72 lines
2.6 KiB
Go
72 lines
2.6 KiB
Go
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package replacefile
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import (
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"fmt"
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"io/ioutil"
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"os"
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"path/filepath"
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)
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// AtomicWriteFile uses a temporary file along with this package's AtomicRename
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// function in order to provide a replacement for ioutil.WriteFile that
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// writes the given file into place as atomically as the underlying operating
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// system can support.
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//
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// The sense of "atomic" meant by this function is that the file at the
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// given filename will either contain the entirety of the previous contents
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// or the entirety of the given data array if opened and read at any point
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// during the execution of the function.
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//
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// On some platforms attempting to overwrite a file that has at least one
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// open filehandle will produce an error. On other platforms, the overwriting
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// will succeed but existing open handles will still refer to the old file,
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// even though its directory entry is no longer present.
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//
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// Although AtomicWriteFile tries its best to avoid leaving behind its
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// temporary file on error, some particularly messy error cases may result
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// in a leftover temporary file.
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func AtomicWriteFile(filename string, data []byte, perm os.FileMode) error {
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dir, file := filepath.Split(filename)
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f, err := ioutil.TempFile(dir, file) // alongside target file and with a similar name
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if err != nil {
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return fmt.Errorf("cannot create temporary file to update %s: %s", filename, err)
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}
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tmpName := f.Name()
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moved := false
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defer func(f *os.File, name string) {
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// Remove the temporary file if it hasn't been moved yet. We're
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// ignoring errors here because there's nothing we can do about
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// them anyway.
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if !moved {
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os.Remove(name)
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}
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}(f, tmpName)
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// We'll try to apply the requested permissions. This may
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// not be effective on all platforms, but should at least work on
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// Unix-like targets and should be harmless elsewhere.
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if err := os.Chmod(tmpName, perm); err != nil {
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return fmt.Errorf("cannot set mode for temporary file %s: %s", tmpName, err)
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}
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// Write the credentials to the temporary file, then immediately close
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// it, whether or not the write succeeds. Note that closing the file here
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// is required because on Windows we can't move a file while it's open.
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_, err = f.Write(data)
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f.Close()
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if err != nil {
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return fmt.Errorf("cannot write to temporary file %s: %s", tmpName, err)
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}
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// Temporary file now replaces the original file, as atomically as
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// possible. (At the very least, we should not end up with a file
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// containing only a partial JSON object.)
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err = AtomicRename(tmpName, filename)
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if err != nil {
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return fmt.Errorf("failed to replace %s with temporary file %s: %s", filename, tmpName, err)
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}
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moved = true
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return nil
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}
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