //
   106  // For embedding a single file, a variable of type string or []byte is often best.
   107  // The FS type enables embedding a tree of files, such as a directory of static
   108  // web server content, as in the example above.
   109  //
   110  // FS implements the io/fs package's FS interface, so it can be used with any package that
   111  // understands file systems, including net/http, text/template, and html/template.
   112  //
   113  // For example, given the content variable in the example above, we can write:
   114  //
   115  //	http.Handle("/static/", http.StripPrefix("/static/", http.FileServer(http.FS(content))))
   116  //
   117  //	template.ParseFS(content, "*.tmpl")
   118  //
   119  // Tools
   120  //
   121  // To support tools that analyze Go packages, the patterns found in //go:embed lines
   122  // are available in “go list” output. See the EmbedPatterns, TestEmbedPatterns,
   123  // and XTestEmbedPatterns fields in the “go help list” output.
   124  //
   125  package embed
   126  
   127  import (
   128  	"errors"
   129  	"io"
   130  	"io/fs"
   131  	"time"
   132  )
   133  
   134  // An FS is a read-only collection of files, usually initialized with a //go:embed directive.
   135  // When declared without a //go:embed directive, an FS is an empty file system.
   136  //
   137  // An FS is a read-only value, so it is safe to use from multiple goroutines
   138  // simultaneously and also safe to assign values of type FS to each other.
   139  //
   140  // FS implements fs.FS, so it can be used with any package that understands
   141  // file system interfaces, including net/http, text/template, and html/template.
   142  //
   143  // See the package documentation for more details about initializing an FS.
   144  type FS struct {
   145  	// The compiler knows the layout of this struct.
   146  	// See cmd/compile/internal/gc's initEmbed.
   147  	//
   148  	// The files list is sorted by name but not by simple string comparison.
   149  	// Instead, each file's name takes the form "dir/elem" or "dir/elem/".
   150  	// The optional trailing slash indicates that the file is itself a directory.
   151  	// The files list is sorted first by dir (if dir is missing, it is taken to be ".")
   152  	// and then by base, so this list of files:
   153  	//
   154  	//	p
   155  	//	q/
   156  	//	q/r
   157  	//	q/s/
   158  	//	q/s/t
   159  	//	q/s/u
   160  	//	q/v
   161  	//	w
   162  	//
   163  	// is actually sorted as:
   164  	//
   165  	//	p       # dir=.    elem=p
   166  	//	q/      # dir=.    elem=q
   167  	//	w/      # dir=.    elem=w
   168  	//	q/r     # dir=q    elem=r
   169  	//	q/s/    # dir=q    elem=s
   170  	//	q/v     # dir=q    elem=v
   171  	//	q/s/t   # dir=q/s  elem=t
   172  	//	q/s/u   # dir=q/s  elem=u
   173  	//
   174  	// This order brings directory contents together in contiguous sections
   175  	// of the list, allowing a directory read to use binary search to find
   176  	// the relevant sequence of entries.
   177  	files *[]file
   178  }
   179  
   180  // split splits the name into dir and elem as described in the
   181  // comment in the FS struct above. isDir reports whether the
   182  // final trailing slash was present, indicating that name is a directory.
   183  func split(name string) (dir, elem string, isDir bool) {
   184  	if name[len(name)-1] == '/' {
   185  		isDir = true
   186  		name = name[:len(name)-1]
   187  	}
   188  	i := len(name) - 1
   189  	for i >= 0 && name[i] != '/' {
   190  		i--
   191  	}
   192  	if i < 0 {
   193  		return ".", name, isDir
   194  	}
   195  	return name[:i], name[i+1:], isDir
   196  }
   197  
   198  // trimSlash trims a trailing slash from name, if present,
   199  // returning the possibly shortened name.
   200  func trimSlash(name string) string {
   201  	if len(name) > 0 && name[len(name)-1] == '/' {
   202  		return name[:len(name)-1]
   203  	}
   204  	return name
   205  }
   206  
   207  var (
   208  	_ fs.ReadDirFS  = FS{}
   209  	_ fs.ReadFileFS = FS{}
   210  )
   211  
   212  // A file is a single file in the FS.
