Cherry-pick buildbot de928a17f52dd91e17fab66d1cd5a60eed6aa360:

third_party/buildbot_8_4p1/buildbot/db/pool.py

 # This file is part of Buildbot.  Buildbot is free software: you can
 # redistribute it and/or modify it under the terms of the GNU General Public
 # License as published by the Free Software Foundation, version 2.
 #
 # This program is distributed in the hope that it will be useful, but WITHOUT
 # ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
 # FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more
 # details.
 #
 # You should have received a copy of the GNU General Public License along with
@@ skipping 32 matching lines @@
             pool_size = engine.optimal_thread_pool_size
 
         threadpool.ThreadPool.__init__(self,
                         minthreads=1,
                         maxthreads=pool_size,
                         name='DBThreadPool')
         self.engine = engine
         if engine.dialect.name == 'sqlite':
             vers = self.get_sqlite_version()
             log.msg("Using SQLite Version %s" % (vers,))
+            if vers < (3,7):
+                log.msg("NOTE: this old version of SQLite does not support "
+                        "WAL journal mode; a busy master may encounter "
+                        "'Database is locked' errors.  Consider upgrading.")
             if vers < (3,3,17):
                 log.msg("NOTE: this old version of SQLite does not support "
                         "multiple simultaneous accesses to the database; "
                         "add the 'pool_size=1' argument to your db url")
             brkn = self.__broken_sqlite = self.detect_bug1810()
             if brkn:
                 log.msg("Applying SQLite workaround from Buildbot bug #1810")
         self._start_evt = reactor.callWhenRunning(self._start)
 
     def _start(self):
@@ skipping 135 matching lines @@
         except:
             return (0,)
 
         if vers_row:
             try:
                 return tuple(map(int, vers_row[0].split('.')))
             except (TypeError, ValueError):
                 return (0,)
         else:
             return (0,)