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Date: Tue, 6 Sep 2016 09:30:45 +0800
From: Fam Zheng <famz@redhat.com>
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References: <1472063464-790-1-git-send-email-silbe@linux.vnet.ibm.com>
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Subject: Re: [Qemu-devel] [PATCH v2 1/7] docker.py: don't hang on large
 docker output
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To: Sascha Silbe <silbe@linux.vnet.ibm.com>
Cc: qemu-devel@nongnu.org, Alex =?iso-8859-1?Q?Benn=E9e?= <alex.bennee@linaro.org>

On Wed, 08/24 20:30, Sascha Silbe wrote:
> Unlike Popen.communicate(), subprocess.call() doesn't read from the
> stdout file descriptor. If the child process produces more output than
> fits into the pipe buffer, it will block indefinitely.
> 
> If we don't intend to consume the output, just send it straight to
> /dev/null to avoid this issue.
> 
> Signed-off-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
> Reviewed-by: Janosch Frank <frankja@linux.vnet.ibm.com>
> ---
> This fixes a hang for me when building the Ubuntu docker image (empty
> docker image cache).
> 
>  tests/docker/docker.py | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/tests/docker/docker.py b/tests/docker/docker.py
> index 222a105..efb2bf4 100755
> --- a/tests/docker/docker.py
> +++ b/tests/docker/docker.py
> @@ -25,6 +25,10 @@ from tarfile import TarFile, TarInfo
>  from StringIO import StringIO
>  from shutil import copy, rmtree
>  
> +
> +DEVNULL = open(os.devnull, 'wb')
> +
> +

Too many blank lines? Otherwise looks good.

>  def _text_checksum(text):
>      """Calculate a digest string unique to the text content"""
>      return hashlib.sha1(text).hexdigest()
> @@ -34,8 +38,7 @@ def _guess_docker_command():
>      commands = [["docker"], ["sudo", "-n", "docker"]]
>      for cmd in commands:
>          if subprocess.call(cmd + ["images"],
> -                           stdout=subprocess.PIPE,
> -                           stderr=subprocess.PIPE) == 0:
> +                           stdout=DEVNULL, stderr=DEVNULL) == 0:
>              return cmd
>      commands_txt = "\n".join(["  " + " ".join(x) for x in commands])
>      raise Exception("Cannot find working docker command. Tried:\n%s" % \
> @@ -98,7 +101,7 @@ class Docker(object):
>  
>      def _do(self, cmd, quiet=True, infile=None, **kwargs):
>          if quiet:
> -            kwargs["stdout"] = subprocess.PIPE
> +            kwargs["stdout"] = DEVNULL
>          if infile:
>              kwargs["stdin"] = infile
>          return subprocess.call(self._command + cmd, **kwargs)
> -- 
> 1.9.1
>