readfile.go

     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 fs
     6  
     7  import "io"
     8  
     9  // ReadFileFS is the interface implemented by a file system
    10  // that provides an optimized implementation of [ReadFile].
    11  type ReadFileFS interface {
    12  	FS
    13  
    14  	// ReadFile reads the named file and returns its contents.
    15  	// A successful call returns a nil error, not io.EOF.
    16  	// (Because ReadFile reads the whole file, the expected EOF
    17  	// from the final Read is not treated as an error to be reported.)
    18  	//
    19  	// The caller is permitted to modify the returned byte slice.
    20  	// This method should return a copy of the underlying data.
    21  	ReadFile(name string) ([]byte, error)
    22  }
    23  
    24  // ReadFile reads the named file from the file system fs and returns its contents.
    25  // A successful call returns a nil error, not [io.EOF].
    26  // (Because ReadFile reads the whole file, the expected EOF
    27  // from the final Read is not treated as an error to be reported.)
    28  //
    29  // If fs implements [ReadFileFS], ReadFile calls fs.ReadFile.
    30  // Otherwise ReadFile calls fs.Open and uses Read and Close
    31  // on the returned [File].
    32  func ReadFile(fsys FS, name string) ([]byte, error) {
    33  	if fsys, ok := fsys.(ReadFileFS); ok {
    34  		return fsys.ReadFile(name)
    35  	}
    36  
    37  	file, err := fsys.Open(name)
    38  	if err != nil {
    39  		return nil, err
    40  	}
    41  	defer file.Close()
    42  
    43  	var size int
    44  	if info, err := file.Stat(); err == nil {
    45  		size64 := info.Size()
    46  		if int64(int(size64)) == size64 {
    47  			size = int(size64)
    48  		}
    49  	}
    50  
    51  	data := make([]byte, 0, size+1)
    52  	for {
    53  		if len(data) >= cap(data) {
    54  			d := append(data[:cap(data)], 0)
    55  			data = d[:len(data)]
    56  		}
    57  		n, err := file.Read(data[len(data):cap(data)])
    58  		data = data[:len(data)+n]
    59  		if err != nil {
    60  			if err == io.EOF {
    61  				err = nil
    62  			}
    63  			return data, err
    64  		}
    65  	}
    66  }
    67
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