Create an infrastructure for parallel computation in PostgreSQL.

This does four basic things.  First, it provides convenience routines
to coordinate the startup and shutdown of parallel workers.  Second,
it synchronizes various pieces of state (e.g. GUCs, combo CID
mappings, transaction snapshot) from the parallel group leader to the
worker processes.  Third, it prohibits various operations that would
result in unsafe changes to that state while parallelism is active.
Finally, it propagates events that would result in an ErrorResponse,
NoticeResponse, or NotifyResponse message being sent to the client
from the parallel workers back to the master, from which they can then
be sent on to the client.

Robert Haas, Amit Kapila, Noah Misch, Rushabh Lathia, Jeevan Chalke.
Suggestions and review from Andres Freund, Heikki Linnakangas, Noah
Misch, Simon Riggs, Euler Taveira, and Jim Nasby.

1 files changed, 23 insertions, 0 deletions

diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index f43aff2d2cb..8727ee3b744 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -5665,6 +5665,20 @@ set_config_option(const char *name, const char *value,
 			elevel = ERROR;
 	}
 
+	/*
+	 * GUC_ACTION_SAVE changes are acceptable during a parallel operation,
+	 * because the current worker will also pop the change.  We're probably
+	 * dealing with a function having a proconfig entry.  Only the function's
+	 * body should observe the change, and peer workers do not share in the
+	 * execution of a function call started by this worker.
+	 *
+	 * Other changes might need to affect other workers, so forbid them.
+	 */
+	if (IsInParallelMode() && changeVal && action != GUC_ACTION_SAVE)
+		ereport(elevel,
+				(errcode(ERRCODE_INVALID_TRANSACTION_STATE),
+				 errmsg("cannot set parameters during a parallel operation")));
+
 	record = find_option(name, true, elevel);
 	if (record == NULL)
 	{
@@ -6969,6 +6983,15 @@ ExecSetVariableStmt(VariableSetStmt *stmt, bool isTopLevel)
 {
 	GucAction	action = stmt->is_local ? GUC_ACTION_LOCAL : GUC_ACTION_SET;
 
+	/*
+	 * Workers synchronize these parameters at the start of the parallel
+	 * operation; then, we block SET during the operation.
+	 */
+	if (IsInParallelMode())
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_TRANSACTION_STATE),
+				 errmsg("cannot set parameters during a parallel operation")));
+
 	switch (stmt->kind)
 	{
 		case VAR_SET_VALUE: