[Bug 229616] bin/dd does not check output's close() return code

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=229616

            Bug ID: 229616
           Summary: bin/dd does not check output's close() return code
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: bin
          Assignee: bugs at FreeBSD.org
          Reporter: tom at hur.st

In dd's main():

        dd_close();
        /*
         * Some devices such as cfi(4) may perform significant amounts
         * of work when a write descriptor is closed.  Close the out
         * descriptor explicitly so that the summary handler (called
         * from an atexit() hook) includes this work.
         */
        close(out.fd);
        exit(0);

dd_close() comments *claim* it flushes output, but this appears to be a lie -
it just finishes off any pending calls to write(), it doesn't ask they be
flushed to disk.

IO errors from previous writes that were later flushed to disk can end up
reported in close(), for example as mentioned in close(2):

    [ENOSPC]           The underlying object did not fit, cached data was lost

Obviously a dd that can exit 0 while also losing data is.. unfortunate.

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