AtomicWriteFile.cs

AtomicWriteFile.cs

/// <summary>
/// Atomically create or replace file at <paramref name="path"/>, with contents provided by
/// <paramref name="write"/>.
/// </summary>
/// <remarks>
/// Although <paramref name="path"/> is atomically updated, the contents of <c>tmpPath</c> are
/// undefined if this method does not return normally.
/// </remarks>
public static void AtomicWriteFile(string path, Action<Stream> write)
{
    // File.Replace() requires tmpPath to be in the same volume as path
    string tmpPath = path + "." + Path.GetRandomFileName();

    using (var tmpStream = new FileStream(tmpPath, FileMode.CreateNew, FileAccess.Write))
    {
        write(tmpStream);

        // Calls https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-flushfilebuffers
        // to ensure that the content of `tmpPath` is persisted to the physical disk before the
        // `File.Replace()` below.
        //
        // This is needed b/c NTFS journals metadata, but not file contents, and prevents a gap
        // where power loss would result in corrupted contents at `path`.
        tmpStream.Flush(flushToDisk: true);
    }

    // Calls https://docs.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-replacefilew
    // to atomically replace `path` with `tmpPath`, while preserving attributes and ACLs of `path`.
    File.Replace(sourceFileName: tmpPath, destinationFileName: path, destinationBackupFileName: null);
}