output/snapcast: implement Drain()

src/output/plugins/snapcast/Client.cxx

@@ -76,6 +76,10 @@ SnapcastClient::LockPopQueue() noexcept
 
 	auto chunk = std::move(chunks.front());
 	chunks.pop();
+
+	if (chunks.empty())
+		output.drain_cond.notify_one();
+
 	return chunk;
 }
 

src/output/plugins/snapcast/Client.hxx

@@ -68,6 +68,13 @@ public:
 	/**
 	 * Caller must lock the mutex.
 	 */
+	bool IsDrained() const noexcept {
+		return chunks.empty();
+	}
+
+	/**
+	 * Caller must lock the mutex.
+	 */
 	void Cancel() noexcept {
 		ClearQueue(chunks);
 	}

src/output/plugins/snapcast/Internal.hxx

@@ -24,6 +24,7 @@
 #include "output/Interface.hxx"
 #include "output/Timer.hxx"
 #include "thread/Mutex.hxx"
+#include "thread/Cond.hxx"
 #include "event/ServerSocket.hxx"
 #include "event/InjectEvent.hxx"
 #include "util/AllocatedArray.hxx"
@@ -82,6 +83,12 @@ public:
 	 */
 	mutable Mutex mutex;
 
+	/**
+	 * This cond is signalled when a #SnapcastClient has an empty
+	 * queue.
+	 */
+	Cond drain_cond;
+
 	SnapcastOutput(EventLoop &_loop, const ConfigBlock &block);
 	~SnapcastOutput() noexcept override;
 
@@ -162,13 +169,19 @@ public:
 
 	size_t Play(const void *chunk, size_t size) override;
 
-	// TODO: void Drain() override;
+	void Drain() override;
 	void Cancel() noexcept override;
 	bool Pause() override;
 
 private:
 	void OnInject() noexcept;
 
+	/**
+	 * Caller must lock the mutex.
+	 */
+	[[gnu::pure]]
+	bool IsDrained() const noexcept;
+
 	/* virtual methods from class ServerSocket */
 	void OnAccept(UniqueSocketDescriptor fd,
 		      SocketAddress address, int uid) noexcept override;

src/output/plugins/snapcast/SnapcastOutputPlugin.cxx

@@ -181,6 +181,9 @@ SnapcastOutput::RemoveClient(SnapcastClient &client) noexcept
 
 	client.unlink();
 	delete &client;
+
+	if (clients.empty())
+		drain_cond.notify_one();
 }
 
 std::chrono::steady_clock::duration
@@ -261,6 +264,26 @@ SnapcastOutput::Pause()
 	return true;
 }
 
+inline bool
+SnapcastOutput::IsDrained() const noexcept
+{
+	if (!chunks.empty())
+		return false;
+
+	for (const auto &client : clients)
+		if (!client.IsDrained())
+			return false;
+
+	return true;
+}
+
+void
+SnapcastOutput::Drain()
+{
+	std::unique_lock<Mutex> protect(mutex);
+	drain_cond.wait(protect, [this]{ return IsDrained(); });
+}
+
 void
 SnapcastOutput::Cancel() noexcept
 {