audio/jack: honour the enable state of the audio device

When the guest closes the audio device we must start dropping input
samples from JACK and zeroing the output buffer samples. Failure to do
so causes sound artifacts during operations such as guest OS reboot, and
causes a hang of the input pipeline breaking it until QEMU is restated.

Closing and reconnecting to JACK was tested during these enable/disable
calls which works well for Linux guests, however Windows re-opens the
audio hardware repeatedly even when doing simple tasks like playing a
system sounds. As such it was decided it is better to feed silence to
JACK while the device is disabled.

Signed-off-by: Geoffrey McRae <geoff@hostfission.com>
Message-id: 20200613040518.38172-6-geoff@hostfission.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

audio/jackaudio.c

@@ -56,7 +56,7 @@ typedef struct QJackClient {
     AudiodevJackPerDirectionOptions *opt;
 
     bool out;
-    bool finished;
+    bool enabled;
     bool connect_ports;
     int  packets;
 
@@ -271,9 +271,17 @@ static int qjack_process(jack_nframes_t nframes, void *arg)
     }
 
     if (c->out) {
-        qjack_buffer_read_l(&c->fifo, buffers, nframes);
+        if (likely(c->enabled)) {
+            qjack_buffer_read_l(&c->fifo, buffers, nframes);
+        } else {
+            for(int i = 0; i < c->nchannels; ++i) {
+                memset(buffers[i], 0, nframes * sizeof(float));
+            }
+        }
     } else {
-        qjack_buffer_write_l(&c->fifo, buffers, nframes);
+        if (likely(c->enabled)) {
+            qjack_buffer_write_l(&c->fifo, buffers, nframes);
+        }
     }
 
     return 0;
@@ -314,8 +322,8 @@ static void qjack_client_recover(QJackClient *c)
     if (c->state == QJACK_STATE_DISCONNECTED &&
         c->packets % 100 == 0) {
 
-        /* if not finished then attempt to recover */
-        if (!c->finished) {
+        /* if enabled then attempt to recover */
+        if (c->enabled) {
             dolog("attempting to reconnect to server\n");
             qjack_client_init(c);
         }
@@ -387,7 +395,6 @@ static int qjack_client_init(QJackClient *c)
     char client_name[jack_client_name_size()];
     jack_options_t options = JackNullOption;
 
-    c->finished      = false;
     c->connect_ports = true;
 
     snprintf(client_name, sizeof(client_name), "%s-%s",
@@ -483,8 +490,10 @@ static int qjack_init_out(HWVoiceOut *hw, struct audsettings *as,
     }
 
     jo->c.out       = true;
+    jo->c.enabled   = false;
     jo->c.nchannels = as->nchannels;
     jo->c.opt       = dev->u.jack.out;
+
     int ret = qjack_client_init(&jo->c);
     if (ret != 0) {
         return ret;
@@ -519,8 +528,10 @@ static int qjack_init_in(HWVoiceIn *hw, struct audsettings *as,
     }
 
     ji->c.out       = false;
+    ji->c.enabled   = false;
     ji->c.nchannels = as->nchannels;
     ji->c.opt       = dev->u.jack.in;
+
     int ret = qjack_client_init(&ji->c);
     if (ret != 0) {
         return ret;
@@ -568,23 +579,25 @@ static void qjack_client_fini(QJackClient *c)
 static void qjack_fini_out(HWVoiceOut *hw)
 {
     QJackOut *jo = (QJackOut *)hw;
-    jo->c.finished = true;
     qjack_client_fini(&jo->c);
 }
 
 static void qjack_fini_in(HWVoiceIn *hw)
 {
     QJackIn *ji = (QJackIn *)hw;
-    ji->c.finished = true;
     qjack_client_fini(&ji->c);
 }
 
 static void qjack_enable_out(HWVoiceOut *hw, bool enable)
 {
+    QJackOut *jo = (QJackOut *)hw;
+    jo->c.enabled = enable;
 }
 
 static void qjack_enable_in(HWVoiceIn *hw, bool enable)
 {
+    QJackIn *ji = (QJackIn *)hw;
+    ji->c.enabled = enable;
 }
 
 static int qjack_thread_creator(jack_native_thread_t *thread,