Limit database max connections by default

Our default of unlimited database connections is not sane, because every database has a limit, and our default should just follow this. Otherwise it will lead to issues every time a small instance gets a high traffic peak.

Part of https://codeberg.org/forgejo/forgejo/issues/3381

The value of 100 is the lowest value from:

- 100 Postgres https://www.postgresql.org/docs/current/runtime-config-connection.html#GUC-MAX-CONNECTIONS
- 151 MySQL https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_max_connections
- 151 MariaDB https://mariadb.com/docs/server/ref/mdb/system-variables/max_connections/

custom/conf/app.example.ini

@@ -407,8 +407,8 @@ USER = root
 ;; Database connection max life time, default is 0 or 3s mysql (See #6804 & #7071 for reasoning)
 ;CONN_MAX_LIFETIME = 3s
 ;;
-;; Database maximum number of open connections, default is 0 meaning no maximum
-;MAX_OPEN_CONNS = 0
+;; Database maximum number of open connections, default is 100 which is the lowest default from Postgres (MariaDB + MySQL default to 151). Ensure you only increase the value if you configured your database server accordingly.
+;MAX_OPEN_CONNS = 100
 ;;
 ;; Whether execute database models migrations automatically
 ;AUTO_MIGRATION = true

modules/setting/database.go

@@ -83,7 +83,7 @@ func loadDBSetting(rootCfg ConfigProvider) {
 		Database.ConnMaxLifetime = sec.Key("CONN_MAX_LIFETIME").MustDuration(0)
 	}
 	Database.ConnMaxIdleTime = sec.Key("CONN_MAX_IDLETIME").MustDuration(0)
-	Database.MaxOpenConns = sec.Key("MAX_OPEN_CONNS").MustInt(0)
+	Database.MaxOpenConns = sec.Key("MAX_OPEN_CONNS").MustInt(100)
 
 	Database.IterateBufferSize = sec.Key("ITERATE_BUFFER_SIZE").MustInt(50)
 	Database.LogSQL = sec.Key("LOG_SQL").MustBool(false)