diff --git a/Lib/test/test_capi.py b/Lib/test/test_capi.py
index 1dd2461..cdd84bb 100644
--- a/Lib/test/test_capi.py
+++ b/Lib/test/test_capi.py
@@ -5,44 +5,51 @@
 from test import test_support
 import _testcapi
 
-for name in dir(_testcapi):
-    if name.startswith('test_'):
-        test = getattr(_testcapi, name)
+def test_main():
+
+    for name in dir(_testcapi):
+        if name.startswith('test_'):
+            test = getattr(_testcapi, name)
+            if test_support.verbose:
+                print "internal", name
+            try:
+                test()
+            except _testcapi.error:
+                raise test_support.TestFailed, sys.exc_info()[1]
+
+    # some extra thread-state tests driven via _testcapi
+    def TestThreadState():
+        import thread
+        import time
+
         if test_support.verbose:
-            print "internal", name
-        try:
-            test()
-        except _testcapi.error:
-            raise test_support.TestFailed, sys.exc_info()[1]
+            print "auto-thread-state"
 
-# some extra thread-state tests driven via _testcapi
-def TestThreadState():
-    import thread
-    import time
+        idents = []
 
-    if test_support.verbose:
-        print "auto-thread-state"
+        def callback():
+            idents.append(thread.get_ident())
 
-    idents = []
+        _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"
 
-    def callback():
-        idents.append(thread.get_ident())
+    try:
+        _testcapi._test_thread_state
+        have_thread_state = True
+    except AttributeError:
+        have_thread_state = False
 
-    _testcapi._test_thread_state(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"
+    if have_thread_state:
+        TestThreadState()
+        import threading
+        t=threading.Thread(target=TestThreadState)
+        t.start()
+        t.join()
 
-try:
-    _testcapi._test_thread_state
-    have_thread_state = True
-except AttributeError:
-    have_thread_state = False
-
-if have_thread_state:
-    TestThreadState()
-    import threading
-    t=threading.Thread(target=TestThreadState)
-    t.start()
+if __name__ == "__main__":
+    test_main()
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 263c61e..60c71d7 100644
--- a/Modules/_testcapimodule.c
+++ b/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. */
@@ -482,7 +481,7 @@
 PyObject *codec_incrementalencoder(PyObject *self, PyObject *args)
 {
 	const char *encoding, *errors = NULL;
-	if (!PyArg_ParseTuple(args, "s|s:test_incrementalencoder", 
+	if (!PyArg_ParseTuple(args, "s|s:test_incrementalencoder",
 			      &encoding, &errors))
 		return NULL;
 	return PyCodec_IncrementalEncoder(encoding, errors);
@@ -492,7 +491,7 @@
 PyObject *codec_incrementaldecoder(PyObject *self, PyObject *args)
 {
 	const char *encoding, *errors = NULL;
-	if (!PyArg_ParseTuple(args, "s|s:test_incrementaldecoder", 
+	if (!PyArg_ParseTuple(args, "s|s:test_incrementaldecoder",
 			      &encoding, &errors))
 		return NULL;
 	return PyCodec_IncrementalDecoder(encoding, errors);
@@ -583,7 +582,17 @@
 
 #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 @@
 	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> 
+	   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