Issue #235: Intermittent "MySQL server has gone away" (conservancy/kallithea)

[André]

New issue 235: Intermittent "MySQL server has gone away"
https://bitbucket.org/conservancy/kallithea/issues/235/intermittent-mysql-server-has-gone-away

André:

Kallithea 0.3.1

This happens from time to time.
To load on the machine (2-core) is minimal.

(repository name replaced by XXXX)
```
2016-07-25 15:02:04.280 INFO  [kallithea.RequestWrapper] IP: 192.168.55.160 Request to /XXXX/info/refs time: 0.002s
2016-07-25 15:02:04.281 ERROR [waitress] Exception when serving /repos/XXXX/info/refs
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/waitress/channel.py", line 337, in service
    task.service()
  File "/usr/local/lib/python2.7/dist-packages/waitress/task.py", line 173, in service
    self.execute()
  File "/usr/local/lib/python2.7/dist-packages/waitress/task.py", line 392, in execute
    app_iter = self.channel.server.application(env, start_response)
  File "/usr/local/lib/python2.7/dist-packages/paste/deploy/config.py", line 291, in __call__
    return self.app(environ, start_response)
  File "/usr/local/lib/python2.7/dist-packages/paste/gzipper.py", line 34, in __call__
    response.gzip_start_response)
  File "/usr/local/lib/python2.7/dist-packages/paste/cascade.py", line 130, in __call__
    return self.apps[-1](environ, start_response)
  File "/usr/local/lib/python2.7/dist-packages/paste/registry.py", line 379, in __call__
    app_iter = self.application(environ, start_response)
  File "/usr/local/lib/python2.7/dist-packages/kallithea/lib/middleware/wrapper.py", line 43, in __call__
    return self.application(environ, start_response)
  File "/usr/local/lib/python2.7/dist-packages/kallithea/lib/middleware/https_fixup.py", line 49, in __call__
    return self.application(environ, custom_start_response)
  File "/usr/local/lib/python2.7/dist-packages/kallithea/lib/base.py", line 312, in __call__
    return self._handle_request(environ, start_response)
  File "/usr/local/lib/python2.7/dist-packages/kallithea/lib/middleware/simplegit.py", line 69, in _handle_request
    if not self._check_ssl(environ):
  File "/usr/local/lib/python2.7/dist-packages/kallithea/lib/base.py", line 257, in _check_ssl
    if str2bool(Ui.get_by_key('push_ssl').ui_value):
  File "/usr/local/lib/python2.7/dist-packages/kallithea/model/db.py", line 355, in get_by_key
    return cls.query().filter(cls.ui_key == key).scalar()
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2215, in scalar
    ret = self.one()
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2184, in one
    ret = list(self)
  File "/usr/local/lib/python2.7/dist-packages/kallithea/lib/caching_query.py", line 83, in __iter__
    return Query.__iter__(self)
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2227, in __iter__
    return self._execute_and_instances(context)
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2242, in _execute_and_instances
    result = conn.execute(querycontext.statement, self._params)
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 1449, in execute
    params)
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 1584, in _execute_clauseelement
    compiled_sql, distilled_params
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 1698, in _execute_context
    context)
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 1691, in _execute_context
    context)
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/engine/default.py", line 331, in do_execute
    cursor.execute(statement, parameters)
  File "/usr/lib/pymodules/python2.7/MySQLdb/cursors.py", line 166, in execute
    self.errorhandler(self, exc, value)
  File "/usr/lib/pymodules/python2.7/MySQLdb/connections.py", line 35, in defaulterrorhandler
    raise errorclass, errorvalue
OperationalError: (OperationalError) (2006, 'MySQL server has gone away') 'SELECT ui.ui_id AS ui_ui_id, ui.ui_section AS ui_ui_section, ui.ui_key AS ui_ui_key, ui.ui_value AS ui_ui_value, ui.ui_active AS ui_ui_active \nFROM ui \nWHERE ui.ui_key = %s' ('push_ssl',)
```