Fix stat file management for resource queues

pgstat_write_statsfile() failed to write the resource queue statistics
to the stat file which in turn makes the pg_stat_resqueues view empty.

Patch by Github user LJoNe with testcase added by me

src/backend/postmaster/pgstat.c

@@ -3059,6 +3059,7 @@ pgstat_write_statsfile(bool permanent)
 	PgStat_StatDBEntry *dbentry;
 	PgStat_StatTabEntry *tabentry;
 	PgStat_StatFuncEntry *funcentry;
+	PgStat_StatQueueEntry *queueentry;
 	FILE	   *fpout;
 	int32		format_id;
 	const char *tmpfile = permanent ? PGSTAT_STAT_PERMANENT_TMPFILE : pgstat_stat_tmpname;
@@ -3133,6 +3134,16 @@ pgstat_write_statsfile(bool permanent)
 		fputc('d', fpout);
 	}
 
+	/*
+	 * Walk through resource queue stats.
+	 */
+	hash_seq_init(&fstat, pgStatQueueHash);
+	while ((queueentry = (PgStat_StatQueueEntry *) hash_seq_search(&fstat)) != NULL)
+	{
+		fputc('Q', fpout);
+		fwrite(queueentry, sizeof(PgStat_StatQueueEntry), 1, fpout);
+	}
+
 	/*
 	 * No more output to be done. Close the temp file and replace the old
 	 * pgstat.stat with it.  The ferror() check replaces testing for error

src/test/regress/expected/gp_toolkit.out

@@ -595,6 +595,25 @@ select * from gp_toolkit.gp_pgdatabase_invalid;
 -----------+----------------+--------------+------------+---------------------
 (0 rows)
 
+-- Test that the statistics on resource queue usage are properly updated and
+-- reflected in the pg_stat_resqueues view
+set stats_queue_level=on;
+create resource queue q with (active_statements = 10);
+create user resqueuetest with resource queue q;
+set role resqueuetest;
+select 1;
+ ?column? 
+----------
+        1
+(1 row)
+
+select n_queries_exec from pg_stat_resqueues where queuename = 'q';
+ n_queries_exec 
+----------------
+              1
+(1 row)
+
+reset role;
 -- GP Readable Data Table
 -- Check that the tables created above are present in gp_toolkit.__gp_user_data_tables_readable
 -- view.

src/test/regress/sql/gp_toolkit.sql

@@ -340,6 +340,15 @@ select * from gp_toolkit.gp_resq_activity where resqrole = 'toolkit_user1';
 -- Should be empty unless there is failure in the segment, it's a view from gp_pgdatabase
 select * from gp_toolkit.gp_pgdatabase_invalid;
 
+-- Test that the statistics on resource queue usage are properly updated and
+-- reflected in the pg_stat_resqueues view
+set stats_queue_level=on;
+create resource queue q with (active_statements = 10);
+create user resqueuetest with resource queue q;
+set role resqueuetest;
+select 1;
+select n_queries_exec from pg_stat_resqueues where queuename = 'q';
+reset role;
 
 -- GP Readable Data Table