Audio: keep rings live, auto-promote loopback->hooked, show the reason

The host used to wait a fixed 1 s for the hook to publish a format and, on
timeout, fall to loopback permanently with no explanation -- which the now
slower (consensus) measurement made common. Restructure the audio thread to
keep the rings live the whole session and alternate: prefer hooked, and while
it isn't ready run loopback (echo) so guests still hear audio, watching the
ring to promote to the no-echo hooked path the instant the hook publishes a
format. Surface the concrete reason loopback is active (no format yet / not
renderable / no ring) in the Audio panel, in amber.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

host/src/audio/audio_loopback.cpp

@@ -111,12 +111,35 @@ std::string AudioMirror::status() const
 	return status_;
 }
 
+std::string AudioMirror::fallback_reason() const
+{
+	std::lock_guard<std::mutex> lock(status_mutex_);
+	return fallback_reason_;
+}
+
 void AudioMirror::set_status(std::string s)
 {
 	std::lock_guard<std::mutex> lock(status_mutex_);
 	status_ = std::move(s);
 }
 
+void AudioMirror::set_fallback_reason(std::string s)
+{
+	std::lock_guard<std::mutex> lock(status_mutex_);
+	fallback_reason_ = std::move(s);
+}
+
+void AudioMirror::enable_capture(AudioRingHeader* const* rings, bool on)
+{
+	for (unsigned i = 0; i < kMaxAudioStreams; ++i)
+	{
+		if (rings[i] != nullptr)
+		{
+			rings[i]->capture_enabled.store(on ? 1u : 0u, std::memory_order_release);
+		}
+	}
+}
+
 bool AudioMirror::start(DWORD pid)
 {
 	stop();
@@ -159,6 +182,7 @@ void AudioMirror::stop()
 	running_.store(false, std::memory_order_release);
 	source_.store(Source::None, std::memory_order_relaxed);
 	buffered_ms_.store(0, std::memory_order_relaxed);
+	set_fallback_reason({});
 	pid_ = 0;
 }
 
@@ -191,10 +215,9 @@ void AudioMirror::thread_main(DWORD pid)
 {
 	const bool com_ok = SUCCEEDED(CoInitializeEx(nullptr, COINIT_MULTITHREADED));
 
-	// Create the shared audio ring the injected hook produces into, and enable
-	// capture. If the hook is present it publishes a format within ~1 s and we
-	// consume the ring (no echo); otherwise we fall back to process loopback.
-	bool handled = false;
+	// Create the shared audio rings the injected hook produces into. The hook counts
+	// frames + measures the format regardless of capture_enabled, so we can keep these
+	// live the whole session and promote loopback -> hooked the moment a format appears.
 	AudioRingHeader* rings[kMaxAudioStreams] = {};
 	bool created_primary = false;
 	for (unsigned i = 0; i < kMaxAudioStreams; ++i)
@@ -203,24 +226,69 @@ void AudioMirror::thread_main(DWORD pid)
 		{
 			rings[i] = audio_ring_shm_[i].as<AudioRingHeader>();
 			audio_ring_init(*rings[i], kAudioRingCapacity);
-			rings[i]->capture_enabled.store(1, std::memory_order_release);
 			created_primary = created_primary || (i == 0);
 		}
 	}
-	if (created_primary)
+
+	if (!created_primary)
 	{
-		set_status("Waiting for render-hook…");
-		if (wait_for_format(rings[0], 1000))
+		// Couldn't create the hook's ring -> loopback only (no promote target).
+		set_fallback_reason("Couldn't create the audio ring; using loopback (echo).");
+		if (!stop_requested())
 		{
-			handled = run_hooked(rings);
+			run_loopback(pid, nullptr);
+		}
+	}
+	else
+	{
+		// Prefer the hooked (no-echo) path. While it isn't ready, run loopback (echo) so
+		// guests still hear audio, but watch the ring and promote to hooked the instant the
+		// hook publishes a format. A short wait first catches the fast cases (exact format /
+		// already-measured) without ever starting the echo.
+		// The guessed-rate path takes a few seconds to reach consensus; loopback covers
+		// that gap and the promote hands off seamlessly.
+		constexpr DWORD kHookWaitMs = 1200;
+		for (;;)
+		{
+			if (stop_requested())
+			{
+				break;
+			}
+			set_status("Waiting for render-hook…");
+			bool watch_for_promote = true;
+			if (wait_for_format(rings[0], kHookWaitMs))
+			{
+				set_fallback_reason({}); // hooked path is taking over
+				if (run_hooked(rings))
+				{
+					break; // ran to a clean stop
+				}
+				if (stop_requested())
+				{
+					break;
+				}
+				// run_hooked failed to initialize (the game's format isn't renderable here).
+				// That won't fix itself, so don't bounce back to it -- stay on loopback.
+				set_fallback_reason("Render-hook format isn't renderable on this endpoint; using loopback (echo).");
+				watch_for_promote = false;
+			}
+			else if (!stop_requested())
+			{
+				set_fallback_reason(
+				    "Render-hook hasn't published a format yet; using loopback (echo) -- will switch to "
+				    "hooked automatically once it does.");
+			}
+
+			enable_capture(rings, false); // game audible locally so loopback can capture it
+			if (!run_loopback(pid, watch_for_promote ? rings[0] : nullptr))
+			{
+				break; // stopped (not a promote)
+			}
+			// Promoted: a format appeared -> loop and try the hooked path again.
 		}
 	}
 
