Revert "Detect too late snapshot"

This reverts commit af5213a.

Author: kvap

contrib/multimaster/multimaster.c

@@ -748,7 +748,6 @@ static void DtmInitialize()
         dtm->initialized = false;
         BgwPoolInit(&dtm->pool, MMExecutor, MMDatabaseName, MMQueueSize);
 		RegisterXactCallback(DtmXactCallback, NULL);
-		RegisterSubXactCallback(DtmSubXactCallback, NULL);
 	}
 	LWLockRelease(AddinShmemInitLock);
 

contrib/pg_dtm/pg_dtm.c

@@ -107,6 +107,7 @@ static SnapshotData DtmSnapshot = { HeapTupleSatisfiesMVCC };
 static bool DtmHasGlobalSnapshot;
 static bool DtmGlobalXidAssigned;
 static int DtmLocalXidReserve;
+static int DtmCurcid;
 static Snapshot DtmLastSnapshot;
 static TransactionManager DtmTM = {
 	DtmGetTransactionStatus,
@@ -613,9 +614,9 @@ static Snapshot DtmGetSnapshot(Snapshot snapshot)
 	}
 	if (TransactionIdIsValid(DtmNextXid) && snapshot != &CatalogSnapshotData)
 	{
-		if (!DtmHasGlobalSnapshot) { 
+		if (!DtmHasGlobalSnapshot && (snapshot != DtmLastSnapshot || DtmCurcid != snapshot->curcid))
 			DtmGlobalGetSnapshot(DtmNextXid, &DtmSnapshot, &dtm->minXid);
-		}
+		DtmCurcid = snapshot->curcid;
 		DtmLastSnapshot = snapshot;
 		DtmMergeWithGlobalSnapshot(snapshot);
 		if (!IsolationUsesXactSnapshot())
@@ -725,7 +726,6 @@ static void DtmInitialize()
 		dtm->nReservedXids = 0;
 		dtm->minXid = InvalidTransactionId;
 		RegisterXactCallback(DtmXactCallback, NULL);
-		RegisterSubXactCallback(DtmSubXactCallback, NULL);
 	}
 	LWLockRelease(AddinShmemInitLock);
 

contrib/pg_tsdtm/pg_dtm.c

@@ -177,7 +177,7 @@ static cid_t dtm_sync(cid_t global_cid)
         SpinLockAcquire(&local->lock);
     }
 #endif
-    return local_cid;
+    return global_cid;
 }
 
 void
@@ -647,10 +647,7 @@ cid_t DtmLocalAccess(DtmCurrentTrans* x, GlobalTransactionId gtid, cid_t global_
         x->is_global = true;
     }
     SpinLockRelease(&local->lock);
-	if (global_cid < local_cid - DtmVacuumDelay*USEC) { 
-		elog(ERROR, "Too old snapshot: requested %ld, current %ld", global_cid, local_cid);
-	}
-    return global_cid;
+    return local_cid;
 }
 
 void DtmLocalBeginPrepare(GlobalTransactionId gtid)