Re: [PATCH v2] pipe_command(): mark stdin descriptor as non-blocking

["René Scharfe" <l.s.r@web.de>]

Am 05.08.2022 um 17:36 schrieb Jeff King:
> On Wed, Aug 03, 2022 at 11:56:13PM +0200, René Scharfe wrote:
>
>> Without that line the added test hangs for me on the Git for Windows
>> SDK on Windows 11.
>
> Hmph. Interesting that it passes in CI, but not on your local setup.
> I wonder why pipes would behave differently. Or perhaps there is even
> some configuration different that means we are still running the perl
> add--interactive there, though I kind of doubt it (and couldn't find
> anything pointing there).
>
> Still, if it fails in at least one spot, it's something we need to deal
> with. On the plus side, once we figure out how to fix it, the
> hand-grenade of "enable_nonblock() does nothing on Windows" will not be
> present anymore. ;)
>
>> With the patch below it fails and reports basically nothing:
>> [...]
>>    not ok 57 - handle very large filtered diff
>>    #
>>    #               git reset --hard &&
>>    #               # The specific number here is not important, but it must
>>    #               # be large enough that the output of "git diff --color"
>>    #               # fills up the pipe buffer. 10,000 results in ~200k of
>>    #               # colored output.
>>    #               test_seq 10000 >test &&
>>    #               test_config interactive.diffFilter cat &&
>>    #               printf y >y &&
>>    #               force_color git add -p >output 2>&1 <y &&
>>    #               git diff-files --exit-code -- test
>>    #
>>    1..57
>>
>> The file "output" contains "error: failed to run 'cat'".  This is
>> add-patch.c::parse_diff() reporting that pipe_command() failed.  So
>> that's not it, yet.  (I don't actually know what I'm doing here.)
>
> That implies that your call to enable_nonblock() succeeded, or
> pipe_command() itself would have complained, too. Maybe instrument
> it like this:
>
> diff --git a/run-command.c b/run-command.c
> index 8ea609d4ae..27e79c928a 100644
> --- a/run-command.c
> +++ b/run-command.c
> @@ -1473,11 +1473,17 @@ int pipe_command(struct child_process *cmd,
>  	}
>
>  	if (pump_io(io, nr) < 0) {
> +		error_errno("pumping io failed");
>  		finish_command(cmd); /* throw away exit code */
>  		return -1;
>  	}
>
> -	return finish_command(cmd);
> +	{
> +		int ret = finish_command(cmd);
> +		if (ret)
> +			error("child returned failure %d", ret);
> +		return ret;
> +	}
>  }
>
>  enum child_state {
>
> Normally we stay pretty quiet there and let the caller report any
> problems, but it lacks enough context to make a more specific error
> report.

This adds "error: pumping io failed: No space left on device" to output.
Which kinda makes sense: With the pipe no longer blocking, there can be
a moment when the buffer is full and writes have to be rejected.  This
condition should be reported with EAGAIN, though.

Adding "if (len < 0 && errno == ENOSPC) continue;" after the xwrite()
call in pump_io_round() lets the test pass.

Perhaps the translation from Windows error code to POSIX is wrong here?

>
>>  int enable_nonblock(int fd)
>>  {
>> +	DWORD mode;
>> +	HANDLE handle = winansi_get_osfhandle(fd);
>> +	if (!handle)
>> +		return -1;
>> +	if (!GetNamedPipeHandleState(handle, &mode, NULL, NULL, NULL, NULL, 0))
>> +		return -1;
>> +	if (mode & PIPE_NOWAIT)
>> +		return 0;
>> +	mode |= PIPE_NOWAIT;
>> +	if (!SetNamedPipeHandleState(handle, &mode, NULL, NULL))
>> +		return -1;
>>  	return 0;
>>  }
>
> This looks plausibly correct to me. ;) We might want to change the name
> of the compat layer to enable_pipe_nonblock(), since one assumes from
> the function names this only works for pipes.

Right, and the "Named" part should be explained: As [1] says: "[...] you
can often pass a handle to an anonymous pipe to a function that requires
a handle to a named pipe.".

And perhaps that's not the right thing to do after all, as [2] says:
"Note that nonblocking mode is supported for compatibility with
Microsoft LAN Manager version 2.0 and should not be used to achieve
asynchronous input and output (I/O) [...]".

[1] https://docs.microsoft.com/en-us/windows/win32/ipc/anonymous-pipe-operations
[2] https://docs.microsoft.com/en-us/windows/win32/api/namedpipeapi/nf-namedpipeapi-setnamedpipehandlestate

>
> Thanks for poking at this.
>
> -Peff