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Source file src/io/fs/readfile.go

Documentation: io/fs

     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  	ReadFile(name string) ([]byte, error)
    19  }
    20  
    21  // ReadFile reads the named file from the file system fs and returns its contents.
    22  // A successful call returns a nil error, not io.EOF.
    23  // (Because ReadFile reads the whole file, the expected EOF
    24  // from the final Read is not treated as an error to be reported.)
    25  //
    26  // If fs implements ReadFileFS, ReadFile calls fs.ReadFile.
    27  // Otherwise ReadFile calls fs.Open and uses Read and Close
    28  // on the returned file.
    29  func ReadFile(fsys FS, name string) ([]byte, error) {
    30  	if fsys, ok := fsys.(ReadFileFS); ok {
    31  		return fsys.ReadFile(name)
    32  	}
    33  
    34  	file, err := fsys.Open(name)
    35  	if err != nil {
    36  		return nil, err
    37  	}
    38  	defer file.Close()
    39  
    40  	var size int
    41  	if info, err := file.Stat(); err == nil {
    42  		size64 := info.Size()
    43  		if int64(int(size64)) == size64 {
    44  			size = int(size64)
    45  		}
    46  	}
    47  
    48  	data := make([]byte, 0, size+1)
    49  	for {
    50  		if len(data) >= cap(data) {
    51  			d := append(data[:cap(data)], 0)
    52  			data = d[:len(data)]
    53  		}
    54  		n, err := file.Read(data[len(data):cap(data)])
    55  		data = data[:len(data)+n]
    56  		if err != nil {
    57  			if err == io.EOF {
    58  				err = nil
    59  			}
    60  			return data, err
    61  		}
    62  	}
    63  }
    64  

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