pulseaudio: Feed audio data in response to write callbacks.

Instead of waiting until the entire buffer from the SDL callback is ready
to be accepted by PulseAudio, we use pa_stream_set_write_callback and
feed some portion of the buffer as callbacks come in asking for more.

This lets us remove the halving of the buffer size during device open,
and also (hopefully) solves several strange hangs that happen in unusual
circumstances.

Fixes #4387
Fixes #2262

src/audio/pulseaudio/SDL_pulseaudio.c

@@ -107,8 +107,6 @@ static int (*PULSEAUDIO_pa_stream_connect_record) (pa_stream *, const char *,
 static pa_stream_state_t (*PULSEAUDIO_pa_stream_get_state) (const pa_stream *);
 static size_t (*PULSEAUDIO_pa_stream_writable_size) (const pa_stream *);
 static size_t (*PULSEAUDIO_pa_stream_readable_size) (const pa_stream *);
-static int (*PULSEAUDIO_pa_stream_begin_write) (pa_stream *, void **, size_t*);
-static int (*PULSEAUDIO_pa_stream_cancel_write) (pa_stream *);
 static int (*PULSEAUDIO_pa_stream_write) (pa_stream *, const void *, size_t,
     pa_free_cb_t, int64_t, pa_seek_mode_t);
 static pa_operation * (*PULSEAUDIO_pa_stream_drain) (pa_stream *,
@@ -119,6 +117,7 @@ static pa_operation * (*PULSEAUDIO_pa_stream_flush) (pa_stream *,
     pa_stream_success_cb_t, void *);
 static int (*PULSEAUDIO_pa_stream_disconnect) (pa_stream *);
 static void (*PULSEAUDIO_pa_stream_unref) (pa_stream *);
+static void (*PULSEAUDIO_pa_stream_set_write_callback)(pa_stream *, pa_stream_request_cb_t, void *);
 
 static int load_pulseaudio_syms(void);
 
@@ -222,8 +221,6 @@ load_pulseaudio_syms(void)
     SDL_PULSEAUDIO_SYM(pa_stream_writable_size);
     SDL_PULSEAUDIO_SYM(pa_stream_readable_size);
     SDL_PULSEAUDIO_SYM(pa_stream_write);
-    SDL_PULSEAUDIO_SYM(pa_stream_begin_write);
-    SDL_PULSEAUDIO_SYM(pa_stream_cancel_write);
     SDL_PULSEAUDIO_SYM(pa_stream_drain);
     SDL_PULSEAUDIO_SYM(pa_stream_disconnect);
     SDL_PULSEAUDIO_SYM(pa_stream_peek);
@@ -232,6 +229,7 @@ load_pulseaudio_syms(void)
     SDL_PULSEAUDIO_SYM(pa_stream_unref);
     SDL_PULSEAUDIO_SYM(pa_channel_map_init_auto);
     SDL_PULSEAUDIO_SYM(pa_strerror);
+    SDL_PULSEAUDIO_SYM(pa_stream_set_write_callback);
     return 0;
 }
 
@@ -359,51 +357,56 @@ ConnectToPulseServer(pa_mainloop **_mainloop, pa_context **_context)
 static void
 PULSEAUDIO_WaitDevice(_THIS)
 {
-    struct SDL_PrivateAudioData *h = this->hidden;
+    /* this is a no-op; we wait in PULSEAUDIO_PlayDevice now. */
+}
 
-    while (SDL_AtomicGet(&this->enabled)) {
-        if (PULSEAUDIO_pa_context_get_state(h->context) != PA_CONTEXT_READY ||
-            PULSEAUDIO_pa_stream_get_state(h->stream) != PA_STREAM_READY ||
-            PULSEAUDIO_pa_mainloop_iterate(h->mainloop, 1, NULL) < 0) {
-            SDL_OpenedAudioDeviceDisconnected(this);
-            return;
-        }
-        if (PULSEAUDIO_pa_stream_writable_size(h->stream) >= (h->mixlen/8)) {
-            return;
-        }
-    }
+static void WriteCallback(pa_stream *p, size_t nbytes, void *userdata)
+{
+    struct SDL_PrivateAudioData *h = (struct SDL_PrivateAudioData *) userdata;
+    /*printf("PULSEAUDIO WRITE CALLBACK! nbytes=%u\n", (unsigned int) nbytes);*/
+    h->bytes_requested += nbytes;
 }
 
