Try to repair at least one segfault on the Mac buildbot,

as diagnosed by Nick Coghlan.

test_capi.py:  A test module should never spawn a thread as
a side effect of being imported.  Because this one did, the
segfault one of its thread tests caused didn't occur until
a few tests after test_regrtest.py thought test_capi was
finished.  Repair that.  Also join() the thread spawned
at the end, so that test_capi is truly finished when
regrtest reports that it's done.

_testcapimodule.c test_thread_state():  this spawns a
couple of non-threading.py threads, passing them a PyObject*
argument, but did nothing to ensure that those threads
finished before returning.  As a result, the PyObject*
_could_ (although this was unlikely) get decref'ed out of
existence before the threads got around to using it.
Added explicit synchronization (via a Python mutex) so
that test_thread_state can reliably wait for its spawned
threads to finish.

Lib/test/test_capi.py

@@ -5,7 +5,9 @@ import sys
 from test import test_support
 import _testcapi
 
-for name in dir(_testcapi):
+def test_main():
+
+    for name in dir(_testcapi):
         if name.startswith('test_'):
             test = getattr(_testcapi, name)
             if test_support.verbose:
@@ -15,8 +17,8 @@ for name in dir(_testcapi):
             except _testcapi.error:
                 raise test_support.TestFailed, sys.exc_info()[1]
 
-# some extra thread-state tests driven via _testcapi
-def TestThreadState():
+    # some extra thread-state tests driven via _testcapi
+    def TestThreadState():
         import thread
         import time
 
@@ -29,20 +31,25 @@ def TestThreadState():
             idents.append(thread.get_ident())
 
         _testcapi._test_thread_state(callback)
+        a = b = callback
         time.sleep(1)
         # Check our main thread is in the list exactly 3 times.
         if idents.count(thread.get_ident()) != 3:
             raise test_support.TestFailed, \
                   "Couldn't find main thread correctly in the list"
 
-try:
+    try:
         _testcapi._test_thread_state
         have_thread_state = True
-except AttributeError:
+    except AttributeError:
         have_thread_state = False
 
-if have_thread_state:
+    if have_thread_state:
         TestThreadState()
         import threading
         t=threading.Thread(target=TestThreadState)
         t.start()
+        t.join()
+
+if __name__ == "__main__":
+    test_main()

Modules/_testcapimodule.c

@@ -10,7 +10,6 @@
 #ifdef WITH_THREAD
 #include "pythread.h"
 #endif /* WITH_THREAD */
-
 static PyObject *TestError;	/* set to exception object in init */
 
 /* Raise TestError with test_name + ": " + msg, and return NULL. */
@@ -583,7 +582,17 @@ raise_exception(PyObject *self, PyObject *args)
 
 #ifdef WITH_THREAD
 
-void _make_call(void *callable)
+/* test_thread_state spawns a thread of its own, and that thread releases
+ * `thread_done` when it's finished.  The driver code has to know when the
+ * thread finishes, because the thread uses a PyObject (the callable) that
+ * may go away when the driver finishes.  The former lack of this explicit
+ * synchronization caused rare segfaults, so rare that they were seen only
+ * on a Mac buildbot (although they were possible on any box).
+ */
+static PyThread_type_lock thread_done = NULL;
+
+static void
+_make_call(void *callable)
 {
 	PyObject *rc;
 	PyGILState_STATE s = PyGILState_Ensure();
@@ -592,32 +601,53 @@ void _make_call(void *callable)
 	PyGILState_Release(s);
 }
 
+/* Same thing, but releases `thread_done` when it returns.  This variant
+ * should be called only from threads spawned by test_thread_state().
+ */
+static void
+_make_call_from_thread(void *callable)
+{
+	_make_call(callable);
+	PyThread_release_lock(thread_done);
+}
+
 static PyObject *
 test_thread_state(PyObject *self, PyObject *args)
 {
 	PyObject *fn;
+
 	if (!PyArg_ParseTuple(args, "O:test_thread_state", &fn))
 		return NULL;
-	/* Ensure Python is setup for threading */
+
+	/* Ensure Python is set up for threading */
 	PyEval_InitThreads();
-	/* Start a new thread for our callback. */
-	PyThread_start_new_thread( _make_call, fn);
+	thread_done = PyThread_allocate_lock();
+	if (thread_done == NULL)
+		return PyErr_NoMemory();
+	PyThread_acquire_lock(thread_done, 1);
+
+	/* Start a new thread with our callback. */
+	PyThread_start_new_thread(_make_call_from_thread, fn);
 	/* Make the callback with the thread lock held by this thread */
 	_make_call(fn);
 	/* Do it all again, but this time with the thread-lock released */
 	Py_BEGIN_ALLOW_THREADS
 	_make_call(fn);
+	PyThread_acquire_lock(thread_done, 1);  /* wait for thread to finish */
 	Py_END_ALLOW_THREADS
+
 	/* And once more with and without a thread
 	   XXX - should use a lock and work out exactly what we are trying
 	   to test <wink>
 	*/
 	Py_BEGIN_ALLOW_THREADS
-	PyThread_start_new_thread( _make_call, fn);
+	PyThread_start_new_thread(_make_call_from_thread, fn);
 	_make_call(fn);
+	PyThread_acquire_lock(thread_done, 1);  /* wait for thread to finish */
 	Py_END_ALLOW_THREADS
-	Py_INCREF(Py_None);
-	return Py_None;
+
+	PyThread_free_lock(thread_done);
+	Py_RETURN_NONE;
 }
 #endif