Fix sqlalchemylogger for use with gunicorn

c2cwsgiutils/sqlalchemylogger/README.md

@@ -63,3 +63,33 @@ if __name__ == '__main__':
     # logs after a timeout
     time.sleep(2)
 ```
+
+## Use with forking server (ex: `gunicorn`)
+
+Snippet for configuration with `gunicorn` and `paste`:
+
+`production.ini`:
+
+```
+[handler_sqlalchemylogger]
+class = c2cwsgiutils.sqlalchemylogger.handlers.SQLAlchemyHandler
+args = ({'url':'postgresql://','tablename':'logs','tableargs': {'schema':'logs'}, 'delay_startup': True},'healthcheck')
+level = NOTSET
+formatter = generic
+propagate = 0
+```
+
+`gunicorn.conf.py`:
+
+```python
+def post_fork(server, worker):
+    import logging
+    logger = logging.getHandlerByName('sqlalchemylogger')
+    logger.start()
+```
+
+The gunicorn must be started with this config file:
+
+```bash
+gunicorn -c gunicorn.conf.py [...]
+```

c2cwsgiutils/sqlalchemylogger/handlers.py

@@ -36,6 +36,29 @@ def __init__(
     ) -> None:
         """Initialize the SQLAlchemyHandler."""
         super().__init__()
+
+        self.started = False
+        if sqlalchemy_url.get("delay_startup", False):
+            self.sqlalchemy_url = sqlalchemy_url
+            self.does_not_contain_expression = does_not_contain_expression
+            self.contains_expression = contains_expression
+        else:
+            self._start(sqlalchemy_url, does_not_contain_expression, contains_expression)
+
+    def start(self) -> None:
+        """Force startup of logger in case of delayed startup."""
+        self._start(self.sqlalchemy_url, self.does_not_contain_expression, self.contains_expression)
+
+    def _start(
+        self,
+        sqlalchemy_url: dict[str, str],
+        does_not_contain_expression: str,
+        contains_expression: str,
+    ) -> None:
+        if self.started:
+            # only start once
+            return
+
         # Initialize DB session
         self.engine = create_engine(sqlalchemy_url["url"])
         self.Log = create_log_class(  # pylint: disable=invalid-name
@@ -55,6 +78,7 @@ def __init__(
             self.addFilter(DoesNotContainExpression(does_not_contain_expression))
         if contains_expression:
             self.addFilter(ContainsExpression(contains_expression))
+        self.started = True
 
     def _processor(self) -> None:
         _LOG.debug("%s: starting processor thread", __name__)