python: make sure spot.automata() terminates the command

Fixes #341.

* python/spot/__init__.py (automata): Rewrite and simplify using
the subprocess context manager.
* tests/python/341.py: New file.
* tests/Makefile.am: Add it.
* NEWS: Mention the issue.

NEWS

@@ -11,6 +11,9 @@ New in spot 2.5.2.dev (not yet released)
   - "autfilt -B --sat-minimize" was incorrectly producing
     transition-based automata.
 
+  - Using spot.automata("cmd...|") to read just a few automata out of
+    an infinite stream would not properly terminate the command.
+
 New in spot 2.5.2 (2018-03-25)
 
   Bugs fixed:

python/spot/__init__.py

@@ -33,6 +33,7 @@ from spot.aux import \
 import subprocess
 import os
 import signal
+from contextlib import suppress as _supress
 
 
 # The parrameters used by default when show() is called on an automaton.
@@ -441,30 +442,33 @@ def automata(*sources, timeout=None, ignore_abort=True,
             else:
                 p = automaton_stream_parser(filename, o)
             a = True
-            while a:
-                # This returns None when we reach the end of the file.
-                a = p.parse(_bdd_dict).aut
-                if a:
-                    yield a
+            # Using proc as a context manager ensures that proc.stdout will be
+            # closed on exit, and the process will be properly waited for.
+            # This is important when running tools that produce an infinite
+            # stream of automata and that must be killed once the generator
+            # returned by spot.automata() is destroyed.  Otherwise, _supress()
+            # is just a dummy context manager that does nothing (Python 3.7
+            # introduces nullcontext() for this purpose, but at the time of
+            # writing we support Python 3.4).
+            mgr = proc if proc else _supress()
+            with mgr:
+                while a:
+                    # This returns None when we reach the end of the file.
+                    a = p.parse(_bdd_dict).aut
+                    if a:
+                        yield a
         finally:
             # Make sure we destroy the parser (p) and the subprocess
-            # (prop) in the correct order...
+            # (prop) in the correct order.
             del p
             if proc is not None:
-                if not a:
-                    # We reached the end of the stream.  Wait for the
-                    # process to finish, so that we get its exit code.
-                    ret = proc.wait()
-                else:
-                    # if a != None, we probably got there through an
-                    # exception, and the subprocess might still be
-                    # running.  Check if an exit status is available
-                    # just in case.
-                    ret = proc.poll()
+                ret = proc.returncode
                 del proc
-                if ret:
+                # Do not complain about the exit code if we are already raising
+                # an exception.
+                if ret and sys.exc_info()[0] is None:
                     raise subprocess.CalledProcessError(ret, filename[:-1])
-    # deleting o explicitely now prevents Python 3.5 from
+    # deleting o explicitly now prevents Python 3.5 from
     # reporting the following error: "<built-in function
     # delete_automaton_parser_options> returned a result with
     # an error set".  It's not clear to me if the bug is in Python

tests/Makefile.am

@@ -348,6 +348,7 @@ TESTS_ipython = \
 # do not consider part of the documentation: those have to start
 # with a _.
 TESTS_python = \
+  python/341.py \
   python/_altscc.ipynb \
   python/_autparserr.ipynb \
   python/_aux.ipynb \

tests/python/341.py

@@ -0,0 +1,35 @@
+# -*- mode: python; coding: utf-8 -*-
+# Copyright (C) 2017 Laboratoire de Recherche et Développement de l'Epita
+# (LRDE).
+#
+# This file is part of Spot, a model checking library.
+#
+# Spot is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# Spot is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+# or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
+# License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import spot
+from subprocess import _active
+
+def two_intersecting_automata():
+    """return two random automata with a non-empty intersection"""
+    g = spot.automata('randaut -A4 -Q5 -n-1 2 |')
+    for a, b in zip(g, g):
+        if a.intersects(b):
+            return a, b
+
+for i in range(5):
+    two_intersecting_automata()
+
+n = len(_active)
+print(n, "active processes")
+assert(n == 0);