I don't think that write on a actual file can randomly return a partial write, unless an actual error has occurred (out of disk space, segfault on the buffer, i/o error, or an interrupt). These are all things that are either under the application control or would need to handled anyway.
I thought it could happen if you received a signal but apparently on linux signals won't interrupt file i/o. which brings back memories of processes getting stuck doing NFS I/O.
