ALSA: fix clear buffer behavior

README.md

@@ -252,8 +252,11 @@ For each backend, do the following:
  0. Run `./sio_list_devices --watch` and make sure it detects when you plug and
     unplug a USB microphone.
  0. Run `./sio_sine` and make sure you hear a sine wave. For backends with raw
-    devices, run `./sio_sine --device id` (where 'id' is a device id you got
-    from `sio_list_devices` and make sure you hear a sine wave.
+    devices, run `./sio_sine --device id --raw` (where 'id' is a device id you
+    got from `sio_list_devices` and make sure you hear a sine wave.
+    - Use 'p' to test pausing, 'u' to test unpausing, 'q' to test cleanup.
+    - 'c' for clear buffer. Clear buffer should not pause the stream and it
+      should also not cause an underflow.
  0. Run `./underflow` and read the testing instructions that it prints.
  0. Run `./sio_microphone` and ensure that it is both recording and playing
     back correctly. If possible use the `--in-device` and `--out-device`

example/sio_sine.c

@@ -241,6 +241,8 @@ int main(int argc, char **argv) {
         } else if (c == 'c') {
             fprintf(stderr, "clear buffer result: %s\n",
                     soundio_strerror(soundio_outstream_clear_buffer(outstream)));
+        } else if (c == 'q') {
+            break;
         } else if (c == '\r' || c == '\n') {
             // ignore
         } else {

src/alsa.cpp

@@ -1064,6 +1064,23 @@ void outstream_thread_run(void *arg) {
                 }
                 if (!osa->thread_exit_flag.test_and_set())
                     return;
+                if (!osa->clear_buffer_flag.test_and_set()) {
+                    if ((err = snd_pcm_drop(osa->handle)) < 0) {
+                        outstream->error_callback(outstream, SoundIoErrorStreaming);
+                        return;
+                    }
+                    if ((err = snd_pcm_reset(osa->handle)) < 0) {
+                        if (err == -EBADFD) {
+                            // If this happens the snd_pcm_drop will have done
+                            // the function of the reset so it's ok that this
+                            // did not work.
+                        } else {
+                            outstream->error_callback(outstream, SoundIoErrorStreaming);
+                            return;
+                        }
+                    }
+                    continue;
+                }
 
                 snd_pcm_sframes_t avail = snd_pcm_avail_update(osa->handle);
                 if (avail < 0) {
@@ -1172,6 +1189,8 @@ static int outstream_open_alsa(SoundIoPrivate *si, SoundIoOutStreamPrivate *os)
     SoundIoOutStream *outstream = &os->pub;
     SoundIoDevice *device = outstream->device;
 
+    osa->clear_buffer_flag.test_and_set();
+
     if (outstream->software_latency == 0.0)
         outstream->software_latency = clamp(device->software_latency_min, 1.0, device->software_latency_max);
 
@@ -1429,9 +1448,7 @@ static int outstream_clear_buffer_alsa(SoundIoPrivate *si,
         SoundIoOutStreamPrivate *os)
 {
     SoundIoOutStreamAlsa *osa = &os->backend_data.alsa;
-    int err;
-    if ((err = snd_pcm_reset(osa->handle)) < 0)
-        return SoundIoErrorStreaming;
+    osa->clear_buffer_flag.clear();
     return 0;
 }
 
@@ -1443,7 +1460,6 @@ static int outstream_pause_alsa(struct SoundIoPrivate *si, struct SoundIoOutStre
 
     int err;
     if ((err = snd_pcm_pause(osa->handle, pause)) < 0) {
-        fprintf(stderr, "alsa pause result: %s\n", snd_strerror(err));
         return SoundIoErrorIncompatibleDevice;
     }
 

src/alsa.hpp

@@ -59,6 +59,7 @@ struct SoundIoOutStreamAlsa {
     int period_size;
     int write_frame_count;
     bool is_paused;
+    atomic_flag clear_buffer_flag;
     SoundIoChannelArea areas[SOUNDIO_MAX_CHANNELS];
 };