output/alsa: implement Interrupt()

This allows canceling the blocking method LockWaitWriteAvailable(),
and thus allows breaking free of misbehaving ALSA drivers, avoiding a
MPD lockup.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/966

NEWS

@@ -1,4 +1,6 @@
 ver 0.22.1 (not yet released)
+* output
+  - alsa: don't deadlock when the ALSA driver is buggy
 
 ver 0.22 (2020/09/23)
 * protocol

src/output/plugins/AlsaOutputPlugin.cxx

@@ -25,6 +25,7 @@
 #include "lib/alsa/PeriodBuffer.hxx"
 #include "lib/alsa/Version.hxx"
 #include "../OutputAPI.hxx"
+#include "../Error.hxx"
 #include "mixer/MixerList.hxx"
 #include "pcm/Export.hxx"
 #include "system/PeriodClock.hxx"
@@ -177,6 +178,15 @@ class AlsaOutput final
 
 	bool drain;
 
+	/**
+	 * Was Interrupt() called?  This will unblock
+	 * LockWaitWriteAvailable().  It will be reset by Cancel() and
+	 * Pause(), as documented by the #AudioOutput interface.
+	 *
+	 * Only initialized while the output is open.
+	 */
+	bool interrupted;
+
 	/**
 	 * This buffer gets allocated after opening the ALSA device.
 	 * It contains silence samples, enough to fill one period (see
@@ -237,9 +247,12 @@ private:
 	void Open(AudioFormat &audio_format) override;
 	void Close() noexcept override;
 
+	void Interrupt() noexcept override;
+
 	size_t Play(const void *chunk, size_t size) override;
 	void Drain() override;
 	void Cancel() noexcept override;
+	bool Pause() noexcept override;
 
 	/**
 	 * Set up the snd_pcm_t object which was opened by the caller.
@@ -728,6 +741,7 @@ AlsaOutput::Open(AudioFormat &audio_format)
 	out_frame_size = pcm_export->GetOutputFrameSize();
 
 	drain = false;
+	interrupted = false;
 
 	size_t period_size = period_frames * out_frame_size;
 	ring_buffer = new boost::lockfree::spsc_queue<uint8_t>(period_size * 4);
@@ -741,6 +755,18 @@ AlsaOutput::Open(AudioFormat &audio_format)
 	error = {};
 }
 
+void
+AlsaOutput::Interrupt() noexcept
+{
+	std::unique_lock<Mutex> lock(mutex);
+
+	/* the "interrupted" flag will prevent
+	   LockWaitWriteAvailable() from actually waiting, and will
+	   instead throw AudioOutputInterrupted */
+	interrupted = true;
+	cond.notify_one();
+}
+
 inline int
 AlsaOutput::Recover(int err) noexcept
 {
@@ -912,6 +938,11 @@ AlsaOutput::CancelInternal() noexcept
 void
 AlsaOutput::Cancel() noexcept
 {
+	{
+		std::unique_lock<Mutex> lock(mutex);
+		interrupted = false;
+	}
+
 	if (!LockIsActive()) {
 		/* early cancel, quick code path without thread
 		   synchronization */
@@ -928,6 +959,17 @@ AlsaOutput::Cancel() noexcept
 		});
 }
 
+bool
+AlsaOutput::Pause() noexcept
+{
+	std::unique_lock<Mutex> lock(mutex);
+	interrupted = false;
+
+	/* not implemented - this override exists only to reset the
+	   "interrupted" flag */
+	return false;
+}
+
 void
 AlsaOutput::Close() noexcept
 {
@@ -956,6 +998,11 @@ AlsaOutput::LockWaitWriteAvailable()
 		if (error)
 			std::rethrow_exception(error);
 
+		if (interrupted)
+			/* a CANCEL command is in flight - don't block
+			   here */
+			throw AudioOutputInterrupted{};
+
 		size_t write_available = ring_buffer->write_available();
 		if (write_available >= min_available) {
 			/* reserve room for one extra block, just in