Release the GIL before any call to async methods (#123)

Fix #122 

When call an async method on Pulsar C++ client, we need to be releasing the GIL to avoid a deadlock between that and the producer lock.

Author: merlimat

src/utils.cc

@@ -21,7 +21,16 @@
 
 void waitForAsyncResult(std::function<void(ResultCallback)> func) {
     auto promise = std::make_shared<std::promise<Result>>();
-    func([promise](Result result) { promise->set_value(result); });
+
+    {
+        // Always call the Pulsar C++ client methods without holding
+        // the GIL. This avoids deadlocks due the sequence of acquiring
+        // mutexes by different threads. eg:
+        // Thread-1: GIL -> producer.lock
+        // Thread-2: producer.lock -> GIL (In a callback)
+        py::gil_scoped_release release;
+        func([promise](Result result) { promise->set_value(result); });
+    }
     internal::waitForResult(*promise);
 }
 

src/utils.h

@@ -49,10 +49,14 @@ inline T waitForAsyncValue(std::function<void(std::function<void(Result, const T
     auto resultPromise = std::make_shared<std::promise<Result>>();
     auto valuePromise = std::make_shared<std::promise<T>>();
 
-    func([resultPromise, valuePromise](Result result, const T& value) {
-        valuePromise->set_value(value);
-        resultPromise->set_value(result);
-    });
+    {
+        py::gil_scoped_release release;
+
+        func([resultPromise, valuePromise](Result result, const T& value) {
+            valuePromise->set_value(value);
+            resultPromise->set_value(result);
+        });
+    }
 
     internal::waitForResult(*resultPromise);
     return valuePromise->get_future().get();