enable multithreading

Python is single-threaded by default and crashes (when using an
unitialized semaphore) respectively blocks (when the main thread still
holds the Python global interpreter lock when it is outside the
pycrosswalk extension).

To avoid this, pycrosswalk must initialize multithreading and be more
careful about entering/leaving methods using Python.

The thread in which XW_Initialize() gets called becomes the main thread.
It must release the global lock when returning to Crosswalk and restore
it when called again. Otherwise Python code called by other threads cannot
run. The assumption is that Crosswalk will always call the extension from
the same thread.

Without this thread support, pycloudeebus in xwalk-launcher does not work:
the main thread is running the glib event loop in which twisted reacts
to D-Bus calls, while the second thread runs a Crosswalk event loop and
would hold the Python lock if we didn't release it.

diff --git a/src/pycrosswalk.c b/src/pycrosswalk.c
index 5e3ebfb..a87bc13 100644 (file)
--- a/src/pycrosswalk.c
+++ b/src/pycrosswalk.c
@@ -28,6 +28,18 @@ static PyObject* g_py_sync_messaging[EXTENSION_MAX];
 static PyObject* g_py_instance_created = NULL;
 static PyObject* g_py_instance_destroyed = NULL;
 
+// The thread in which XW_Initialize() gets called becomes the main thread.
+// It must release the global lock when returning to Crosswalk and restore
+// it when called again. Otherwise Python code called by other threads cannot
+// run. The assumption is that Crosswalk will always call the extension from
+// the same thread.
+//
+// Without this thread support, pycloudeebus in xwalk-launcher does not work:
+// the main thread is running the glib event loop in which twisted reacts
+// to D-Bus calls, while the second thread runs a Crosswalk event loop and
+// would hold the Python lock if we didn't release it.
+static PyThreadState* g_py_save_state;
+
 static char* g_extension_name = NULL;
 static char* g_javascript_api = NULL;
 
@@ -174,6 +186,8 @@ static PyObject* py_set_instance_destroyed_callback(PyObject* self, PyObject* ar
 static char* py_handle_message(XW_Instance instance,
                                PyObject* callback,
                                const char* message) {
+  char* pass_string = NULL;
+  PyEval_RestoreThread(g_py_save_state);
   PyObject* instance_object = PyLong_FromLong((long) instance);
   PyObject* message_object = PyUnicode_FromString(message);
   PyObject* args = PyTuple_Pack(2, instance_object, message_object);
@@ -182,7 +196,7 @@ static char* py_handle_message(XW_Instance instance,
 
   if (!args) {
     PyErr_Print();
-    return NULL;
+    goto done;
   }
 
 #ifdef LOGGING
@@ -193,10 +207,9 @@ static char* py_handle_message(XW_Instance instance,
 
   if (!result_object) {
     PyErr_Print();
-    return NULL;
+    goto done;
   }
 
-  char* pass_string = NULL;
 
 #if PY_MAJOR_VERSION >= 3
   char* result_string = result_object != Py_None ? PyUnicode_AsUTF8(result_object) : NULL;
@@ -214,6 +227,8 @@ static char* py_handle_message(XW_Instance instance,
 
   Py_DECREF(result_object);
 
+ done:
+  g_py_save_state = PyEval_SaveThread();
   return pass_string;
 }
 
@@ -245,14 +260,14 @@ static char* py_handle_instance(XW_Instance instance, PyObject* callback) {
     fprintf(stderr, "Handle instance (created/destroyed) not set!\n");
     return;
   }
-
+  PyEval_RestoreThread(g_py_save_state);
   PyObject* instance_object = PyLong_FromLong((long) instance);
   PyObject* args = PyTuple_Pack(1, instance_object);
   Py_DECREF(instance_object);
 
   if (!args) {
     PyErr_Print();
-    return;
+    goto done;
   }
 
   PyObject* result_object = PyObject_CallObject(callback, args);
@@ -261,10 +276,13 @@ static char* py_handle_instance(XW_Instance instance, PyObject* callback) {
   if (!result_object)
     PyErr_Print();
 
+ done:
+  g_py_save_state = PyEval_SaveThread();
   return;
 }
 
 static void xw_handle_shutdown(XW_Extension extension) {
+  PyEval_RestoreThread(g_py_save_state);
   Py_Finalize();
 }
 
@@ -367,6 +385,9 @@ int32_t XW_Initialize(XW_Extension extension, XW_GetInterface get_interface) {
   dlclose(handle);
 
   Py_Initialize();
+  PyEval_InitThreads();
+
+  int32_t result = XW_ERROR;
 
 #if PY_MAJOR_VERSION >= 3
   PyObject* xwalk_module = PyModule_Create(&PyXWalkModule);
@@ -414,5 +435,9 @@ int32_t XW_Initialize(XW_Extension extension, XW_GetInterface get_interface) {
   g_xw_sync_messaging = get_interface(XW_INTERNAL_SYNC_MESSAGING_INTERFACE);
   g_xw_sync_messaging->Register(extension, xw_handle_sync_message);
 
-  return XW_OK;
+  result = XW_OK;
+
+ done:
+  g_py_save_state = PyEval_SaveThread();
+  return result;
 }