Avoid deadlock with subprocess

Heed Python's subprocess module documentation warnings about deadlock
when using PIPE for a Popen object's stderr with Popen.wait() and
Popen.stderr.read() and use Popen.communicate() instead.

Fixes major#126.

supernova/supernova.py

@@ -40,8 +40,8 @@ def execute_executable(nova_args, env_vars):
                                stdout=sys.stdout,
                                stderr=subprocess.PIPE,
                                env=env_vars)
-    process.wait()
-    return process
+    stderr = process.communicate()[1]
+    return process, stderr
 
 
 def check_for_debug(supernova_args, nova_args):
@@ -88,17 +88,15 @@ def check_for_bypass_url(raw_creds, nova_args):
     return nova_args
 
 
-def handle_stderr(stderr_pipe):
+def handle_stderr(stderr):
     """
     Takes stderr from the command's output and displays it AFTER the stdout
     is printed by run_command().
     """
-    stderr_output = stderr_pipe.read()
-
-    if len(stderr_output) > 0:
+    if len(stderr) > 0:
         click.secho("\n__ Error Output {0}".format('_'*62), fg='white',
                     bold=True)
-        click.echo(stderr_output)
+        click.echo(stderr)
 
     return True
 
@@ -141,10 +139,10 @@ def run_command(nova_creds, nova_args, supernova_args):
     # In other news, I hate how python 2.6 does unicode.
     nova_args.insert(0, supernova_args['executable'])
     nova_args = [nova_arg.strip() for nova_arg in nova_args]
-    process = execute_executable(nova_args, env_vars)
+    process, stderr = execute_executable(nova_args, env_vars)
 
     # If the user asked us to be quiet, then let's not print stderr
     if not supernova_args.get('quiet'):
-        handle_stderr(process.stderr)
+        handle_stderr(stderr)
 
     return process.returncode

tests/test_supernova.py

@@ -96,10 +96,9 @@ def test_debug_check_with_heat(self):
         expected_nova_args = ['-d ']
         assert result == expected_nova_args
 
-    def test_handle_stderr(self, tmpdir, capsys):
-        p = tmpdir.mkdir("sub").join("stderr.txt")
-        p.write("This would be in the stderr pipe")
-        result = supernova.handle_stderr(p)
+    def test_handle_stderr(self, capsys):
+        s = "This would be in the stderr pipe"
+        result = supernova.handle_stderr(s)
         out, err = capsys.readouterr()
-        assert "stderr pipe" in out
+        assert s in out
         assert result