output/MultipleOutputs: simplify locking in CheckPipe()

Instead of keeping all open outputs locked, let ClearTailChunk() stall
playback until MultipleOutputs::CheckPipe() has updated the MusicPipe.

src/output/Control.hxx

@@ -404,8 +404,29 @@ public:
 	gcc_pure
 	bool LockIsChunkConsumed(const MusicChunk &chunk) const noexcept;
 
+	/**
+	 * There's only one chunk left in the pipe (#pipe), and all
+	 * audio outputs have consumed it already.  Clear the
+	 * reference.
+	 *
+	 * This stalls playback to give the caller a chance to shift
+	 * the #MusicPipe without getting disturbed; after this,
+	 * LockAllowPlay() must be called to resume playback.
+	 */
 	void ClearTailChunk(const MusicChunk &chunk) noexcept {
+		if (!IsOpen())
+			return;
+
 		source.ClearTailChunk(chunk);
+		allow_play = false;
+	}
+
+	/**
+	 * Locking wrapper for ClearTailChunk().
+	 */
+	void LockClearTailChunk(const MusicChunk &chunk) noexcept {
+		const std::lock_guard<Mutex> lock(mutex);
+		ClearTailChunk(chunk);
 	}
 
 	void LockPlay() noexcept;

src/output/MultipleOutputs.cxx

@@ -271,35 +271,10 @@ MultipleOutputs::IsChunkConsumed(const MusicChunk *chunk) const noexcept
 	return true;
 }
 
-inline void
-MultipleOutputs::ClearTailChunk(const MusicChunk *chunk,
-				bool *locked) noexcept
-{
-	assert(chunk->next == nullptr);
-	assert(pipe->Contains(chunk));
-
-	for (unsigned i = 0, n = outputs.size(); i != n; ++i) {
-		auto &ao = *outputs[i];
-
-		/* this mutex will be unlocked by the caller when it's
-		   ready */
-		ao.mutex.lock();
-		locked[i] = ao.IsOpen();
-
-		if (!locked[i]) {
-			ao.mutex.unlock();
-			continue;
-		}
-
-		ao.ClearTailChunk(*chunk);
-	}
-}
-
 unsigned
 MultipleOutputs::CheckPipe() noexcept
 {
 	const MusicChunk *chunk;
-	bool locked[outputs.size()];
 
 	assert(pipe != nullptr);
 
@@ -320,18 +295,18 @@ MultipleOutputs::CheckPipe() noexcept
 		if (is_tail)
 			/* this is the tail of the pipe - clear the
 			   chunk reference in all outputs */
-			ClearTailChunk(chunk, locked);
+			for (const auto &ao : outputs)
+				ao->LockClearTailChunk(*chunk);
 
 		/* remove the chunk from the pipe */
 		const auto shifted = pipe->Shift();
 		assert(shifted.get() == chunk);
 
 		if (is_tail)
-			/* unlock all audio outputs which were locked
-			   by clear_tail_chunk() */
-			for (unsigned i = 0, n = outputs.size(); i != n; ++i)
-				if (locked[i])
-					outputs[i]->mutex.unlock();
+			/* resume playback which has been suspended by
+			   LockClearTailChunk() */
+			for (const auto &ao : outputs)
+				ao->LockAllowPlay();
 
 		/* chunk is automatically returned to the buffer by
 		   ~MusicChunkPtr() */

src/output/MultipleOutputs.hxx

@@ -171,13 +171,6 @@ private:
 	 */
 	bool IsChunkConsumed(const MusicChunk *chunk) const noexcept;
 
-	/**
-	 * There's only one chunk left in the pipe (#pipe), and all
-	 * audio outputs have consumed it already.  Clear the
-	 * reference.
-	 */
-	void ClearTailChunk(const MusicChunk *chunk, bool *locked) noexcept;
-
 	/* virtual methods from class PlayerOutputs */
 	void EnableDisable() override;
 	void Open(const AudioFormat audio_format) override;