Check for trigger context before accessing data

The ts_hypertable_insert_blocker function was accessing data from the
trigger context before it was tested that a trigger context actually
exists. This led to a crash when the function was called directly.

Fixes: #6819

src/hypertable.c

@@ -1318,12 +1318,12 @@ TS_FUNCTION_INFO_V1(ts_hypertable_insert_blocker);
 Datum
 ts_hypertable_insert_blocker(PG_FUNCTION_ARGS)
 {
-	TriggerData *trigdata = (TriggerData *) fcinfo->context;
-	const char *relname = get_rel_name(trigdata->tg_relation->rd_id);
-
 	if (!CALLED_AS_TRIGGER(fcinfo))
 		elog(ERROR, "insert_blocker: not called by trigger manager");
 
+	TriggerData *trigdata = (TriggerData *) fcinfo->context;
+	const char *relname = get_rel_name(trigdata->tg_relation->rd_id);
+
 	if (ts_guc_restoring)
 		ereport(ERROR,
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),

test/expected/triggers.out

@@ -443,3 +443,6 @@ ERROR:  trigger with transition tables not supported on hypertables
 CREATE TRIGGER t4 AFTER DELETE ON transition_test REFERENCING OLD TABLE AS old_trans FOR EACH ROW EXECUTE FUNCTION test_trigger();
 ERROR:  trigger with transition tables not supported on hypertables
 \set ON_ERROR_STOP 1
+-- Test insert blocker trigger does not crash when called directly
+SELECT _timescaledb_functions.insert_blocker();
+ERROR:  insert_blocker: not called by trigger manager

test/sql/triggers.sql

@@ -324,4 +324,6 @@ CREATE TRIGGER t3 AFTER UPDATE ON transition_test REFERENCING NEW TABLE AS new_t
 CREATE TRIGGER t4 AFTER DELETE ON transition_test REFERENCING OLD TABLE AS old_trans FOR EACH ROW EXECUTE FUNCTION test_trigger();
 \set ON_ERROR_STOP 1
 
+-- Test insert blocker trigger does not crash when called directly
+SELECT _timescaledb_functions.insert_blocker();