1.7.8: write fails with EAGAIN

[Robert Wruck]

>> I replaced the cygwin1.dll with cygwin1-20110309.dll and now a single
>> write() with 78 MB never returns but seems to write repeatedly to the
>> pipe (file was 5GB when I hit Ctrl-C).

Just terminating the loop after write_overlapped_fallback is not enough.
When the size to write exceeds MAX_OVERLAPPED_WRITE_LEN and WriteFile 
fails for MAX_OVERLAPPED_WRITE_LEN, write() will always return 0.

In case of overlapped_error, nbytes should be set to -1.

I'd suggest to remove write_overlapped_fallback and instead decrement 
'len' on each failure. Since write is not guaranteed to write the whole 
data, this should be OK.

       do
	{
	  bool res = WriteFile (get_output_handle (), ptr, len, &nbytes,
				get_overlapped ());
	  switch (wait_overlapped (res, true, &nbytes, is_nonblocking (), 
(size_t) len))
	    {
	    case overlapped_fallback:
		  len /= 4; /* feel free to tune */
		  if (!len) {
		    set_errno (ENOSPC); /* Can't write anything */
		    nbytes = (DWORD) -1; /* Don't forget */
		    keep_looping = false;
		    break;
		  }
	      /* fall through intentionally */;
	    case overlapped_signal:
	      keep_looping = true;
	      break;
	    default:	/* Added to quiet gcc */
	    case overlapped_error:
	      nbytes = (DWORD) -1; /* Don't forget */
	      /* fall through intentionally */;
	    case overlapped_success:
	      keep_looping = false;
	      break;
	    }
	}
       while (keep_looping);

Result (from the application's perspective) is:

writing 78954543 bytes...
result is 19738635, errno is 0
writing 59215908 bytes...
result is 14803977, errno is 0
writing 44411931 bytes...
result is 11102982, errno is 0
writing 33308949 bytes...
result is 8327237, errno is 0
writing 24981712 bytes...
result is 24981712, errno is 0

-Robert

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