WriteFileRequest oneof

[rakyll]

FileSystemService.WriteRequest should either accept path + mode or a FileChunk per incoming message. Introducing the oneof will help with the ambiguity of what happens when all the fields are set.

It could look like:

// Metadata required to initialize the file write stream.
message FileMetadata {
  // Absolute or workspace-relative path of the file to write.
  string path = 1;
  // Optional POSIX file mode permission (e.g. 0644 or 0755).
  uint32 mode = 2;
}

// Streamed request chunk for writing data to a file.
message WriteFileRequest {
  oneof request {
    // Sent only on the first message to initialize the stream.
    FileMetadata metadata = 1;
    
    // Sent on all subsequent messages containing the file data.
    FileChunk chunk = 2;
  }
}