src/Pure/Tools/build_process.scala

       val table = make_table("workers",
         List(worker_uuid, build_uuid, hostname, java_pid, java_start, start, stamp, stop, serial))
 
       val serial_max = serial.copy(expr = "MAX(" + serial.ident + ")")
     }
+
+    def read_serial(db: SQL.Database): Long =
+      db.execute_query_statementO[Long](
+        Workers.table.select(List(Workers.serial_max)),
+        res => res.long(Workers.serial)
+      ).getOrElse(0L)
 
     def read_workers(
       db: SQL.Database,
       build_uuid: String = "",
       worker_uuid: String = ""

               stop = res.get_date(Workers.stop),
               serial = res.long(Workers.serial))
           })
     }
 
-    def serial_max(db: SQL.Database): Long =
-      db.execute_query_statementO[Long](
-        Workers.table.select(List(Workers.serial_max)),
-        res => res.long(Workers.serial)
-      ).getOrElse(0L)
-
     def start_worker(
       db: SQL.Database,
       worker_uuid: String,
       build_uuid: String,
       hostname: String,

       db: SQL.Database,
       worker_uuid: String,
       hostname: String,
       state: State
     ): State = {
-      val serial0 = serial_max(db)
-      if (serial0 == state.serial) state
+      val serial_db = read_serial(db)
+      if (serial_db == state.serial) state
       else {
-        val serial = serial0 max state.serial
+        val serial = serial_db max state.serial
         stamp_worker(db, worker_uuid, serial)
 
         val numa_next = Host.Data.read_numa_next(db, hostname)
         val sessions = pull1(read_sessions_domain(db), read_sessions(db, _), state.sessions)
         val workers = read_workers(db)