Re: [PATCH v1 1/2] git-p4: print stderr if P4 read_pipe operation fails

From: Junio C Hamano <gitster@pobox.com>
Date: Sep 9, 2015, 16:00 UTC
Message-ID: <xmqqbndb7fc0.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <1441629723-90604-2-git-send-email-larsxschneider@gmail.com>

larsxschneider@gmail.com writes:

Show 25 quoted lines

> From: Lars Schneider <larsxschneider@gmail.com>
>
> If read_pipe crashes then the caller can inspect the error and handle
> it appropriately.
>
> Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
> ---
>  git-p4.py | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/git-p4.py b/git-p4.py
> index 073f87b..36a4bcb 100755
> --- a/git-p4.py
> +++ b/git-p4.py
> @@ -134,11 +134,11 @@ def read_pipe(c, ignore_error=False):
>          sys.stderr.write('Reading pipe: %s\n' % str(c))
>  
>      expand = isinstance(c,basestring)
> -    p = subprocess.Popen(c, stdout=subprocess.PIPE, shell=expand)
> +    p = subprocess.Popen(c, stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=expand)
>      pipe = p.stdout
>      val = pipe.read()
>      if p.wait() and not ignore_error:
> -        die('Command failed: %s' % str(c))
> +        die('Command failed: %s\nError: %s' % (str(c), p.stderr.read()))

I don't know enough about the callers of this helper function to tell offhand if that is an issue, but this looks unsafe depending on what the process on the other side of these pipes are doing.

If it attempts to spew a lot on its standard error stream first and then write some to its standard output, I would not be surprised it would get stuck waiting for us to read and drain its standard error before it can proceed to write to its standard output, and in the meantime we would be waiting for it to say something on its standard output, no?