pg_upgrade: Parallelize retrieving extension updates.

This commit makes use of the new task framework in pg_upgrade to
parallelize retrieving the set of extensions that should be updated
with the ALTER EXTENSION command after upgrade.  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

Author: nathan-bossart

src/bin/pg_upgrade/version.c

@@ -139,72 +139,76 @@ old_9_6_invalidate_hash_indexes(ClusterInfo *cluster, bool check_mode)
 		check_ok();
 }
 
+/*
+ * Callback function for processing results of query for
+ * report_extension_updates()'s UpgradeTask.  If the query returned any rows,
+ * write the details to the report file.
+ */
+static void
+process_extension_updates(DbInfo *dbinfo, PGresult *res, void *arg)
+{
+	bool		db_used = false;
+	int			ntups = PQntuples(res);
+	int			i_name = PQfnumber(res, "name");
+	UpgradeTaskReport *report = (UpgradeTaskReport *) arg;
+
+	AssertVariableIsOfType(&process_extension_updates, UpgradeTaskProcessCB);
+
+	for (int rowno = 0; rowno < ntups; rowno++)
+	{
+		if (report->file == NULL &&
+			(report->file = fopen_priv(report->path, "w")) == NULL)
+			pg_fatal("could not open file \"%s\": %m", report->path);
+		if (!db_used)
+		{
+			PQExpBufferData connectbuf;
+
+			initPQExpBuffer(&connectbuf);
+			appendPsqlMetaConnect(&connectbuf, dbinfo->db_name);
+			fputs(connectbuf.data, report->file);
+			termPQExpBuffer(&connectbuf);
+			db_used = true;
+		}
+		fprintf(report->file, "ALTER EXTENSION %s UPDATE;\n",
+				quote_identifier(PQgetvalue(res, rowno, i_name)));
+	}
+}
+
 /*
  * report_extension_updates()
  *	Report extensions that should be updated.
  */
 void
 report_extension_updates(ClusterInfo *cluster)
 {
-	int			dbnum;
-	FILE	   *script = NULL;
-	char	   *output_path = "update_extensions.sql";
+	UpgradeTaskReport report;
+	UpgradeTask *task = upgrade_task_create();
+	const char *query = "SELECT name "
+		"FROM pg_available_extensions "
+		"WHERE installed_version != default_version";
 
 	prep_status("Checking for extension updates");
 
-	for (dbnum = 0; dbnum < cluster->dbarr.ndbs; dbnum++)
-	{
-		PGresult   *res;
-		bool		db_used = false;
-		int			ntups;
-		int			rowno;
-		int			i_name;
-		DbInfo	   *active_db = &cluster->dbarr.dbs[dbnum];
-		PGconn	   *conn = connectToServer(cluster, active_db->db_name);
-
-		/* find extensions needing updates */
-		res = executeQueryOrDie(conn,
-								"SELECT name "
-								"FROM pg_available_extensions "
-								"WHERE installed_version != default_version"
-			);
+	report.file = NULL;
+	strcpy(report.path, "update_extensions.sql");
 
-		ntups = PQntuples(res);
-		i_name = PQfnumber(res, "name");
-		for (rowno = 0; rowno < ntups; rowno++)
-		{
-			if (script == NULL && (script = fopen_priv(output_path, "w")) == NULL)
-				pg_fatal("could not open file \"%s\": %m", output_path);
-			if (!db_used)
-			{
-				PQExpBufferData connectbuf;
+	upgrade_task_add_step(task, query, process_extension_updates,
+						  true, &report);
 
-				initPQExpBuffer(&connectbuf);
-				appendPsqlMetaConnect(&connectbuf, active_db->db_name);
-				fputs(connectbuf.data, script);
-				termPQExpBuffer(&connectbuf);
-				db_used = true;
-			}
-			fprintf(script, "ALTER EXTENSION %s UPDATE;\n",
-					quote_identifier(PQgetvalue(res, rowno, i_name)));
-		}
+	upgrade_task_run(task, cluster);
+	upgrade_task_free(task);
 
-		PQclear(res);
-
-		PQfinish(conn);
-	}
-
-	if (script)
+	if (report.file)
 	{
-		fclose(script);
+		fclose(report.file);
 		report_status(PG_REPORT, "notice");
 		pg_log(PG_REPORT, "\n"
 			   "Your installation contains extensions that should be updated\n"
 			   "with the ALTER EXTENSION command.  The file\n"
 			   "    %s\n"
 			   "when executed by psql by the database superuser will update\n"
 			   "these extensions.",
-			   output_path);
+			   report.path);
 	}
 	else
 		check_ok();