Rearrange logrep worker's snapshot handling some more.

It turns out that worker.c's code path for TRUNCATE was also
careless about establishing a snapshot while executing user-defined
code, allowing the checks added by commit 84f5c2908 to fail when
a trigger is fired in that context.

We could just wrap Push/PopActiveSnapshot around the truncate call,
but it seems better to establish a policy of holding a snapshot
throughout execution of a replication step.  To help with that and
possible future requirements, replace the previous ensure_transaction
calls with pairs of begin/end_replication_step calls.

Per report from Mark Dilger.  Back-patch to v11, like the previous
changes.

Discussion: https://postgr.es/m/B4A3AF82-79ED-4F4C-A4E5-CD2622098972@enterprisedb.com

src/backend/replication/logical/worker.c

@@ -138,30 +138,41 @@ should_apply_changes_for_rel(LogicalRepRelMapEntry *rel)
 }
 
 /*
- * Make sure that we started local transaction.
+ * Begin one step (one INSERT, UPDATE, etc) of a replication transaction.
  *
- * Also switches to ApplyMessageContext as necessary.
+ * Start a transaction, if this is the first step (else we keep using the
+ * existing transaction).
+ * Also provide a global snapshot and ensure we run in ApplyMessageContext.
  */
-static bool
+static void
-ensure_transaction(void)
+begin_replication_step(void)
 {
-	if (IsTransactionState())
+	SetCurrentStatementStartTimestamp();
+
+	if (!IsTransactionState())
 	{
-		SetCurrentStatementStartTimestamp();
+		StartTransactionCommand();
-
+		maybe_reread_subscription();
-		if (CurrentMemoryContext != ApplyMessageContext)
-			MemoryContextSwitchTo(ApplyMessageContext);
-
-		return false;
 	}
 
-	SetCurrentStatementStartTimestamp();
+	PushActiveSnapshot(GetTransactionSnapshot());
-	StartTransactionCommand();
-
-	maybe_reread_subscription();
 
 	MemoryContextSwitchTo(ApplyMessageContext);
-	return true;
+}
+
+/*
+ * Finish up one step of a replication transaction.
+ * Callers of begin_replication_step() must also call this.
+ *
+ * We don't close out the transaction here, but we should increment
+ * the command counter to make the effects of this step visible.
+ */
+static void
+end_replication_step(void)
+{
+	PopActiveSnapshot();
+
+	CommandCounterIncrement();
 }
 
 
@@ -178,13 +189,6 @@ create_estate_for_relation(LogicalRepRelMapEntry *rel)
 	ResultRelInfo *resultRelInfo;
 	RangeTblEntry *rte;
 
-	/*
-	 * Input functions may need an active snapshot, as may AFTER triggers
-	 * invoked during finish_estate.  For safety, ensure an active snapshot
-	 * exists throughout all our usage of the executor.
-	 */
-	PushActiveSnapshot(GetTransactionSnapshot());
-
 	estate = CreateExecutorState();
 
 	rte = makeNode(RangeTblEntry);
@@ -222,7 +226,6 @@ finish_estate(EState *estate)
 	/* Cleanup. */
 	ExecResetTupleTable(estate->es_tupleTable, false);
 	FreeExecutorState(estate);
-	PopActiveSnapshot();
 }
 
 /*
@@ -612,7 +615,7 @@ apply_handle_insert(StringInfo s)
 	TupleTableSlot *remoteslot;
 	MemoryContext oldctx;
 
-	ensure_transaction();
+	begin_replication_step();
 
 	relid = logicalrep_read_insert(s, &newtup);
 	rel = logicalrep_rel_open(relid, RowExclusiveLock);
@@ -623,6 +626,7 @@ apply_handle_insert(StringInfo s)
 		 * transaction so it's safe to unlock it.
 		 */
 		logicalrep_rel_close(rel, RowExclusiveLock);
+		end_replication_step();
 		return;
 	}
 
@@ -650,7 +654,7 @@ apply_handle_insert(StringInfo s)
 
 	logicalrep_rel_close(rel, NoLock);
 
-	CommandCounterIncrement();
+	end_replication_step();
 }
 
 /*
@@ -708,7 +712,7 @@ apply_handle_update(StringInfo s)
 	bool		found;
 	MemoryContext oldctx;
 
-	ensure_transaction();
+	begin_replication_step();
 
 	relid = logicalrep_read_update(s, &has_oldtup, &oldtup,
 								   &newtup);
@@ -720,6 +724,7 @@ apply_handle_update(StringInfo s)
 		 * transaction so it's safe to unlock it.
 		 */
 		logicalrep_rel_close(rel, RowExclusiveLock);
+		end_replication_step();
 		return;
 	}
 
@@ -825,7 +830,7 @@ apply_handle_update(StringInfo s)
 
 	logicalrep_rel_close(rel, NoLock);
 
-	CommandCounterIncrement();
+	end_replication_step();
 }
 
 /*
@@ -847,7 +852,7 @@ apply_handle_delete(StringInfo s)
 	bool		found;
 	MemoryContext oldctx;
 
-	ensure_transaction();
+	begin_replication_step();
 
 	relid = logicalrep_read_delete(s, &oldtup);
 	rel = logicalrep_rel_open(relid, RowExclusiveLock);
@@ -858,6 +863,7 @@ apply_handle_delete(StringInfo s)
 		 * transaction so it's safe to unlock it.
 		 */
 		logicalrep_rel_close(rel, RowExclusiveLock);
+		end_replication_step();
 		return;
 	}
 
@@ -920,7 +926,7 @@ apply_handle_delete(StringInfo s)
 
 	logicalrep_rel_close(rel, NoLock);
 
-	CommandCounterIncrement();
+	end_replication_step();
 }
 
 /*
@@ -941,7 +947,7 @@ apply_handle_truncate(StringInfo s)
 	ListCell   *lc;
 	LOCKMODE	lockmode = AccessExclusiveLock;
 
-	ensure_transaction();
+	begin_replication_step();
 
 	remote_relids = logicalrep_read_truncate(s, &cascade, &restart_seqs);
 
@@ -982,7 +988,7 @@ apply_handle_truncate(StringInfo s)
 		logicalrep_rel_close(rel, NoLock);
 	}
 
-	CommandCounterIncrement();
+	end_replication_step();
 }