pg_upgrade: Parallelize retrieving loadable libraries.

This commit makes use of the new task framework in pg_upgrade to
parallelize retrieving the names of all libraries referenced by
non-built-in C functions.  This step will now process multiple
databases concurrently when pg_upgrade's --jobs option is provided
a value greater than 1.

Reviewed-by: Daniel Gustafsson, Ilya Gladyshev
Discussion: https://postgr.es/m/20240516211638.GA1688936%40nathanxps13

src/bin/pg_upgrade/function.c

@@ -42,6 +42,30 @@ library_name_compare(const void *p1, const void *p2)
 					  ((const LibraryInfo *) p2)->dbnum);
 }
 
+/*
+ * Private state for get_loadable_libraries()'s UpgradeTask.
+ */
+struct loadable_libraries_state
+{
+	PGresult  **ress;			/* results for each database */
+	int			totaltups;		/* number of tuples in all results */
+};
+
+/*
+ * Callback function for processing results of query for
+ * get_loadable_libraries()'s UpgradeTask.  This function stores the results
+ * for later use within get_loadable_libraries().
+ */
+static void
+process_loadable_libraries(DbInfo *dbinfo, PGresult *res, void *arg)
+{
+	struct loadable_libraries_state *state = (struct loadable_libraries_state *) arg;
+
+	AssertVariableIsOfType(&process_loadable_libraries, UpgradeTaskProcessCB);
+
+	state->ress[dbinfo - old_cluster.dbarr.dbs] = res;
+	state->totaltups += PQntuples(res);
+}
 
 /*
  * get_loadable_libraries()
@@ -54,47 +78,41 @@ library_name_compare(const void *p1, const void *p2)
 void
 get_loadable_libraries(void)
 {
-	PGresult  **ress;
 	int			totaltups;
 	int			dbnum;
 	int			n_libinfos;
+	UpgradeTask *task = upgrade_task_create();
+	struct loadable_libraries_state state;
+	char	   *query;
 
-	ress = (PGresult **) pg_malloc(old_cluster.dbarr.ndbs * sizeof(PGresult *));
-	totaltups = 0;
+	state.ress = (PGresult **) pg_malloc(old_cluster.dbarr.ndbs * sizeof(PGresult *));
+	state.totaltups = 0;
 
-	/* Fetch all library names, removing duplicates within each DB */
-	for (dbnum = 0; dbnum < old_cluster.dbarr.ndbs; dbnum++)
-	{
-		DbInfo	   *active_db = &old_cluster.dbarr.dbs[dbnum];
-		PGconn	   *conn = connectToServer(&old_cluster, active_db->db_name);
+	query = psprintf("SELECT DISTINCT probin "
+					 "FROM pg_catalog.pg_proc "
+					 "WHERE prolang = %u AND "
+					 "probin IS NOT NULL AND "
+					 "oid >= %u",
+					 ClanguageId,
+					 FirstNormalObjectId);
 
-		/*
-		 * Fetch all libraries containing non-built-in C functions in this DB.
-		 */
-		ress[dbnum] = executeQueryOrDie(conn,
-										"SELECT DISTINCT probin "
-										"FROM pg_catalog.pg_proc "
-										"WHERE prolang = %u AND "
-										"probin IS NOT NULL AND "
-										"oid >= %u;",
-										ClanguageId,
-										FirstNormalObjectId);
-		totaltups += PQntuples(ress[dbnum]);
-
-		PQfinish(conn);
-	}
+	upgrade_task_add_step(task, query, process_loadable_libraries,
+						  false, &state);
+
+	upgrade_task_run(task, &old_cluster);
+	upgrade_task_free(task);
 
 	/*
 	 * Allocate memory for required libraries and logical replication output
 	 * plugins.
 	 */
-	n_libinfos = totaltups + count_old_cluster_logical_slots();
+	n_libinfos = state.totaltups + count_old_cluster_logical_slots();
 	os_info.libraries = (LibraryInfo *) pg_malloc(sizeof(LibraryInfo) * n_libinfos);
 	totaltups = 0;
 
 	for (dbnum = 0; dbnum < old_cluster.dbarr.ndbs; dbnum++)
 	{
-		PGresult   *res = ress[dbnum];
+		PGresult   *res = state.ress[dbnum];
 		int			ntups;
 		int			rowno;
 		LogicalSlotInfoArr *slot_arr = &old_cluster.dbarr.dbs[dbnum].slot_arr;
@@ -129,7 +147,8 @@ get_loadable_libraries(void)
 		}
 	}
 
-	pg_free(ress);
+	pg_free(state.ress);
+	pg_free(query);
 
 	os_info.num_libraries = totaltups;
 }