livesync: Add sync property for allowing to output buffers as soon as they arrive

By default livesync will wait for each buffer on the clock. If sync is
set to false, it will output buffers immediately once they're available
and only waits on the clock for outputting gap filler buffers.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1635>

docs/plugins/gst_plugins_cache.json

@@ -3410,6 +3410,18 @@
                         "readable": true,
                         "type": "gboolean",
                         "writable": true
+                    },
+                    "sync": {
+                        "blurb": "Synchronize buffers to the clock",
+                        "conditionally-available": false,
+                        "construct": false,
+                        "construct-only": false,
+                        "controllable": false,
+                        "default": "true",
+                        "mutable": "ready",
+                        "readable": true,
+                        "type": "gboolean",
+                        "writable": true
                     }
                 },
                 "rank": "none"

utils/livesync/src/livesync/imp.rs

@@ -89,6 +89,9 @@ struct State {
     /// See `PROP_SINGLE_SEGMENT`
     single_segment: bool,
 
+    /// See `PROP_SYNC`
+    sync: bool,
+
     /// Latency reported by upstream
     upstream_latency: Option<gst::ClockTime>,
 
@@ -162,6 +165,7 @@ struct State {
 const PROP_LATENCY: &str = "latency";
 const PROP_LATE_THRESHOLD: &str = "late-threshold";
 const PROP_SINGLE_SEGMENT: &str = "single-segment";
+const PROP_SYNC: &str = "sync";
 
 const PROP_IN: &str = "in";
 const PROP_DROP: &str = "drop";
@@ -181,6 +185,7 @@ impl Default for State {
             latency: DEFAULT_LATENCY,
             late_threshold: DEFAULT_LATE_THRESHOLD,
             single_segment: false,
+            sync: true,
             upstream_latency: None,
             playing: false,
             eos: false,
@@ -291,7 +296,7 @@ impl ObjectSubclass for LiveSync {
 
 impl ObjectImpl for LiveSync {
     fn properties() -> &'static [glib::ParamSpec] {
-        static PROPERTIES: Lazy<[glib::ParamSpec; 7]> = Lazy::new(|| {
+        static PROPERTIES: Lazy<[glib::ParamSpec; 8]> = Lazy::new(|| {
             [
                 glib::ParamSpecUInt64::builder(PROP_LATENCY)
                     .nick("Latency")
@@ -318,6 +323,11 @@ impl ObjectImpl for LiveSync {
                     .blurb("Timestamp buffers and eat segments so as to appear as one segment")
                     .mutable_ready()
                     .build(),
+                glib::ParamSpecBoolean::builder(PROP_SYNC)
+                    .nick("Sync")
+                    .blurb("Synchronize buffers to the clock")
+                    .mutable_ready()
+                    .build(),
                 glib::ParamSpecUInt64::builder(PROP_IN)
                     .nick("Frames input")
                     .blurb("Number of incoming frames accepted")
@@ -369,6 +379,10 @@ impl ObjectImpl for LiveSync {
                 state.single_segment = value.get().unwrap();
             }
 
+            PROP_SYNC => {
+                state.sync = value.get().unwrap();
+            }
+
             _ => unimplemented!(),
         }
     }
@@ -379,6 +393,7 @@ impl ObjectImpl for LiveSync {
             PROP_LATENCY => state.latency.to_value(),
             PROP_LATE_THRESHOLD => state.late_threshold.to_value(),
             PROP_SINGLE_SEGMENT => state.single_segment.to_value(),
+            PROP_SYNC => state.sync.to_value(),
             PROP_IN => state.num_in.to_value(),
             PROP_DROP => state.num_drop.to_value(),
             PROP_OUT => state.num_out.to_value(),
@@ -950,6 +965,14 @@ impl LiveSync {
         state.in_timestamp = timestamp;
         self.cond.notify_all();
 
+        // If we're not strictly syncing to the clock but output buffers as soon as they arrive
+        // then also wake up the source pad task now in case it's waiting on the clock.
+        if !state.sync {
+            if let Some(clock_id) = state.clock_id.take() {
+                clock_id.unschedule();
+            }
+        }
+
         Ok(gst::FlowSuccess::Ok)
     }
 
@@ -1008,7 +1031,12 @@ impl LiveSync {
         }
         state.srcresult?;
 
-        if let Some(out_timestamp) = state.out_timestamp {
+        // Synchronize to the clock if requested to do so, or when the queue is currently empty
+        // and we might have to introduce a gap buffer.
+        if let Some(out_timestamp) = (state.sync || state.queue.is_empty())
+            .then_some(state.out_timestamp)
+            .flatten()
+        {
             let sync_ts = out_timestamp.end;
 
             let element = self.obj();
@@ -1038,12 +1066,8 @@ impl LiveSync {
                 if jitter.is_negative() {"-"} else {""},
                 gst::ClockTime::from_nseconds(jitter.unsigned_abs()));
 
-            if res == Err(gst::ClockError::Unscheduled) {
-                return Err(gst::FlowError::Flushing);
-            }
-
-            state.srcresult?;
             state.clock_id = None;
+            state.srcresult?;
         }
 
         let in_item = state.queue.pop_front();