Report the ring PipeWire was given, not a racing measurement of it

poll()'s one-shot quantum report sized the ring as free_space() + available().
Those are two separate samples of a position the realtime callback owns, and
free_space() derives from available() itself, so a process() that consumes
between the two makes the sum understate the ring by whatever it took.

That figure decides which line is printed. Understated far enough it reports
`tight`, and further still `starves` -- a WARN telling the operator their
stream will underrun on every cycle and to raise --buffer-ms, about a stream
that is sized correctly. Precisely the case the warning exists to distinguish.

The configured size is deterministic and already factored out, so ask
pipewire_ring_frames() for it; ring_capacity_() builds the ring from the same
call, so the reported figure is the one the ring was actually given.

The guard moves from bytes_per_frame_ to rate_ with it: nothing is divided
here now, and rate_ is what the report needs to be meaningful.

Reported by Copilot on PR #27.

Author: chrisuthe

src/pipewire_sink.cpp

@@ -565,9 +565,12 @@ void PipeWireSink::poll(int64_t now_ms) {
     const uint32_t quantum = this->quantum_frames_.load(std::memory_order_relaxed);
     if (quantum != 0 && !this->quantum_logged_.exchange(true, std::memory_order_relaxed)) {
         const std::lock_guard<std::mutex> lock(this->mutex_);
-        if (this->bytes_per_frame_ != 0) {
-            const size_t ring_frames = (this->ring_.free_space() + this->ring_.available()) /
-                                       this->bytes_per_frame_;
+        if (this->rate_ != 0) {
+            // The size the ring was built at, rather than a measurement of it. free_space() and
+            // available() are two samples of a position the realtime callback owns, and it can
+            // consume between them -- their sum then understates the ring, which here would mean
+            // warning an operator to raise --buffer-ms on a stream that is sized correctly.
+            const size_t ring_frames = pipewire_ring_frames(this->rate_, this->buffer_ms_);
             const PipeWireQuantumFit fit = pipewire_quantum_fit(ring_frames, quantum, this->rate_);
             if (fit.starves) {
                 // Broken rather than merely tight, and not something this player can negotiate