[PATCH] Cygwin: pipe: Fix SSH hang with non-cygwin pipe reader

[Takashi Yano]

On Fri, 27 Jun 2025 12:25:18 +0200 (CEST)
Johannes Schindelin wrote:
> Hi Takashi,
> Good explanation, thank you for improving your commit message writing
> skill.

Thanks.

> > Also, pipe_data_available() returns PDA_UNKNOWN rather than 1 when the
> > pipe space estimation fails so that select() and raw_write() can perform
> > appropriate fallback handling.
> 
> This looks unrelated? Would this not rather be in a separate patch, to
> make it substantially easier to review for correctness?

I'll make a separate patch.

> P.S.: One thing that strikes me as immediately concerning is this part:
> 
> > -	  if (avail < 1)	/* error or pipe closed */
> > +	  if (avail == PDA_UNKNOWN && real_non_blocking_mode)
> > +	    avail = len1;
> 
> That means that the next loop iteration will call `NtWriteFile()` with
> `len1` bytes (`len1` now being identical to `avail`), even if `len1` can
> be substantially larger than `PIPE_BUF` (in my tests, it got stuck at
> `len1 == 2097152` in some instances), which is highly likely to be
> undesirable.

I don't think so. avail = len1 is performed only when real_non_blocking_mode.
In real non blocking mode, we can call NtWriteFile() with larger data than
actual pipe space without blocking.

When you observed `len1 == 2097152`, perhaps avail was 1, I guess.

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>