gst: gstpipewireclock: Return a valid clock before stream start

When a gstreamer pipeline transits to playing state, it sets the
base_time of all elements to the internal time of the current clock. At
that point, the pipewire stream has not yet started and the
gstpipewireclock returns last_time which is initialized to 0 in
gstpipewirestream. This leads to a incorrect base_time in the gstreamer
element and various synchronization issues.

The use-case for last_time is not really clear to me. My basic guesswork
is: If a stream is no longer streaming the internal clock should pause
at that time and return last_time. So this patch keeps this behaviour
in place and only ensures that a valid time is returned when the stream
is not yet started and last_time is not in the future.

To keep the time scaling logic in place, a start time is recorded when
the stream was started, to properly match stream time and clock
monotonic.

A gstreamer pipeline that can be used to replicate the issue is:
GST_DEBUG="pipeline:5,GST_CLOCK:6,pipewiresrc:6" gst-launch-1.0 \
pipewiresrc name=video target-object=<some video target> ! \
video/x-raw,format=UYVY !  fakesink \
pipewiresrc name=audio target-object=<some audio target> ! \
audio/x-raw ! f akesink 2>&1 | \
grep PTS

Signed-off-by: Stefan Klug <stefan.klug@ideasonboard.com>

---

Changes in v2:
- Drop incorrect logic in case s == NULL
- Keep clock scaling in place

src/gst/gstpipewireclock.c

@@ -22,6 +22,7 @@ gst_pipewire_clock_new (GstPipeWireStream *stream, GstClockTime last_time)
   g_weak_ref_set (&clock->stream, stream);
   clock->last_time = last_time;
   clock->time_offset = last_time;
+  clock->start_time_valid = false;
 
   return GST_CLOCK_CAST (clock);
 }
@@ -38,14 +39,26 @@ gst_pipewire_clock_get_internal_time (GstClock * clock)
     return pclock->last_time;
 
   now = pw_stream_get_nsec(s->pwstream);
+
 #if 1
   struct pw_time t;
   if (s->pwstream == NULL ||
       pw_stream_get_time_n (s->pwstream, &t, sizeof(t)) < 0 ||
-      t.rate.denom == 0)
-    return pclock->last_time;
+      t.rate.denom == 0) {
+        pclock->start_time_valid = false;
+        if (now < pclock->last_time)
+          return pclock->last_time;
+        return now;
+  }
 
-  result = gst_util_uint64_scale (t.ticks, GST_SECOND * t.rate.num, t.rate.denom);
+  uint64_t elapsed = gst_util_uint64_scale (t.ticks, GST_SECOND * t.rate.num, t.rate.denom);
+
+  if (!pclock->start_time_valid) {
+    pclock->start_time = t.now - elapsed;
+    pclock->start_time_valid = true;
+  }
+
+  result = pclock->start_time + elapsed;
   result += now - t.now;
 
   result += pclock->time_offset;

src/gst/gstpipewireclock.h

@@ -22,6 +22,8 @@ struct _GstPipeWireClock {
   GWeakRef stream;
 
   GstClockTime last_time;
+  GstClockTime start_time;
+  bool start_time_valid;
   GstClockTimeDiff time_offset;
 };