[core, ios, qt, android] Close race condition in RunLoop (issue #9620)

Because a message we queue from the foreground may cause the background to complete, exit, and tear down the AsyncTask, we have to block queue processing until we've finished our call to AsyncTask::send().
Broadening the scope of a mutex is scary, but I audited the code of our four implementations of AsyncTask and I don't see any way this could cause a deadlock.

platform/android/src/run_loop.cpp

@@ -217,8 +217,10 @@ LOOP_HANDLE RunLoop::getLoopHandle() {
 }
 
 void RunLoop::push(std::shared_ptr<WorkTask> task) {
-    withMutex([&] { queue.push(std::move(task)); });
-    impl->wake();
+    withMutex([&] {
+        queue.push(std::move(task));
+        impl->wake();
+    });
 }
 
 void RunLoop::run() {

platform/darwin/src/run_loop.cpp

@@ -30,8 +30,10 @@ RunLoop::~RunLoop() {
 }
 
 void RunLoop::push(std::shared_ptr<WorkTask> task) {
-    withMutex([&] { queue.push(std::move(task)); });
-    impl->async->send();
+    withMutex([&] {
+        queue.push(std::move(task));
+        impl->async->send();
+    });
 }
 
 void RunLoop::run() {

platform/default/run_loop.cpp

@@ -130,8 +130,10 @@ LOOP_HANDLE RunLoop::getLoopHandle() {
 }
 
 void RunLoop::push(std::shared_ptr<WorkTask> task) {
-    withMutex([&] { queue.push(std::move(task)); });
-    impl->async->send();
+    withMutex([&] {
+        queue.push(std::move(task));
+        impl->async->send();
+    });
 }
 
 void RunLoop::run() {

platform/qt/src/run_loop.cpp

@@ -53,8 +53,10 @@ LOOP_HANDLE RunLoop::getLoopHandle() {
 }
 
 void RunLoop::push(std::shared_ptr<WorkTask> task) {
-    withMutex([&] { queue.push(task); });
-    impl->async->send();
+    withMutex([&] {
+        queue.push(std::move(task));
+        impl->async->send();
+    });
 }
 
 void RunLoop::run() {