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

diff --git a/src/backend/replication/logical/worker.c b/src/backend/replication/logical/worker.c
index f8c52b4ccc1b47456b1d8b91f0c3350e1abb142a..e00cc423494e03e09cce7ff634d783fbabb42f60 100644 (file)
--- a/src/backend/replication/logical/worker.c
+++ b/src/backend/replication/logical/worker.c
@@ -154,30 +154,41 @@ 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
-ensure_transaction(void)
+static void
+begin_replication_step(void)
 {
-       if (IsTransactionState())
-       {
-               SetCurrentStatementStartTimestamp();
-
-               if (CurrentMemoryContext != ApplyMessageContext)
-                       MemoryContextSwitchTo(ApplyMessageContext);
+       SetCurrentStatementStartTimestamp();
 
-               return false;
+       if (!IsTransactionState())
+       {
+               StartTransactionCommand();
+               maybe_reread_subscription();
        }
 
-       SetCurrentStatementStartTimestamp();
-       StartTransactionCommand();
-
-       maybe_reread_subscription();
+       PushActiveSnapshot(GetTransactionSnapshot());
 
        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();
 }
 
 
@@ -194,13 +205,6 @@ 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);
@@ -241,7 +245,6 @@ finish_estate(EState *estate)
        /* Cleanup. */
        ExecResetTupleTable(estate->es_tupleTable, false);
        FreeExecutorState(estate);
-       PopActiveSnapshot();
 }
 
 /*
@@ -631,7 +634,7 @@ 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);
@@ -642,6 +645,7 @@ apply_handle_insert(StringInfo s)
                 * transaction so it's safe to unlock it.
                 */
                logicalrep_rel_close(rel, RowExclusiveLock);
+               end_replication_step();
                return;
        }
 
@@ -668,7 +672,7 @@ apply_handle_insert(StringInfo s)
 
        logicalrep_rel_close(rel, NoLock);
 
-       CommandCounterIncrement();
+       end_replication_step();
 }
 
 /*
@@ -727,7 +731,7 @@ apply_handle_update(StringInfo s)
        bool            found;
        MemoryContext oldctx;
 
-       ensure_transaction();
+       begin_replication_step();
 
        relid = logicalrep_read_update(s, &has_oldtup, &oldtup,
                                                                   &newtup);
@@ -739,6 +743,7 @@ apply_handle_update(StringInfo s)
                 * transaction so it's safe to unlock it.
                 */
                logicalrep_rel_close(rel, RowExclusiveLock);
+               end_replication_step();
                return;
        }
 
@@ -840,7 +845,7 @@ apply_handle_update(StringInfo s)
 
        logicalrep_rel_close(rel, NoLock);
 
-       CommandCounterIncrement();
+       end_replication_step();
 }
 
 /*
@@ -862,7 +867,7 @@ 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);
@@ -873,6 +878,7 @@ apply_handle_delete(StringInfo s)
                 * transaction so it's safe to unlock it.
                 */
                logicalrep_rel_close(rel, RowExclusiveLock);
+               end_replication_step();
                return;
        }
 
@@ -934,7 +940,7 @@ apply_handle_delete(StringInfo s)
 
        logicalrep_rel_close(rel, NoLock);
 
-       CommandCounterIncrement();
+       end_replication_step();
 }
 
 /*
@@ -955,7 +961,7 @@ apply_handle_truncate(StringInfo s)
        ListCell   *lc;
        LOCKMODE        lockmode = AccessExclusiveLock;
 
-       ensure_transaction();
+       begin_replication_step();
 
        remote_relids = logicalrep_read_truncate(s, &cascade, &restart_seqs);
 
@@ -996,7 +1002,7 @@ apply_handle_truncate(StringInfo s)
                logicalrep_rel_close(rel, NoLock);
        }
 
-       CommandCounterIncrement();
+       end_replication_step();
 }