github.com/keybase/client/go@v0.0.0-20241007131713-f10651d043c8/kbfs/ioutil/write_serialized_file.go (about)

     1  // Copyright 2017 Keybase Inc. All rights reserved.
     2  // Use of this source code is governed by a BSD
     3  // license that can be found in the LICENSE file.
     4  
     5  package ioutil
     6  
     7  import (
     8  	"io"
     9  	"os"
    10  
    11  	"github.com/pkg/errors"
    12  )
    13  
    14  // WriteSerializedFile writes (or overwrites) `data` into `filename`.
    15  // If `filename` doesn't exist, it's created with `perm` permissions.
    16  // If `filename` does exist, the data is first overwritten to the
    17  // file, and then the file is truncated to the length of the data.  If
    18  // the data represents a serialized data structure where the length is
    19  // explicitly stated in, or implicitly calculated from, the data
    20  // itself on a read, then this is approximately an atomic write
    21  // (without the performance overhead of writing to a temp file and
    22  // renaming it).  NOTE: it's technically possible a partial OS write
    23  // could lead to a corrupted file, though in practice this seems much
    24  // more rare than os.WriteFile() leaving behind an empty file.
    25  func WriteSerializedFile(
    26  	filename string, data []byte, perm os.FileMode) (err error) {
    27  	// Don't use os.WriteFile because it truncates the file first,
    28  	// and if there's a crash it will leave the file in an unknown
    29  	// state.
    30  	f, err := OpenFile(filename, os.O_WRONLY|os.O_CREATE, 0600)
    31  	if err != nil {
    32  		return err
    33  	}
    34  	defer func() {
    35  		closeErr := f.Close()
    36  		if err == nil {
    37  			err = errors.WithStack(closeErr)
    38  		}
    39  	}()
    40  	// Overwrite whatever data is there and then truncate.
    41  	n, err := f.Write(data)
    42  	if err != nil {
    43  		return errors.WithStack(err)
    44  	} else if n < len(data) {
    45  		return errors.WithStack(io.ErrShortWrite)
    46  	}
    47  
    48  	return f.Truncate(int64(len(data)))
    49  }