[systemd-backend] implicit closing journal descriptor by stop filter.

Partially cherry-picked from 0.10 (d153555a07)

ChangeLog

@@ -15,6 +15,8 @@ releases.
 
 
 ### Fixes
+* Fix for systemd-backend: fail2ban hits the ulimit (out of file descriptors), see gh-991.
+  Partially back-ported from v.0.10.
 
 ### New Features
 

fail2ban/server/filtersystemd.py

@@ -291,6 +291,13 @@ class FilterSystemd(JournalFilter): # pragma: systemd no cover
 				except FailManagerEmpty:
 					self.failManager.cleanup(MyTime.time())
 
+		# close journal:
+		try:
+			if self.__journal:
+				self.__journal.close()
+		except Exception as e: # pragma: no cover
+			logSys.error("Close journal failed: %r", e,
+				exc_info=logSys.getEffectiveLevel()<=logging.DEBUG)
 		logSys.debug((self.jail is not None and self.jail.name
                       or "jailless") +" filter terminated")
 		return True