configsession: T3259: avoid deadlock when data fills stdout pipe

If the subprocess is producing enough data (in this case showConfig on a
large config file), then the construction:

p = subprocess.Popen(.., stdout=subprocess.PIPE, ..)
p.wait()

will deadlock with the subprocess waiting for data to be consumed, while
the Python process waits for its termination. So consume data, then wait
for termination.

1 files changed, 2 insertions, 2 deletions

diff --git a/python/vyos/configsession.py b/python/vyos/configsession.py
index 82b9355a3..670e6c7fc 100644
--- a/python/vyos/configsession.py
+++ b/python/vyos/configsession.py
@@ -129,9 +129,9 @@ class ConfigSession(object):
 
     def __run_command(self, cmd_list):
         p = subprocess.Popen(cmd_list, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, env=self.__session_env)
+        (stdout_data, stderr_data) = p.communicate()
+        output = stdout_data.decode()
         result = p.wait()
-        output = p.stdout.read().decode()
-        p.communicate()
         if result != 0:
             raise ConfigSessionError(output)
         return output