thread-mainloop: Add API for running a callback unlocked

This adds API to allow clients to schedule a callback in the mainloop
thread without the mainloop lock being held. This is meant for a case
where the client might be dealing with locking its own objects in
addition to the mainloop thread itself. In this case, it might need ton
control the locking order of the two, to match the order in other
threads, as it might not always be able to allow for its objects to be
locked after the mainloop thread lock.

src/map-file

@@ -360,6 +360,7 @@ pa_threaded_mainloop_get_retval;
 pa_threaded_mainloop_in_thread;
 pa_threaded_mainloop_lock;
 pa_threaded_mainloop_new;
+pa_threaded_mainloop_once_unlocked;
 pa_threaded_mainloop_set_name;
 pa_threaded_mainloop_signal;
 pa_threaded_mainloop_start;

src/pulse/thread-mainloop.c

@@ -44,6 +44,7 @@
 struct pa_threaded_mainloop {
     pa_mainloop *real_mainloop;
     volatile int n_waiting, n_waiting_for_accept;
+    pa_atomic_t in_once_unlocked;
 
     pa_thread* thread;
     pa_mutex* mutex;
@@ -174,8 +175,8 @@ void pa_threaded_mainloop_stop(pa_threaded_mainloop *m) {
 void pa_threaded_mainloop_lock(pa_threaded_mainloop *m) {
     pa_assert(m);
 
-    /* Make sure that this function is not called from the helper thread */
-    pa_assert(!m->thread || !pa_thread_is_running(m->thread) || !in_worker(m));
+    /* Make sure that this function is not called from the helper thread, unless it is unlocked */
+    pa_assert(!m->thread || !pa_thread_is_running(m->thread) || !in_worker(m) || pa_atomic_load(&m->in_once_unlocked));
 
     pa_mutex_lock(m->mutex);
 }
@@ -183,8 +184,8 @@ void pa_threaded_mainloop_lock(pa_threaded_mainloop *m) {
 void pa_threaded_mainloop_unlock(pa_threaded_mainloop *m) {
     pa_assert(m);
 
-    /* Make sure that this function is not called from the helper thread */
-    pa_assert(!m->thread || !pa_thread_is_running(m->thread) || !in_worker(m));
+    /* Make sure that this function is not called from the helper thread, unless it is unlocked */
+    pa_assert(!m->thread || !pa_thread_is_running(m->thread) || !in_worker(m) || pa_atomic_load(&m->in_once_unlocked));
 
     pa_mutex_unlock(m->mutex);
 }
@@ -258,3 +259,43 @@ void pa_threaded_mainloop_set_name(pa_threaded_mainloop *m, const char *name) {
     if (m->thread)
         pa_thread_set_name(m->thread, m->name);
 }
+
+typedef struct {
+    pa_threaded_mainloop *mainloop;
+    void (*callback)(pa_threaded_mainloop *m, void *userdata);
+    void *userdata;
+} once_unlocked_data;
+
+static void once_unlocked_cb(pa_mainloop_api *api, void *userdata) {
+    once_unlocked_data *data = userdata;
+
+    pa_assert(userdata);
+
+    pa_atomic_store(&data->mainloop->in_once_unlocked, 1);
+    pa_mutex_unlock(data->mainloop->mutex);
+
+    data->callback(data->mainloop, data->userdata);
+
+    pa_mutex_lock(data->mainloop->mutex);
+    pa_atomic_store(&data->mainloop->in_once_unlocked, 0);
+}
+
+void pa_threaded_mainloop_once_unlocked(pa_threaded_mainloop *m, void (*callback)(pa_threaded_mainloop *m, void *userdata),
+        void *userdata) {
+    pa_mainloop_api *api;
+    once_unlocked_data *data;
+
+    pa_assert(m);
+    pa_assert(callback);
+    /* Make sure that this function is not called from the helper thread */
+    pa_assert((m->thread && !pa_thread_is_running(m->thread)) || !in_worker(m));
+
+    api = pa_mainloop_get_api(m->real_mainloop);
+    data = pa_xnew0(once_unlocked_data, 1);
+
+    data->mainloop = m;
+    data->callback = callback;
+    data->userdata = userdata;
+
+    pa_mainloop_api_once(api, once_unlocked_cb, data);
+}

src/pulse/thread-mainloop.h

@@ -312,6 +312,16 @@ int pa_threaded_mainloop_in_thread(pa_threaded_mainloop *m);
 /** Sets the name of the thread. \since 5.0 */
 void pa_threaded_mainloop_set_name(pa_threaded_mainloop *m, const char *name);
 
+/** Runs the given callback in the mainloop thread without the lock held. The
+ * caller is responsible for ensuring that PulseAudio data structures are only
+ * accessed in a thread-safe way (that is, APIs that take pa_context and
+ * pa_stream are not thread-safe, and should not accessed without some
+ * synchronisation). This is the only situation in which
+ * pa_threaded_mainloop_lock() and pa_threaded_mainloop_unlock() may be used
+ * in the mainloop thread context. \since 13.0 */
+void pa_threaded_mainloop_once_unlocked(pa_threaded_mainloop *m, void (*callback)(pa_threaded_mainloop *m, void *userdata),
+        void *userdata);
+
 PA_C_DECL_END
 
 #endif

src/tests/thread-mainloop-test.c

@@ -30,8 +30,9 @@
 #include <pulse/util.h>
 #include <pulse/thread-mainloop.h>
 
-#include <pulsecore/macro.h>
 #include <pulsecore/core-rtclock.h>
+#include <pulsecore/macro.h>
+#include <pulsecore/mutex.h>
 
 static void tcb(pa_mainloop_api *a, pa_time_event *e, const struct timeval *tv, void *userdata) {
     pa_assert_se(pa_threaded_mainloop_in_thread(userdata));
@@ -40,6 +41,12 @@ static void tcb(pa_mainloop_api *a, pa_time_event *e, const struct timeval *tv,
     fprintf(stderr, "TIME EVENT END\n");
 }
 
+static void ocb(pa_threaded_mainloop *m, void *userdata) {
+    pa_threaded_mainloop_lock(m);
+    pa_threaded_mainloop_signal(m, 0);
+    pa_threaded_mainloop_unlock(m);
+}
+
 START_TEST (thread_mainloop_test) {
     pa_mainloop_api *a;
     pa_threaded_mainloop *m;
@@ -69,6 +76,17 @@ START_TEST (thread_mainloop_test) {
     fprintf(stderr, "waiting 5s (sleep)\n");
     pa_msleep(5000);
 
+    /* Test pa_threaded_mainloop_once_unlocked() */
+    pa_threaded_mainloop_lock(m);
+
+    fprintf(stderr, "scheduling unlocked callback\n");
+    pa_threaded_mainloop_once_unlocked(m, ocb, NULL);
+
+    pa_threaded_mainloop_wait(m);
+    fprintf(stderr, "got unlocked callback\n");
+
+    pa_threaded_mainloop_unlock(m);
+
     pa_threaded_mainloop_stop(m);
 
     pa_threaded_mainloop_free(m);