Re: [PATCH] mingw: handle writes to non-blocking pipe

["René Scharfe" <l.s.r@web.de>]

Am 10.08.2022 um 22:02 schrieb Jeff King:
> On Wed, Aug 10, 2022 at 07:39:40AM +0200, René Scharfe wrote:
>
>> write() on Windows reports ENOSPC when writing to a non-blocking pipe
>> whose buffer is full and rejects writes bigger than the buffer outright.
>> Change the error code to EAGAIN and try a buffer-sized partial write to
>> comply with POSIX and the expections of our Git-internal callers.
>
> Bearing in mind that I have no qualifications for reviewing
> Windows-specific patches, this seems like the right thing to be doing
> from the behavior you saw.
>
> One question:
>
>> +	if (result < 0 && errno == ENOSPC) {
>> +		/* check if fd is a non-blocking pipe */
>> +		HANDLE h = (HANDLE) _get_osfhandle(fd);
>> +		DWORD s;
>> +		if (GetFileType(h) == FILE_TYPE_PIPE &&
>> +		    GetNamedPipeHandleState(h, &s, NULL, NULL, NULL, NULL, 0) &&
>> +		    (s & PIPE_NOWAIT)) {
>> +			DWORD obuflen;
>> +			if (!GetNamedPipeInfo(h, NULL, &obuflen, NULL, NULL))
>> +				obuflen = PIPE_BUFFER_SIZE;
>> +			if (len > obuflen)
>> +				return mingw_write(fd, buf, obuflen);
>> +			errno = EAGAIN;
>> +		}
>
> OK, so we call GetNamedPipeInfo() to find the size of the pipe buffer.
> It's unclear to me from Microsoft's docs if that is the _total_ size, or
> if it's the remaining available size. Hopefully the latter, since none
> of this works otherwise. ;)
>
> But two corner cases:
>
>   - If we fail to get the size, we guess that it's the maximum. Is this
>     dangerous? I'm not sure why the call would fail, but if for some
>     reason it did fail and we can't make forward progress, would we
>     enter an infinite recursion of mingw_write()? Would it be safer to
>     bail with EAGAIN in such a case (through granted, that probably just
>     puts us into an infinite loop in xwrite())?

AFAIU it's the total size, not the available space.  I think I confused
it with PIPE_BUF, which we should use instead.

Alternatively we could retry with ever smaller sizes, down to one byte,
to avoid EAGAIN as much as possible.  Sounds costly, though.

>
>   - According to the docs:
>
>       If the buffer size is zero, the buffer is allocated as needed.
>
>     If we see this case, we'd then call mingw_write() with 0 bytes,
>     which I imagine also makes no forward progress (though maybe we
>     eventually return a successful 0-byte write?).

Ah, yes, forgot that case.

René