Add some assertions in syncrep.c

A couple of routines assume that the LWLock SyncRepLock needs to be
taken, so add a couple of assertions to be sure of that.  Also, when
waiting for a given LSN at transaction commit, the code implied that the
syncrep queue cleanup happens while holding interrupts, but the code
never checked after that.

Author: Michael Paquier
Reviewed-by: Fujii Masao, Kyotaro Horiguchi, Dongming Liu
Discussion: https://postgr.es/m/a0806273-8bbb-43b3-bbe1-c45a58f6ae21.lingce.ldm@alibaba-inc.com

src/backend/replication/syncrep.c

@@ -149,6 +149,13 @@ SyncRepWaitForLSN(XLogRecPtr lsn, bool commit)
 	const char *old_status;
 	int			mode;
 
+	/*
+	 * This should be called while holding interrupts during a transaction
+	 * commit to prevent the follow-up shared memory queue cleanups to be
+	 * influenced by external interruptions.
+	 */
+	Assert(InterruptHoldoffCount > 0);
+
 	/* Cap the level for anything other than commit to remote flush only. */
 	if (commit)
 		mode = SyncRepWaitMode;
@@ -516,6 +523,8 @@ SyncRepReleaseWaiters(void)
 /*
  * Calculate the synced Write, Flush and Apply positions among sync standbys.
  *
+ * The caller must hold SyncRepLock.
+ *
  * Return false if the number of sync standbys is less than
  * synchronous_standby_names specifies. Otherwise return true and
  * store the positions into *writePtr, *flushPtr and *applyPtr.
@@ -529,6 +538,8 @@ SyncRepGetSyncRecPtr(XLogRecPtr *writePtr, XLogRecPtr *flushPtr,
 {
 	List	   *sync_standbys;
 
+	Assert(LWLockHeldByMe(SyncRepLock));
+
 	*writePtr = InvalidXLogRecPtr;
 	*flushPtr = InvalidXLogRecPtr;
 	*applyPtr = InvalidXLogRecPtr;
@@ -688,6 +699,8 @@ cmp_lsn(const void *a, const void *b)
 List *
 SyncRepGetSyncStandbys(bool *am_sync)
 {
+	Assert(LWLockHeldByMe(SyncRepLock));
+
 	/* Set default result */
 	if (am_sync != NULL)
 		*am_sync = false;
@@ -992,7 +1005,7 @@ SyncRepGetStandbyPriority(void)
  * Pass all = true to wake whole queue; otherwise, just wake up to
  * the walsender's LSN.
  *
- * Must hold SyncRepLock.
+ * The caller must hold SyncRepLock in exclusive mode.
  */
 static int
 SyncRepWakeQueue(bool all, int mode)
@@ -1003,6 +1016,7 @@ SyncRepWakeQueue(bool all, int mode)
 	int			numprocs = 0;
 
 	Assert(mode >= 0 && mode < NUM_SYNC_REP_WAIT_MODE);
+	Assert(LWLockHeldByMeInMode(SyncRepLock, LW_EXCLUSIVE));
 	Assert(SyncRepQueueIsOrderedByLSN(mode));
 
 	proc = (PGPROC *) SHMQueueNext(&(WalSndCtl->SyncRepQueue[mode]),