docker.py: don't hang on large docker output

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>
Message-Id: <1473192351-601-2-git-send-email-silbe@linux.vnet.ibm.com>
Signed-off-by: Fam Zheng <famz@redhat.com>

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')
+
+
 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)