audio: drain the AudioOut2 queue at an exact 48 kHz grain rate

src/libs/libAudio2.cpp

@@ -262,6 +262,11 @@ static AudioOut2Submission& audioout2_submission(const AudioOut2ContextState& co
 	return g_audioout2_submissions[&context - g_audioout2_contexts.data()];
 }
 
+// Ticks of a sixth of a microsecond: one 48 kHz sample lasts exactly 125 of them.
+static uint64_t audioout2_clock() {
+	return LibKernel::KernelGetProcessTime() * 6;
+}
+
 static uint32_t audioout2_grain_micros(uint32_t grains) {
 	const auto sample_count = (grains == 0 ? 512u : grains);
 	return std::max<uint32_t>((sample_count * 1000000u) / 48000u, 1000u);
@@ -331,25 +336,23 @@ static void audioout2_update_context_locked(AudioOut2ContextState* state) {
 		return;
 	}
 
-	const auto now = LibKernel::KernelGetProcessTime();
+	const auto now = audioout2_clock();
 	if (state->last_update == 0 || state->queued == 0) {
 		state->last_update = now;
 		return;
 	}
-
+	if (now <= state->last_update) {
-	const auto grain_micros = static_cast<uint64_t>(audioout2_grain_micros(state->num_grains));
-	if (now <= state->last_update || grain_micros == 0) {
 		return;
 	}
 
-	const auto elapsed = now - state->last_update;
+	const auto grain   = uint64_t {state->num_grains} * 125;
-	const auto drained = std::min<uint64_t>(state->queued, elapsed / grain_micros);
+	const auto drained = std::min<uint64_t>(state->queued, (now - state->last_update) / grain);
 	if (drained == 0) {
 		return;
 	}
 
 	state->queued -= static_cast<uint32_t>(drained);
-	state->last_update += drained * grain_micros;
+	state->last_update += drained * grain;
 	if (state->queued == 0) {
 		state->last_update = now;
 	}
@@ -527,7 +530,7 @@ int KYTY_SYSV_ABI AudioOut2ContextCreate(const AudioOut2ContextParam* params, vo
 	state->handle              = *ctx;
 	state->queue_depth         = params->queue_depth == 0 ? 4u : params->queue_depth;
 	state->num_grains          = num_grains;
-	state->last_update         = LibKernel::KernelGetProcessTime();
+	state->last_update         = audioout2_clock();
 	state->object_audio_handle = object_audio_handle;
 	g_audioout2_state_mutex.Unlock();
 
@@ -615,7 +618,7 @@ int KYTY_SYSV_ABI AudioOut2ContextPush(AudioOut2ContextHandle ctx, uint32_t bloc
 			sleep_micros = audioout2_grain_micros(state->num_grains);
 			if (state->queued < state->queue_depth || use_device_clock) {
 				if (state->queued == 0) {
-					state->last_update = LibKernel::KernelGetProcessTime();
+					state->last_update = audioout2_clock();
 				}
 				if (state->queued < state->queue_depth) {
 					state->queued++;

tests/AudioOut2PortTests.cpp

@@ -35,6 +35,8 @@ int                               g_open_waiters = 0;
 bool                              g_block_opens  = false;
 bool                              g_block_outputs = false;
 std::vector<int>                  g_blocked_output;
+std::atomic_uint64_t              g_process_time {0};
+std::atomic_bool                  g_process_time_frozen {false};
 
 void Check(bool value, const char* text) {
 	if (!value) {
@@ -465,6 +467,27 @@ void TestPushConsumesPcm() {
 	AudioOut2::AudioOut2ContextDestroy(context);
 }
 
+void TestQueueDrainsAtExactGrainRate() {
+	// Three 512-sample grains play in exactly 32000 us.
+	const auto context = CreateContext();
+	g_process_time_frozen = true;
+	const auto start      = g_process_time.load();
+	for (int push = 0; push < 3; push++) {
+		Check(AudioOut2::AudioOut2ContextPush(context, 0) == OK, "async push failed");
+	}
+	const auto queued_after = [&](uint64_t micros) {
+		g_process_time  = start + micros;
+		uint32_t queued = 0;
+		Check(AudioOut2::AudioOut2ContextGetQueueLevel(context, &queued, nullptr) == OK,
+		      "queue-level query failed");
+		return queued;
+	};
+	Check(queued_after(31999) == 1, "the third grain drained before it finished playing");
+	Check(queued_after(32000) == 0, "three grains did not drain in their playing time");
+	g_process_time_frozen = false;
+	AudioOut2::AudioOut2ContextDestroy(context);
+}
+
 void TestPcmCopiedBeforeScratchBufferReuse() {
 	const auto context = CreateContext();
 	const auto param   = MakeParam();
@@ -746,8 +769,7 @@ uint32_t AudioOutOutputs(const OutputParam* params, uint32_t num, bool blocking)
 namespace Libs::LibKernel {
 
 uint64_t KYTY_SYSV_ABI KernelGetProcessTime() {
-	static std::atomic_uint64_t now {0};
+	return g_process_time_frozen ? g_process_time.load() : g_process_time.fetch_add(1000);
-	return now.fetch_add(1000);
 }
 
 } // namespace Libs::LibKernel
@@ -764,6 +786,7 @@ int main() {
 	TestAsynchronousDevicePushKeepsQueueBounded();
 	TestHandleWithoutPcmDoesNotBypassQueue();
 	TestPushConsumesPcm();
+	TestQueueDrainsAtExactGrainRate();
 	TestPcmCopiedBeforeScratchBufferReuse();
 	TestPortGainAndValidation();
 	TestObjectGainAndAmbisonics();