Allow adjusting session_authorization and role in parallel workers.

The code intends to allow GUCs to be set within parallel workers
via function SET clauses, but not otherwise.  However, doing so fails
for "session_authorization" and "role", because the assign hooks for
those attempt to set the subsidiary "is_superuser" GUC, and that call
falls foul of the "not otherwise" prohibition.  We can't switch to
using GUC_ACTION_SAVE for this, so instead add a new GUC variable
flag GUC_ALLOW_IN_PARALLEL to mark is_superuser as being safe to set
anyway.  (This is okay because is_superuser has context PGC_INTERNAL
and thus only hard-wired calls can change it.  We'd need more thought
before applying the flag to other GUCs; but maybe there are other
use-cases.)  This isn't the prettiest fix perhaps, but other
alternatives we thought of would be much more invasive.

While here, correct a thinko in commit 059de3ca4: when rejecting
a GUC setting within a parallel worker, we should return 0 not -1
if the ereport doesn't longjmp.  (This seems to have no consequences
right now because no caller cares, but it's inconsistent.)  Improve
the comments to try to forestall future confusion of the same kind.

Despite the lack of field complaints, this seems worth back-patching.
Thanks to Nathan Bossart for the idea to invent a new flag,
and for review.

Discussion: https://postgr.es/m/2833457.1723229039@sss.pgh.pa.us

src/backend/utils/misc/guc.c

@@ -1231,7 +1231,7 @@ static struct config_bool ConfigureNamesBool[] =
 		{"is_superuser", PGC_INTERNAL, UNGROUPED,
 			gettext_noop("Shows whether the current user is a superuser."),
 			NULL,
-			GUC_REPORT | GUC_NO_SHOW_ALL | GUC_NO_RESET_ALL | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+			GUC_REPORT | GUC_NO_SHOW_ALL | GUC_NO_RESET_ALL | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_ALLOW_IN_PARALLEL
 		},
 		&session_auth_is_superuser,
 		false,
@@ -7562,10 +7562,12 @@ parse_and_validate_value(struct config_generic *record,
  *
  * Return value:
  *	+1: the value is valid and was successfully applied.
- *	0:	the name or value is invalid (but see below).
- *	-1: the value was not applied because of context, priority, or changeVal.
+ *	0:	the name or value is invalid, or it's invalid to try to set
+ *		this GUC now; but elevel was less than ERROR (see below).
+ *	-1: no error detected, but the value was not applied, either
+ *		because changeVal is false or there is some overriding setting.
  *
- * If there is an error (non-existing option, invalid value) then an
+ * If there is an error (non-existing option, invalid value, etc) then an
  * ereport(ERROR) is thrown *unless* this is called for a source for which
  * we don't want an ERROR (currently, those are defaults, the config file,
  * and per-database or per-user settings, as well as callers who specify
@@ -7645,6 +7647,10 @@ set_config_option_ext(const char *name, const char *value,
 			elevel = ERROR;
 	}
 
+	record = find_option(name, true, false, elevel);
+	if (record == NULL)
+		return 0;
+
 	/*
 	 * GUC_ACTION_SAVE changes are acceptable during a parallel operation,
 	 * because the current worker will also pop the change.  We're probably
@@ -7652,19 +7658,19 @@ set_config_option_ext(const char *name, const char *value,
 	 * body should observe the change, and peer workers do not share in the
 	 * execution of a function call started by this worker.
 	 *
+	 * Also allow normal setting if the GUC is marked GUC_ALLOW_IN_PARALLEL.
+	 *
 	 * Other changes might need to affect other workers, so forbid them.
 	 */
-	if (IsInParallelMode() && changeVal && action != GUC_ACTION_SAVE)
+	if (IsInParallelMode() && changeVal && action != GUC_ACTION_SAVE &&
+		(record->flags & GUC_ALLOW_IN_PARALLEL) == 0)
 	{
 		ereport(elevel,
 				(errcode(ERRCODE_INVALID_TRANSACTION_STATE),
-				 errmsg("cannot set parameters during a parallel operation")));
-		return -1;
-	}
-
-	record = find_option(name, true, false, elevel);
-	if (record == NULL)
+				 errmsg("parameter \"%s\" cannot be set during a parallel operation",
+						name)));
 		return 0;
+	}
 
 	/*
 	 * Check if the option can be set at this time. See guc.h for the precise

src/include/utils/guc.h

@@ -238,6 +238,7 @@ typedef enum
  * available via 'postgres -C' if the server is not running.
  */
 #define GUC_RUNTIME_COMPUTED  0x200000
+#define GUC_ALLOW_IN_PARALLEL 0x400000	/* allow setting in parallel mode */
 
 #define GUC_UNIT				(GUC_UNIT_MEMORY | GUC_UNIT_TIME)
 

src/test/regress/expected/select_parallel.out

@@ -1219,3 +1219,36 @@ SELECT 1 FROM tenk1_vw_sec
 (9 rows)
 
 rollback;
+-- test that function option SET ROLE works in parallel workers.
+create role regress_parallel_worker;
+create function set_and_report_role() returns text as
+  $$ select current_setting('role') $$ language sql parallel safe
+  set role = regress_parallel_worker;
+create function set_role_and_error(int) returns int as
+  $$ select 1 / $1 $$ language sql parallel safe
+  set role = regress_parallel_worker;
+set force_parallel_mode = 0;
+select set_and_report_role();
+   set_and_report_role   
+-------------------------
+ regress_parallel_worker
+(1 row)
+
+select set_role_and_error(0);
+ERROR:  division by zero
+CONTEXT:  SQL function "set_role_and_error" statement 1
+set force_parallel_mode = 1;
+select set_and_report_role();
+   set_and_report_role   
+-------------------------
+ regress_parallel_worker
+(1 row)
+
+select set_role_and_error(0);
+ERROR:  division by zero
+CONTEXT:  SQL function "set_role_and_error" statement 1
+parallel worker
+reset force_parallel_mode;
+drop function set_and_report_role();
+drop function set_role_and_error(int);
+drop role regress_parallel_worker;

src/test/regress/sql/select_parallel.sql

@@ -462,3 +462,26 @@ SELECT 1 FROM tenk1_vw_sec
   WHERE (SELECT sum(f1) FROM int4_tbl WHERE f1 < unique1) < 100;
 
 rollback;
+
+-- test that function option SET ROLE works in parallel workers.
+create role regress_parallel_worker;
+
+create function set_and_report_role() returns text as
+  $$ select current_setting('role') $$ language sql parallel safe
+  set role = regress_parallel_worker;
+
+create function set_role_and_error(int) returns int as
+  $$ select 1 / $1 $$ language sql parallel safe
+  set role = regress_parallel_worker;
+
+set force_parallel_mode = 0;
+select set_and_report_role();
+select set_role_and_error(0);
+set force_parallel_mode = 1;
+select set_and_report_role();
+select set_role_and_error(0);
+reset force_parallel_mode;
+
+drop function set_and_report_role();
+drop function set_role_and_error(int);
+drop role regress_parallel_worker;