Introduce GUC_NO_RESET flag.

Previously, the transaction-property GUCs such as transaction_isolation
could be reset after starting a transaction, because we marked them
as GUC_NO_RESET_ALL but still allowed a targeted RESET.  That leads to
assertion failures or worse, because those properties aren't supposed
to change after we've acquired a transaction snapshot.

There are some NO_RESET_ALL variables for which RESET is okay, so
we can't just redefine the semantics of that flag.  Instead introduce
a separate GUC_NO_RESET flag.  Mark "seed", as well as the transaction
property GUCs, as GUC_NO_RESET.

We have to disallow GUC_ACTION_SAVE as well as straight RESET, because
otherwise a function having a "SET transaction_isolation" clause can
still break things: the end-of-function restore action is equivalent
to a RESET.

No back-patch, as it's conceivable that someone is doing something
this patch will forbid (like resetting one of these GUCs at transaction
start, or "CREATE FUNCTION ... SET transaction_read_only = 1") and not
running into problems with it today.  Given how long we've had this
issue and not noticed, the side effects in non-assert builds can't be
too serious.

Per bug #17385 from Andrew Bille.

Masahiko Sawada

Discussion: https://postgr.es/m/[email protected]

3 files changed, 27 insertions, 8 deletions

diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 8c2e08fad6a..ff082fc3d95 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -3243,6 +3243,26 @@ set_config_option_ext(const char *name, const char *value,
 		}
 	}
 
+	/* Disallow resetting and saving GUC_NO_RESET values */
+	if (record->flags & GUC_NO_RESET)
+	{
+		if (value == NULL)
+		{
+			ereport(elevel,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("parameter \"%s\" cannot be reset", name)));
+			return 0;
+		}
+		if (action == GUC_ACTION_SAVE)
+		{
+			ereport(elevel,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("parameter \"%s\" cannot be set locally in functions",
+							name)));
+			return 0;
+		}
+	}
+
 	/*
 	 * Should we set reset/stacked values?	(If so, the behavior is not
 	 * transactional.)	This is done either when we get a default value from
diff --git a/src/backend/utils/misc/guc_funcs.c b/src/backend/utils/misc/guc_funcs.c
index 3d2df18659b..ffc71726f91 100644
--- a/src/backend/utils/misc/guc_funcs.c
+++ b/src/backend/utils/misc/guc_funcs.c
@@ -141,9 +141,6 @@ ExecSetVariableStmt(VariableSetStmt *stmt, bool isTopLevel)
 				WarnNoTransactionBlock(isTopLevel, "SET LOCAL");
 			/* fall through */
 		case VAR_RESET:
-			if (strcmp(stmt->name, "transaction_isolation") == 0)
-				WarnNoTransactionBlock(isTopLevel, "RESET TRANSACTION");
-
 			(void) set_config_option(stmt->name,
 									 NULL,
 									 (superuser() ? PGC_SUSET : PGC_USERSET),
@@ -539,7 +536,7 @@ ShowAllGUCConfig(DestReceiver *dest)
 Datum
 pg_settings_get_flags(PG_FUNCTION_ARGS)
 {
-#define MAX_GUC_FLAGS	5
+#define MAX_GUC_FLAGS	6
 	char	   *varname = TextDatumGetCString(PG_GETARG_DATUM(0));
 	struct config_generic *record;
 	int			cnt = 0;
@@ -554,6 +551,8 @@ pg_settings_get_flags(PG_FUNCTION_ARGS)
 
 	if (record->flags & GUC_EXPLAIN)
 		flags[cnt++] = CStringGetTextDatum("EXPLAIN");
+	if (record->flags & GUC_NO_RESET)
+		flags[cnt++] = CStringGetTextDatum("NO_RESET");
 	if (record->flags & GUC_NO_RESET_ALL)
 		flags[cnt++] = CStringGetTextDatum("NO_RESET_ALL");
 	if (record->flags & GUC_NO_SHOW_ALL)
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index ab3140ff3af..fda3f9befb0 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -1505,7 +1505,7 @@ struct config_bool ConfigureNamesBool[] =
 		{"transaction_read_only", PGC_USERSET, CLIENT_CONN_STATEMENT,
 			gettext_noop("Sets the current transaction's read-only status."),
 			NULL,
-			GUC_NO_RESET_ALL | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_NO_RESET | GUC_NO_RESET_ALL | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
 		},
 		&XactReadOnly,
 		false,
@@ -1524,7 +1524,7 @@ struct config_bool ConfigureNamesBool[] =
 		{"transaction_deferrable", PGC_USERSET, CLIENT_CONN_STATEMENT,
 			gettext_noop("Whether to defer a read-only serializable transaction until it can be executed with no possible serialization failures."),
 			NULL,
-			GUC_NO_RESET_ALL | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_NO_RESET | GUC_NO_RESET_ALL | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
 		},
 		&XactDeferrable,
 		false,
@@ -3606,7 +3606,7 @@ struct config_real ConfigureNamesReal[] =
 		{"seed", PGC_USERSET, UNGROUPED,
 			gettext_noop("Sets the seed for random-number generation."),
 			NULL,
-			GUC_NO_SHOW_ALL | GUC_NO_RESET_ALL | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_NO_SHOW_ALL | GUC_NO_RESET | GUC_NO_RESET_ALL | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
 		},
 		&phony_random_seed,
 		0.0, -1.0, 1.0,
@@ -4557,7 +4557,7 @@ struct config_enum ConfigureNamesEnum[] =
 		{"transaction_isolation", PGC_USERSET, CLIENT_CONN_STATEMENT,
 			gettext_noop("Sets the current transaction's isolation level."),
 			NULL,
-			GUC_NO_RESET_ALL | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_NO_RESET | GUC_NO_RESET_ALL | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
 		},
 		&XactIsoLevel,
 		XACT_READ_COMMITTED, isolation_level_options,