   213  // It implements fs.FileInfo and fs.DirEntry.
   214  type file struct {
   215  	// The compiler knows the layout of this struct.
   216  	// See cmd/compile/internal/gc's initEmbed.
   217  	name string
   218  	data string
   219  	hash [16]byte // truncated SHA256 hash
   220  }
   221  
   222  var (
   223  	_ fs.FileInfo = (*file)(nil)
   224  	_ fs.DirEntry = (*file)(nil)
   225  )
   226  
   227  func (f *file) Name() string               { _, elem, _ := split(f.name); return elem }
   228  func (f *file) Size() int64                { return int64(len(f.data)) }
   229  func (f *file) ModTime() time.Time         { return time.Time{} }
   230  func (f *file) IsDir() bool                { _, _, isDir := split(f.name); return isDir }
   231  func (f *file) Sys() interface{}           { return nil }
   232  func (f *file) Type() fs.FileMode          { return f.Mode().Type() }
   233  func (f *file) Info() (fs.FileInfo, error) { return f, nil }
   234  
   235  func (f *file) Mode() fs.FileMode {
   236  	if f.IsDir() {
   237  		return fs.ModeDir | 0555
   238  	}
   239  	return 0444
   240  }
   241  
   242  // dotFile is a file for the root directory,
   243  // which is omitted from the files list in a FS.
   244  var dotFile = &file{name: "./"}
   245  
   246  // lookup returns the named file, or nil if it is not present.
   247  func (f FS) lookup(name string) *file {
   248  	if !fs.ValidPath(name) {
   249  		// The compiler should never emit a file with an invalid name,
   250  		// so this check is not strictly necessary (if name is invalid,
   251  		// we shouldn't find a match below), but it's a good backstop anyway.
   252  		return nil
   253  	}
   254  	if name == "." {
   255  		return dotFile
   256  	}
   257  	if f.files == nil {
   258  		return nil
   259  	}
   260  
   261  	// Binary search to find where name would be in the list,
   262  	// and then check if name is at that position.
   263  	dir, elem, _ := split(name)
   264  	files := *f.files
   265  	i := sortSearch(len(files), func(i int) bool {
   266  		idir, ielem, _ := split(files[i].name)
   267  		return idir > dir || idir == dir && ielem >= elem
   268  	})
   269  	if i < len(files) && trimSlash(files[i].name) == name {
   270  		return &files[i]
   271  	}
   272  	return nil
   273  }
   274  
   275  // readDir returns the list of files corresponding to the directory dir.
   276  func (f FS) readDir(dir string) []file {
   277  	if f.files == nil {
   278  		return nil
   279  	}
   280  	// Binary search to find where dir starts and ends in the list
   281  	// and then return that slice of the list.
   282  	files := *f.files
   283  	i := sortSearch(len(files), func(i int) bool {
   284  		idir, _, _ := split(files[i].name)
   285  		return idir >= dir
   286  	})
   287  	j := sortSearch(len(files), func(j int) bool {
   288  		jdir, _, _ := split(files[j].name)
   289  		return jdir > dir
   290  	})
   291  	return files[i:j]
   292  }
   293  
   294  // Open opens the named file for reading and returns it as an fs.File.
   295  func (f FS) Open(name string) (fs.File, error) {
   296  	file := f.lookup(name)
   297  	if file == nil {
   298  		return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrNotExist}
   299  	}
   300  	if file.IsDir() {
   301  		return &openDir{file, f.readDir(name), 0}, nil
   302  	}
   303  	return &openFile{file, 0}, nil
   304  }
   305  
   306  // ReadDir reads and returns the entire named directory.
   307  func (f FS) ReadDir(name string) ([]fs.DirEntry, error) {
   308  	file, err := f.Open(name)
   309  	if err != nil {
   310  		return nil, err
   311  	}
   312  	dir, ok := file.(*openDir)
   313  	if !ok {
   314  		return nil, &fs.PathError{Op: "read", Path: name, Err: errors.New("not a directory")}
   315  	}
   316  	list := make([]fs.DirEntry, len(dir.files))
   317  	for i := range list {
   318  		list[i] = &dir.files[i]
   319  	}
   320  	return list, nil
   321  }
   322  
   323  // ReadFile reads and returns the content of the named file.
   324  func (f FS) ReadFile(name string) ([]byte, error) {
   325  	file, err := f.Open(name)
   326  	if err != nil {
   327  		return nil, err
   328  	}
   329  	ofile, ok := file.(*openFile)
   330  	if !ok {
   331  		return nil, &fs.PathError{Op: "read", Path: name, Err: errors.New("is a directory")}
   332  	}
   333  	return []byte(ofile.f.data), nil
   334  }
   335  
   336  // An openFile is a regular file open for reading.
   337  type openFile struct {
   338  	f      *file // the file itself
   339  	offset int64 // current read offset
   340  }
   341  
   342  func (f *openFile) Close() error               { return nil }
   343  func (f *openFile) Stat() (fs.FileInfo, error) { return f.f, nil }
   344  
   345  func (f *openFile) Read(b []byte) (int, error) {
   346  	if f.offset >= int64(len(f.f.data)) {
   347  		return 0, io.EOF
   348  	}
   349  	if f.offset < 0 {
   350  		return 0, &fs.PathError{Op: "read", Path: f.f.name, Err: fs.ErrInvalid}
   351  	}
   352  	n := copy(b, f.f.data[f.offset:])
   353  	f.offset += int64(n)
   354  	return n, nil
   355  }
   356  
   357  func (f *openFile) Seek(offset int64, whence int) (int64, error) {
   358  	switch whence {
   359  	case 0:
   360  		// offset += 0
   361  	case 1:
   362  		offset += f.offset
   363  	case 2:
   364  		offset += int64(len(f.f.data))
   365  	}
   366  	if offset < 0 || offset > int64(len(f.f.data)) {
   367  		return 0, &fs.PathError{Op: "seek", Path: f.f.name, Err: fs.ErrInvalid}
   368  	}
   369  	f.offset = offset
   370  	return offset, nil
   371  }
   372  
   373  // An openDir is a directory open for reading.
   374  type openDir struct {
   375  	f      *file  // the directory file itself
   376  	files  []file // the directory contents
   377  	offset int    // the read offset, an index into the files slice
   378  }
   379  
   380  func (d *openDir) Close() error               { return nil }
   381  func (d *openDir) Stat() (fs.FileInfo, error) { return d.f, nil }
   382  
   383  func (d *openDir) Read([]byte) (int, error) {
   384  	return 0, &fs.PathError{Op: "read", Path: d.f.name, Err: errors.New("is a directory")}
   385  }
   386  
   387  func (d *openDir) ReadDir(count int) ([]fs.DirEntry, error) {
   388  	n := len(d.files) - d.offset
   389  	if count > 0 && n > count {
   390  		n = count
   391  	}
   392  	if n == 0 {
   393  		if count <= 0 {
   394  			return nil, nil
   395  		}
   396  		return nil, io.EOF
   397  	}
   398  	list := make([]fs.DirEntry, n)
   399  	for i := range list {
   400  		list[i] = &d.files[d.offset+i]
   401  	}
   402  	d.offset += n
   403  	return list, nil
   404  }
   405  
   406  // sortSearch is like sort.Search, avoiding an import.
   407  func sortSearch(n int, f func(int) bool) int {
   408  	// Define f(-1) == false and f(n) == true.
   409  	// Invariant: f(i-1) == false, f(j) == true.
   410  	i, j := 0, n
   411  	for i < j {
   412  		h := int(uint(i+j) >> 1) // avoid overflow when computing h
   413  		// i ≤ h < j
   414  		if !f(h) {
   415  			i = h + 1 // preserves f(i-1) == false
   416  		} else {
   417  			j = h // preserves f(j) == true
   418  		}
   419  	}
   420  	// i == j, f(i-1) == false, and f(j) (= f(i)) == true  =>  answer is i.
   421  	return i
   422  }
   423  

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