-	if (!handled && !stop_requested())
-	{
-		run_loopback(pid);
-	}
-
+	enable_capture(rings, false);
 	for (auto& shm : audio_ring_shm_)
 	{
 		shm.reset();
@@ -245,6 +313,7 @@ void AudioMirror::thread_main(DWORD pid)
 bool AudioMirror::run_hooked(AudioRingHeader* const* rings)
 {
 	AudioRingHeader* primary = rings[0];
+	enable_capture(rings, true); // hook silences the game + pushes frames into the rings
 	auto disable_all = [&] {
 		for (unsigned i = 0; i < kMaxAudioStreams; ++i)
 		{
@@ -515,9 +584,10 @@ bool AudioMirror::run_hooked(AudioRingHeader* const* rings)
 	return true;
 }
 
-void AudioMirror::run_loopback(DWORD pid)
+bool AudioMirror::run_loopback(DWORD pid, AudioRingHeader* promote_ring)
 {
 	source_.store(Source::Loopback, std::memory_order_relaxed);
+	bool promote = false;
 
 	IMMDeviceEnumerator* enumerator = nullptr;
 	IMMDevice* endpoint = nullptr;
@@ -643,6 +713,14 @@ void AudioMirror::run_loopback(DWORD pid)
 				set_status(capture.status());
 				break;
 			}
+			// Auto-promote: the hook published a format -> hand back so the caller switches
+			// to the no-echo hooked path (the rings stayed live the whole time).
+			if (promote_ring != nullptr && audio_ring_format_ready(*promote_ring))
+			{
+				set_status("Render-hook ready -- switching to hooked (no echo)…");
+				promote = true;
+				break;
+			}
 
 			UINT32 padding = 0;
 			if (FAILED(render_client->GetCurrentPadding(&padding)))
@@ -706,6 +784,7 @@ void AudioMirror::run_loopback(DWORD pid)
 	{
 		CloseHandle(render_event);
 	}
+	return promote; // true = hook caught up, caller should switch to hooked
 }
 
 } // namespace coop

host/src/audio/audio_loopback.hpp

@@ -95,16 +95,25 @@ public:
 
 	[[nodiscard]] std::string status() const;
 
+	// Why the loopback (echo) path is active instead of the hooked one, for the Audio
+	// panel. Empty when on the hooked path or before any fallback decision.
+	[[nodiscard]] std::string fallback_reason() const;
+
 private:
 	void thread_main(DWORD pid);
 	// Returns true if it owned the session to a clean stop; false if setup failed
 	// and the caller should fall back to the loopback path. `rings[0]` is the primary
 	// stream; additional non-null rings are mixed in.
 	bool run_hooked(AudioRingHeader* const* rings);
-	void run_loopback(DWORD pid);
+	// Loopback (echo) capture. If `promote_ring` is non-null, returns true the moment
+	// that ring's format becomes ready (the hook caught up -> caller promotes to hooked);
+	// returns false when stopped. With a null ring it only returns false (on stop).
+	bool run_loopback(DWORD pid, AudioRingHeader* promote_ring);
 	bool wait_for_format(AudioRingHeader* ring, DWORD timeout_ms);
+	static void enable_capture(AudioRingHeader* const* rings, bool on);
 	bool stop_requested() const;
 	void set_status(std::string s);
+	void set_fallback_reason(std::string s);
 
 	std::thread thread_;
 	HANDLE stop_event_ = nullptr;
@@ -120,6 +129,7 @@ private:
 
 	mutable std::mutex status_mutex_;
 	std::string status_;
+	std::string fallback_reason_; // why loopback is active (shown in the panel); guarded by status_mutex_
 };
 
 } // namespace coop

host/src/audio_panel.cpp

@@ -137,6 +137,16 @@ void AudioPanel::draw_ui(const HookStatusView& status, bool debug_details)
 		ImGui::TextWrapped("%s", mirror_status.c_str());
 	}
 
+	// Why we're on loopback instead of the no-echo hooked path (empty when hooked). Amber
+	// because it's a degraded-but-working state that auto-resolves when the hook catches up.
+	const std::string reason = mirror_.fallback_reason();
+	if (!reason.empty())
+	{
+		ImGui::PushStyleColor(ImGuiCol_Text, kAmber);
+		ImGui::TextWrapped("Why loopback: %s", reason.c_str());
+		ImGui::PopStyleColor();
+	}
+
 	// Only the loopback path leaves the game audible locally (the echo); the
 	// hooked path silences it, so don't warn there.
 	if (mirror_.source() == AudioMirror::Source::Loopback)