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/17385-9ee529fb091f0ce5@postgresql.org

Author: tglsfdc

doc/src/sgml/func.sgml

@@ -24353,6 +24353,12 @@ SELECT collation for ('foo' COLLATE "de_DE");
        <command>SHOW ALL</command> commands.
       </entry>
      </row>
+     <row>
+      <entry><literal>NO_RESET</literal></entry>
+      <entry>Parameters with this flag do not support
+      <command>RESET</command> commands.
+      </entry>
+     </row>
      <row>
       <entry><literal>NO_RESET_ALL</literal></entry>
       <entry>Parameters with this flag are excluded from

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

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)

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,

src/include/utils/guc.h

@@ -207,6 +207,7 @@ typedef enum
 #define GUC_LIST_INPUT			0x0001	/* input can be list format */
 #define GUC_LIST_QUOTE			0x0002	/* double-quote list elements */
 #define GUC_NO_SHOW_ALL			0x0004	/* exclude from SHOW ALL */
+#define GUC_NO_RESET		  0x400000	/* disallow RESET and SAVE */
 #define GUC_NO_RESET_ALL		0x0008	/* exclude from RESET ALL */
 #define GUC_REPORT				0x0010	/* auto-report changes to client */
 #define GUC_NOT_IN_SAMPLE		0x0020	/* not in postgresql.conf.sample */

src/pl/plpgsql/src/expected/plpgsql_transaction.out

@@ -576,16 +576,15 @@ BEGIN
     PERFORM 1;
     RAISE INFO '%', current_setting('transaction_isolation');
     COMMIT;
-    SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
-    RESET TRANSACTION ISOLATION LEVEL;
+    SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
     PERFORM 1;
     RAISE INFO '%', current_setting('transaction_isolation');
     COMMIT;
 END;
 $$;
 INFO:  read committed
 INFO:  repeatable read
-INFO:  read committed
+INFO:  serializable
 -- error cases
 DO LANGUAGE plpgsql $$
 BEGIN

src/pl/plpgsql/src/sql/plpgsql_transaction.sql

@@ -481,8 +481,7 @@ BEGIN
     PERFORM 1;
     RAISE INFO '%', current_setting('transaction_isolation');
     COMMIT;
-    SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
-    RESET TRANSACTION ISOLATION LEVEL;
+    SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
     PERFORM 1;
     RAISE INFO '%', current_setting('transaction_isolation');
     COMMIT;

src/test/regress/expected/guc.out

@@ -839,6 +839,7 @@ SELECT pg_settings_get_flags('does_not_exist');
 
 CREATE TABLE tab_settings_flags AS SELECT name, category,
     'EXPLAIN'          = ANY(flags) AS explain,
+    'NO_RESET'         = ANY(flags) AS no_reset,
     'NO_RESET_ALL'     = ANY(flags) AS no_reset_all,
     'NO_SHOW_ALL'      = ANY(flags) AS no_show_all,
     'NOT_IN_SAMPLE'    = ANY(flags) AS not_in_sample,
@@ -906,4 +907,12 @@ SELECT name FROM tab_settings_flags
 ------
 (0 rows)
 
+-- NO_RESET implies NO_RESET_ALL.
+SELECT name FROM tab_settings_flags
+  WHERE no_reset AND NOT no_reset_all
+  ORDER BY 1;
+ name 
+------
+(0 rows)
+
 DROP TABLE tab_settings_flags;

src/test/regress/expected/transactions.out

@@ -44,6 +44,40 @@ SELECT * FROM xacttest;
  777 | 777.777
 (5 rows)
 
+-- Test that transaction characteristics cannot be reset.
+BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
+SELECT COUNT(*) FROM xacttest;
+ count 
+-------
+     5
+(1 row)
+
+RESET transaction_isolation; -- error
+ERROR:  parameter "transaction_isolation" cannot be reset
+END;
+BEGIN TRANSACTION READ ONLY;
+SELECT COUNT(*) FROM xacttest;
+ count 
+-------
+     5
+(1 row)
+
+RESET transaction_read_only; -- error
+ERROR:  parameter "transaction_read_only" cannot be reset
+END;
+BEGIN TRANSACTION DEFERRABLE;
+SELECT COUNT(*) FROM xacttest;
+ count 
+-------
+     5
+(1 row)
+
+RESET transaction_deferrable; -- error
+ERROR:  parameter "transaction_deferrable" cannot be reset
+END;
+CREATE FUNCTION errfunc() RETURNS int LANGUAGE SQL AS 'SELECT 1'
+SET transaction_read_only = on; -- error
+ERROR:  parameter "transaction_read_only" cannot be set locally in functions
 -- Read-only tests
 CREATE TABLE writetest (a int);
 CREATE TEMPORARY TABLE temptest (a int);

src/test/regress/sql/guc.sql

@@ -324,6 +324,7 @@ SELECT pg_settings_get_flags(NULL);
 SELECT pg_settings_get_flags('does_not_exist');
 CREATE TABLE tab_settings_flags AS SELECT name, category,
     'EXPLAIN'          = ANY(flags) AS explain,
+    'NO_RESET'         = ANY(flags) AS no_reset,
     'NO_RESET_ALL'     = ANY(flags) AS no_reset_all,
     'NO_SHOW_ALL'      = ANY(flags) AS no_show_all,
     'NOT_IN_SAMPLE'    = ANY(flags) AS not_in_sample,
@@ -360,4 +361,8 @@ SELECT name FROM tab_settings_flags
 SELECT name FROM tab_settings_flags
   WHERE no_show_all AND NOT not_in_sample
   ORDER BY 1;
+-- NO_RESET implies NO_RESET_ALL.
+SELECT name FROM tab_settings_flags
+  WHERE no_reset AND NOT no_reset_all
+  ORDER BY 1;
 DROP TABLE tab_settings_flags;