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55e6f64977
This originated in the cliconfig code to write out credentials files. The Windows implementation of this in particular was quite onerous to get right because it needs a very specific sequence of operations to avoid running into exclusive file locks, and so by factoring this out with only cosmetic modification we can avoid repeating all of that engineering effort for other atomic file writing use-cases.
72 lines
2.6 KiB
Go
72 lines
2.6 KiB
Go
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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