Prohibit parallel query when the isolation level is serializable.

In order for this to be safe, the code which hands true serializability
will need to taught that the SIRead locks taken by a parallel worker
pertain to the same transaction as those taken by the parallel leader.
Some further changes may be needed as well.  Until the necessary
adaptations are made, don't generate parallel plans in serializable
mode, and if a previously-generated parallel plan is used after
serializable mode has been activated, run it serially.

This fixes a bug in commit 7aea8e4f.

src/backend/access/transam/parallel.c

@@ -135,6 +135,14 @@ CreateParallelContext(parallel_worker_main_type entrypoint, int nworkers)
 	if (dynamic_shared_memory_type == DSM_IMPL_NONE)
 		nworkers = 0;
 
+	/*
+	 * If we are running under serializable isolation, we can't use
+	 * parallel workers, at least not until somebody enhances that mechanism
+	 * to be parallel-aware.
+	 */
+	if (IsolationIsSerializable())
+		nworkers = 0;
+
 	/* We might be running in a short-lived memory context. */
 	oldcontext = MemoryContextSwitchTo(TopTransactionContext);
 

src/backend/optimizer/plan/planner.c

@@ -20,6 +20,7 @@
 
 #include "access/htup_details.h"
 #include "access/parallel.h"
+#include "access/xact.h"
 #include "executor/executor.h"
 #include "executor/nodeAgg.h"
 #include "foreign/fdwapi.h"
@@ -210,11 +211,20 @@ standard_planner(Query *parse, int cursorOptions, ParamListInfo boundParams)
 	 * a parallel worker.  We might eventually be able to relax this
 	 * restriction, but for now it seems best not to have parallel workers
 	 * trying to create their own parallel workers.
+	 *
+	 * We can't use parallelism in serializable mode because the predicate
+	 * locking code is not parallel-aware.  It's not catastrophic if someone
+	 * tries to run a parallel plan in serializable mode; it just won't get
+	 * any workers and will run serially.  But it seems like a good heuristic
+	 * to assume that the same serialization level will be in effect at plan
+	 * time and execution time, so don't generate a parallel plan if we're
+	 * in serializable mode.
 	 */
 	glob->parallelModeOK = (cursorOptions & CURSOR_OPT_PARALLEL_OK) != 0 &&
 		IsUnderPostmaster && dynamic_shared_memory_type != DSM_IMPL_NONE &&
 		parse->commandType == CMD_SELECT && !parse->hasModifyingCTE &&
 		parse->utilityStmt == NULL && !IsParallelWorker() &&
+		!IsolationIsSerializable() &&
 		!contain_parallel_unsafe((Node *) parse);
 
 	/*