fix(audio): per-port A/V start barrier so video+audio FIFOs begin at the same instant (fixes fixed A/V offset)

services/capture/deltacast-bridge/main.c

@@ -50,6 +50,19 @@ static void on_signal(int s) { (void)s; atomic_store(&g_stop, 1); }
  * rather than stopping the port, so the thread survives ffmpeg restarts. */
 static atomic_int g_port_stop[MAX_PORTS];
 
+/* ── Per-port A/V start barrier ──────────────────────────────────────────
+ * Video and audio are two independent FIFOs that ffmpeg opens at slightly
+ * different times (input 0 video, then input 1 audio). Whichever thread's
+ * reader connects first would otherwise start pumping data immediately,
+ * giving its stream a head start → a fixed A/V offset that never recovers
+ * (raw streams carry no timestamps, so ffmpeg PTS-zeroes whatever each stream
+ * delivers first). To start both streams from the SAME instant, each thread
+ * publishes "my FIFO reader is connected" and then BOTH spin until the other
+ * side is also connected before writing their first real sample/frame.
+ * Reset to 0 each time a fresh pair of readers connects. */
+static atomic_int g_video_ready[MAX_PORTS];
+static atomic_int g_audio_ready[MAX_PORTS];
+
 /* ── Stream type by port index (non-contiguous SDK enum) ────────────── */
 static ULONG rx_streamtype(unsigned port) {
     switch (port) {
@@ -252,6 +265,28 @@ static void *audio_thread(void *arg) {
         #endif
         fcntl(fd, F_SETPIPE_SZ, 1024 * 1024);
 
+        /* A/V start barrier: announce ready, then wait (up to ~5s) for the
+         * video reader so both streams begin at the same instant. While
+         * waiting we drain+discard hardware audio slots so the board audio
+         * queue doesn't overflow and the first sample we write lines up with
+         * the first video frame. */
+        atomic_store(&g_audio_ready[ps->port], 1);
+        {
+            int waited = 0;
+            while (!atomic_load(&g_video_ready[ps->port])
+                   && !atomic_load(&g_stop) && !atomic_load(&g_port_stop[ps->port])
+                   && waited < 5000) {
+                if (have_vhd_audio) {
+                    HANDLE ds = NULL;
+                    if (VHD_LockSlotHandle(stream, &ds) == VHDERR_NOERROR)
+                        VHD_UnlockSlotHandle(ds);  /* discard pre-roll audio */
+                }
+                struct timespec t = {0, 1000000L}; nanosleep(&t, NULL); waited++;
+            }
+            fprintf(stderr, "[audio:%u] A/V barrier released (video_ready=%d)\n",
+                    ps->port, atomic_load(&g_video_ready[ps->port]));
+        }
+
         /* Reset wall-clock baseline after potentially blocking on open().
          * Only used for the SILENCE fallback path (no hardware audio). */
         struct timespec next;
@@ -343,6 +378,8 @@ static void *audio_thread(void *arg) {
         }
 
         close(fd);
+        /* Reader gone — clear ready so the next session re-syncs at the barrier. */
+        atomic_store(&g_audio_ready[ps->port], 0);
     }
 
     if (stream) {
@@ -383,6 +420,26 @@ static void *video_thread(void *arg) {
             }
         }
 
+        /* A/V start barrier: announce ready, then wait (up to ~5s) for the
+         * audio reader so both streams begin at the same instant. While
+         * waiting we drain+discard incoming video slots so the board queue
+         * doesn't overflow and so the FIRST frame we actually write is the one
+         * captured at the moment audio also starts. */
+        atomic_store(&g_video_ready[ps->port], 1);
+        {
+            int waited = 0;
+            while (!atomic_load(&g_audio_ready[ps->port])
+                   && !atomic_load(&g_stop) && !atomic_load(&g_port_stop[ps->port])
+                   && waited < 5000) {
+                HANDLE ds = NULL;
+                if (VHD_LockSlotHandle(ps->video_stream, &ds) == VHDERR_NOERROR)
+                    VHD_UnlockSlotHandle(ds);   /* discard pre-roll frame */
+                struct timespec t = {0, 1000000L}; nanosleep(&t, NULL); waited++;
+            }
+            fprintf(stderr, "[video:%u] A/V barrier released (audio_ready=%d)\n",
+                    ps->port, atomic_load(&g_audio_ready[ps->port]));
+        }
+
         HANDLE slot = NULL;
         int fatal = 0;
         while (!atomic_load(&g_stop) && !atomic_load(&g_port_stop[ps->port])) {
@@ -424,6 +481,8 @@ static void *video_thread(void *arg) {
         }
 
         close(fd);
+        /* Reader gone — clear ready so the next session re-syncs at the barrier. */
+        atomic_store(&g_video_ready[ps->port], 0);
         if (fatal) break;
     }
 
@@ -610,8 +669,12 @@ int main(int argc, char *argv[]) {
     memset(ps, 0, sizeof(ps));
     int active_count = 0;
 
-    /* Initialise per-port stop flags. */
+    /* Initialise per-port stop + A/V barrier flags. */
-    for (int pi = 0; pi < MAX_PORTS; pi++) atomic_store(&g_port_stop[pi], 0);
+    for (int pi = 0; pi < MAX_PORTS; pi++) {
+        atomic_store(&g_port_stop[pi], 0);
+        atomic_store(&g_video_ready[pi], 0);
+        atomic_store(&g_audio_ready[pi], 0);
+    }
 
     for (int pi = 0; pi < port_count; pi++) {
         if (!locked[pi]) continue;