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

> On Thu, Aug 11, 2022 at 12:34:46AM +0200, René Scharfe wrote:
>
>>> 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.
>
> Hmm. If it's giving us the total size, that seems like it may fail in a
> case like this:
>
>   - we write a smaller amount to the pipe, say 7168 bytes. That leaves
>     1024 bytes in the buffer. The reader reads nothing yet.
>
>   - now we try to write another 4096 bytes. That's too big, so we get
>     ENOSPC. But when we ask for the pipe size, it tells us 8192. So we
>     _don't_ try to do a partial write of the remaining size, and return
>     EAGAIN. But now we've made no forward progress (i.e., even though
>     poll() said we could write, we don't, and we end up in the xwrite
>     loop).
>
> So we really do want to try to get a partial write. Even a single byte
> means we are making forward progress.
>
>> Alternatively we could retry with ever smaller sizes, down to one byte,
>> to avoid EAGAIN as much as possible.  Sounds costly, though.
>
> It's definitely not optimal, but it may not be too bad. If we cut the
> size in half each time, then at worst we make log2(N) extra write
> attempts, and we end up with a partial write within 50% of the optimal
> size.