 static void
 PULSEAUDIO_PlayDevice(_THIS)
 {
-    /* Write the audio data */
     struct SDL_PrivateAudioData *h = this->hidden;
-    if (SDL_AtomicGet(&this->enabled)) {
-        if (PULSEAUDIO_pa_stream_write(h->stream, h->pabuf, h->mixlen, NULL, 0LL, PA_SEEK_RELATIVE) < 0) {
+    int available = h->mixlen;
+    int written = 0;
+    int cpy;
+
+    /*printf("PULSEAUDIO PLAYDEVICE START! mixlen=%d\n", available);*/
+
+    while (SDL_AtomicGet(&this->enabled) && (available > 0)) {
+        cpy = SDL_min(h->bytes_requested, available);
+        if (cpy) {
+            if (PULSEAUDIO_pa_stream_write(h->stream, h->mixbuf + written, cpy, NULL, 0LL, PA_SEEK_RELATIVE) < 0) {
+                SDL_OpenedAudioDeviceDisconnected(this);
+                return;
+            }
+            /*printf("PULSEAUDIO FEED! nbytes=%u\n", (unsigned int) cpy);*/
+            h->bytes_requested -= cpy;
+            written += cpy;
+            available -= cpy;
+        }
+
+        /* let WriteCallback fire if necessary. */
+        /*printf("PULSEAUDIO ITERATE!\n");*/
+        if (PULSEAUDIO_pa_context_get_state(h->context) != PA_CONTEXT_READY ||
+            PULSEAUDIO_pa_stream_get_state(h->stream) != PA_STREAM_READY ||
+            PULSEAUDIO_pa_mainloop_iterate(h->mainloop, 1, NULL) < 0) {
             SDL_OpenedAudioDeviceDisconnected(this);
+            return;
         }
     }
+
+    /*printf("PULSEAUDIO PLAYDEVICE END! written=%d\n", written);*/
 }
 
 static Uint8 *
 PULSEAUDIO_GetDeviceBuf(_THIS)
 {
-    struct SDL_PrivateAudioData *h = this->hidden;
-    size_t nbytes = h->mixlen;
-    int ret;
-
-    ret = PULSEAUDIO_pa_stream_begin_write(h->stream, &h->pabuf, &nbytes);
-
-    if (ret != 0) {
-        /* fall back it intermediate buffer */
-        h->pabuf = h->mixbuf;
-    } else if (nbytes < h->mixlen) {
-        PULSEAUDIO_pa_stream_cancel_write(h->stream);
-        h->pabuf = h->mixbuf;
-    }
-
-    return (Uint8 *)h->pabuf;
+    return this->hidden->mixbuf;
 }
 
 
@@ -604,9 +607,6 @@ PULSEAUDIO_OpenDevice(_THIS, const char *devname)
     paspec.format = format;
 
     /* Calculate the final parameters for this audio specification */
-#ifdef PA_STREAM_ADJUST_LATENCY
-    this->spec.samples /= 2; /* Mix in smaller chunck to avoid underruns */
-#endif
     SDL_CalculateAudioSpec(&this->spec);
 
     /* Allocate mixing buffer */
@@ -623,22 +623,12 @@ PULSEAUDIO_OpenDevice(_THIS, const char *devname)
     paspec.rate = this->spec.freq;
 
     /* Reduced prebuffering compared to the defaults. */
-#ifdef PA_STREAM_ADJUST_LATENCY
     paattr.fragsize = this->spec.size;
-    /* 2x original requested bufsize */
-    paattr.tlength = h->mixlen * 4;
+    paattr.tlength = h->mixlen;
     paattr.prebuf = -1;
     paattr.maxlength = -1;
-    /* -1 can lead to pa_stream_writable_size() >= mixlen never being true */
-    paattr.minreq = h->mixlen;
-    flags = PA_STREAM_ADJUST_LATENCY;
-#else
-    paattr.fragsize = this->spec.size;
-    paattr.tlength = h->mixlen*2;
-    paattr.prebuf = h->mixlen*2;
-    paattr.maxlength = h->mixlen*2;
-    paattr.minreq = h->mixlen;
-#endif
+    paattr.minreq = -1;
+    flags |= PA_STREAM_ADJUST_LATENCY;
 
     if (ConnectToPulseServer(&h->mainloop, &h->context) < 0) {
         return SDL_SetError("Could not connect to PulseAudio server");
@@ -675,6 +665,7 @@ PULSEAUDIO_OpenDevice(_THIS, const char *devname)
     if (iscapture) {
         rc = PULSEAUDIO_pa_stream_connect_record(h->stream, h->device_name, &paattr, flags);
     } else {
+        PULSEAUDIO_pa_stream_set_write_callback(h->stream, WriteCallback, h);
         rc = PULSEAUDIO_pa_stream_connect_playback(h->stream, h->device_name, &paattr, flags, NULL, NULL);
     }
 

src/audio/pulseaudio/SDL_pulseaudio.h

@@ -43,11 +43,7 @@ struct SDL_PrivateAudioData
     Uint8 *mixbuf;
     int mixlen;
 
-    /* Pointer to the actual buffer in use in the current
-       GetDeviceBuf() -> PlayDevice() iteration.
-       Can be either the pointer returned by pa_stream_begin_write()
-       or mixbuf */
-    void *pabuf;
+    int bytes_requested;  /* bytes of data the hardware wants _now_. */
 
     const Uint8 *capturebuf;
     int capturelen;