[PATCH 2/9] audio/audio: fix video playback slowdown with spiceaudio

["Volker Rümelin" <vr_qemu@t-online.de>]

This patch allows the audio backends get_buffer_out() functions
to drop audio data and mitigates a bug reported on the qemu-devel
mailing list.

https://lists.nongnu.org/archive/html/qemu-devel/2020-09/msg03832.html

The new rules for the variables buf and size returned by
get_buffer_out() are:
size == 0: Downstream playback buffer is full. Retry later.
size > 0, buf != NULL: Copy size bytes to buf for playback.
size > 0, buf == NULL: Drop size bytes.

The audio playback rate with spiceaudio for the no audio case is
too fast, but that's what we had before commit fb35c2cec5
"audio/dsound: fix invalid parameters error". The complete fix
comes with the next patch.

Reported-by: Qi Zhou <atmgnd@outlook.com>
Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
---
 audio/audio.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/audio/audio.c b/audio/audio.c
index ce8c6dec5f..ad3f57b4c4 100644
--- a/audio/audio.c
+++ b/audio/audio.c
@@ -1091,12 +1091,15 @@ static size_t audio_pcm_hw_run_out(HWVoiceOut *hw, size_t live)
     while (live) {
         size_t size, decr, proc;
         void *buf = hw->pcm_ops->get_buffer_out(hw, &size);
-        if (!buf || size == 0) {
+
+        if (size == 0) {
             break;
         }
 
         decr = MIN(size / hw->info.bytes_per_frame, live);
-        audio_pcm_hw_clip_out(hw, buf, decr);
+        if (buf) {
+            audio_pcm_hw_clip_out(hw, buf, decr);
+        }
         proc = hw->pcm_ops->put_buffer_out(hw, buf,
                                            decr * hw->info.bytes_per_frame) /
             hw->info.bytes_per_frame;
-- 
2.26.2