[obs] scheduler: serialize cadence controls

Track the LGMP serial for each accepted frame-schedule message and wait
for its acknowledgement before sending another. Preserve pending RESET,
IMMEDIATE, and feedback state after definite enqueue failures.

Use bounded retry polling and request one fresh immediate frame after an
ambiguous pointer-queue fault recovers.

obs/frame_scheduler.c

@@ -24,13 +24,79 @@
 
 #include <string.h>
 
-#define FRAME_SCHEDULER_LEASE_MS        1000U
-#define FRAME_SCHEDULER_RENEW_NS        250000000ULL
-#define FRAME_SCHEDULER_FEEDBACK_NS     50000000ULL
-#define FRAME_SCHEDULER_TICK_GRACE_NS   500000000ULL
-#define FRAME_SCHEDULER_TARGET_SLACK_NS 1000000ULL
-#define FRAME_SCHEDULER_MIN_PERIOD_NS   2000000ULL
-#define FRAME_SCHEDULER_MAX_PERIOD_NS   1000000000ULL
+#define FRAME_SCHEDULER_LEASE_MS          1000U
+#define FRAME_SCHEDULER_RENEW_NS          250000000ULL
+#define FRAME_SCHEDULER_FEEDBACK_NS       50000000ULL
+#define FRAME_SCHEDULER_TICK_GRACE_NS     500000000ULL
+#define FRAME_SCHEDULER_TARGET_SLACK_NS   1000000ULL
+#define FRAME_SCHEDULER_MIN_PERIOD_NS     2000000ULL
+#define FRAME_SCHEDULER_MAX_PERIOD_NS     1000000000ULL
+#define FRAME_SCHEDULER_CONTROL_RETRY_NS  10000000ULL
+#define FRAME_SCHEDULER_CONTROL_RETRY_MAX 250000000ULL
+
+static void lgFrameSchedulerControlBackoff(
+    LGFrameScheduler * scheduler, uint64_t now)
+{
+  uint64_t delay = scheduler->controlRetryDelay;
+  if (!delay)
+    delay = FRAME_SCHEDULER_CONTROL_RETRY_NS;
+
+  scheduler->nextControlCheck = now + delay;
+  scheduler->controlRetryDelay =
+    delay < FRAME_SCHEDULER_CONTROL_RETRY_MAX / 2 ?
+      delay * 2 : FRAME_SCHEDULER_CONTROL_RETRY_MAX;
+}
+
+static bool lgFrameSchedulerControlReady(LGFrameScheduler * scheduler,
+    PLGMPClientQueue queue, uint64_t now)
+{
+  if (!scheduler->controlPending)
+    return now >= scheduler->nextControlCheck;
+
+  if (now < scheduler->nextControlCheck)
+    return false;
+
+  uint32_t serial;
+  const LGMP_STATUS status = lgmpClientGetSerial(queue, &serial);
+  if (status != LGMP_OK ||
+      (int32_t)(serial - scheduler->controlSerial) < 0)
+  {
+    if (status != LGMP_OK)
+      scheduler->controlFaulted = true;
+    lgFrameSchedulerControlBackoff(scheduler, now);
+    return false;
+  }
+
+  scheduler->controlPending    = false;
+  scheduler->nextControlCheck  = 0;
+  scheduler->controlRetryDelay = FRAME_SCHEDULER_CONTROL_RETRY_NS;
+  if (scheduler->controlFaulted)
+  {
+    scheduler->controlFaulted   = false;
+    scheduler->immediatePending = true;
+  }
+  return true;
+}
+
+static bool lgFrameSchedulerSend(LGFrameScheduler * scheduler,
+    PLGMPClientQueue queue, const KVMFRFrameSchedule * message,
+    uint64_t now)
+{
+  uint32_t serial;
+  if (lgmpClientSendData(
+        queue, message, sizeof(*message), &serial) != LGMP_OK)
+  {
+    lgFrameSchedulerControlBackoff(scheduler, now);
+    return false;
+  }
+
+  scheduler->controlPending    = true;
+  scheduler->controlSerial     = serial;
+  scheduler->nextControlCheck  =
+    now + FRAME_SCHEDULER_CONTROL_RETRY_NS;
+  scheduler->controlRetryDelay = FRAME_SCHEDULER_CONTROL_RETRY_NS;
+  return true;
+}
 
 static void lgFrameSchedulerResetFeedback(LGFrameScheduler * scheduler)
 {
@@ -51,9 +117,10 @@ void lgFrameSchedulerInit(LGFrameScheduler * scheduler, bool supported,
     uint32_t clientID)
 {
   memset(scheduler, 0, sizeof(*scheduler));
-  scheduler->supported        = supported;
-  scheduler->immediatePending = supported;
-  scheduler->clientID         = clientID;
+  scheduler->supported         = supported;
+  scheduler->immediatePending  = supported;
+  scheduler->clientID          = clientID;
+  scheduler->controlRetryDelay = FRAME_SCHEDULER_CONTROL_RETRY_NS;
 }
 
 void lgFrameSchedulerSetActive(LGFrameScheduler * scheduler, bool active)
@@ -175,6 +242,9 @@ void lgFrameSchedulerUpdate(LGFrameScheduler * scheduler,
   if (!scheduler->supported || !queue)
     return;
 
+  if (!lgFrameSchedulerControlReady(scheduler, queue, now))
+    return;
+
   const bool tickStale = !scheduler->lastTick ||
     now - scheduler->lastTick > FRAME_SCHEDULER_TICK_GRACE_NS;
   if (!scheduler->sourceActive || tickStale || !scheduler->period)
@@ -189,7 +259,7 @@ void lgFrameSchedulerUpdate(LGFrameScheduler * scheduler,
       .flags      = KVMFR_FRAME_SCHEDULE_RELEASE,
     };
 
-    if (lgmpClientSendData(queue, &message, sizeof(message), NULL) != LGMP_OK)
+    if (!lgFrameSchedulerSend(scheduler, queue, &message, now))
       return;
 
     scheduler->active           = false;
@@ -234,7 +304,7 @@ void lgFrameSchedulerUpdate(LGFrameScheduler * scheduler,
     .lease                  = FRAME_SCHEDULER_LEASE_MS,
   };
 
-  if (lgmpClientSendData(queue, &message, sizeof(message), NULL) != LGMP_OK)
+  if (!lgFrameSchedulerSend(scheduler, queue, &message, now))
     return;
 
   scheduler->active           = true;

obs/frame_scheduler.h

@@ -34,11 +34,16 @@ typedef struct LGFrameScheduler
   bool     resetPending;
   bool     immediatePending;
   bool     feedbackDirty;
+  bool     controlPending;
+  bool     controlFaulted;
   uint32_t clientID;
   uint32_t generation;
+  uint32_t controlSerial;
   uint64_t period;
   uint64_t lastSend;
   uint64_t lastTick;
+  uint64_t nextControlCheck;
+  uint64_t controlRetryDelay;
 
   int64_t  phaseError;
   uint32_t feedbackFrameSerial;