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

[Junio C Hamano <gitster@pobox.com>]

larsxschneider@gmail.com writes:

> 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?