manager: fix Manager::createSinkClient()

Manager::createSinkClient() returns a nullptr set shared_ptr
when called with an id already associated to a sink client.
This is not the waiting behaviour: returns the existing client.

This patch fixes that and also add documentation to the public
API to explicit this behaviour.

Change-Id: Idc9a0b4f7d7a06e2bdafe965891469903f0d5d99
(cherry picked from commit a06588ed)

src/manager.cpp

@@ -2862,10 +2862,9 @@ Manager::createSinkClient(const std::string& id, bool mixer)
 {
     const auto& iter = sinkMap_.find(id);
     if (iter != std::end(sinkMap_)) {
-        if (iter->second.expired())
-            sinkMap_.erase(iter);
-        else
-            return nullptr;
+        if (auto sink = iter->second.lock())
+            return sink;
+        sinkMap_.erase(iter); // remove expired weak_ptr
     }
 
     auto sink = std::make_shared<video::SinkClient>(id, mixer);

src/manager.h

@@ -994,8 +994,24 @@ class Manager {
         void scheduleTask(std::shared_ptr<Runnable> task, std::chrono::steady_clock::time_point when);
 
 #ifdef RING_VIDEO
+        /**
+         * Create a new SinkClient instance, store it in an internal cache as a weak_ptr
+         * and return it as a shared_ptr. If a SinkClient is already stored for the given id,
+         * this method returns this instance.
+         * @param id SinkClient identifier as a string. Default is empty.
+         * @param mixer true if the SinkCient is the sink of a VideoMixer node. Default is false.
+         * @return share_ptr<SinkClient> A shared pointer on the created instance.
+         */
         std::shared_ptr<video::SinkClient> createSinkClient(const std::string& id="", bool mixer=false);
+
+        /**
+         * Return an existing SinkClient instance as a shared_ptr associated to the given identifier.
+         * Return an empty shared_ptr (nullptr) if nothing found.
+         * @param id SinkClient identifier as a string.
+         * @return share_ptr<SinkClient> A shared pointer on the found instance. Empty if not found.
+         */
         std::shared_ptr<video::SinkClient> getSinkClient(const std::string& id);
+
         VideoManager& getVideoManager() const { return *videoManager_; }
 #endif // RING_VIDEO