audio: slightly simplify pull underrun message printing

A previous commit moved the underrun reporting to report_underruns(),
and called it from get_space(). One reason was that I worried about
printing a log message from a "realtime" callback, so I tried to move it
out of the way. (Though there's little justification other than a bad
feeling. While an older version of the pull code tried to avoid any
mutexes at all in the callback to accommodate "requirements" from APIs
like jackaudio, we gave up on that. Nobody has complained yet.)

Simplify this and move underrun reporting back to the callback. But
instead of printing the message from there, move the message into the
playloop. Change the message slightly, because ao->log is inaccessible,
and without the log prefix (e.g. "[ao/alsa]"), some context is missing.

audio/out/ao.c

@@ -469,10 +469,7 @@ void ao_hotplug_event(struct ao *ao)
 // Returns whether this call actually set a new underrun flag.
 bool ao_underrun_event(struct ao *ao)
 {
-    bool new_underrun = ao_add_events(ao, AO_EVENT_UNDERRUN);
-    if (new_underrun)
-        MP_WARN(ao, "Device underrun detected.\n");
-    return new_underrun;
+    return ao_add_events(ao, AO_EVENT_UNDERRUN);
 }
 
 bool ao_chmap_sel_adjust(struct ao *ao, const struct mp_chmap_sel *s,

audio/out/pull.c

@@ -83,23 +83,10 @@ static void set_state(struct ao *ao, int new_state)
     }
 }
 
-static void report_underruns(struct ao *ao)
-{
-    struct ao_pull_state *p = ao->api_priv;
-
-    int underflow = atomic_fetch_and(&p->underflow, 0);
-    if (underflow) {
-        if (ao_underrun_event(ao))
-            MP_VERBOSE(ao, "Audio underrun by %d samples.\n", underflow);
-    }
-}
-
 static int get_space(struct ao *ao)
 {
     struct ao_pull_state *p = ao->api_priv;
 
-    report_underruns(ao);
-
     // Since the reader will read the last plane last, its free space is the
     // minimum free space across all planes.
     return mp_ring_available(p->buffers[ao->num_planes - 1]) / ao->sstride;
@@ -133,7 +120,9 @@ static int play(struct ao *ao, void **data, int samples, int flags)
     bool draining = write_samples == samples && (flags & AOPLAY_FINAL_CHUNK);
     atomic_store(&p->draining, draining);
 
-    report_underruns(ao);
+    int underflow = atomic_fetch_and(&p->underflow, 0);
+    if (underflow)
+        MP_DBG(ao, "Audio underrun by %d samples.\n", underflow);
 
     return write_samples;
 }
@@ -165,8 +154,10 @@ int ao_read_data(struct ao *ao, void **data, int samples, int64_t out_time_us)
     int buffered_bytes = mp_ring_buffered(p->buffers[0]);
     bytes = MPMIN(buffered_bytes, full_bytes);
 
-    if (full_bytes > bytes && !atomic_load(&p->draining))
+    if (full_bytes > bytes && !atomic_load(&p->draining)) {
         atomic_fetch_add(&p->underflow, (full_bytes - bytes) / ao->sstride);
+        ao_underrun_event(ao);
+    }
 
     if (bytes > 0)
         atomic_store(&p->end_time_us, out_time_us);

player/audio.c

@@ -859,8 +859,11 @@ void fill_audio_out_buffers(struct MPContext *mpctx)
 
     int playsize = ao_get_space(mpctx->ao);
 
-    if (ao_query_and_reset_events(mpctx->ao, AO_EVENT_UNDERRUN))
+    if (ao_query_and_reset_events(mpctx->ao, AO_EVENT_UNDERRUN)) {
+        if (!ao_c->underrun)
+            MP_WARN(mpctx, "Audio device underrun detected.\n");
         ao_c->underrun = true;
+    }
 
     // Stop feeding data if an underrun happened. Something else needs to
     // "unblock" audio after underrun. handle_update_cache() does this and can