fixed restoring sane environment (via stop/start) if invariant check failed: bypass possible errors in stop (if start/check succeeded hereafter);

test cases extended to cover such situation.
Closes gh-1997

fail2ban/server/action.py

@@ -750,7 +750,10 @@ class CommandAction(ActionBase):
 					#    otherwise stop/start will theoretically remove all the bans,
 					#    but the tickets are still in BanManager, so in case of new failures
 					#    it will not be banned, because "already banned" will happen.
-					self.stop()
+					try:
+						self.stop()
+					except RuntimeError: # bypass error in stop (if start/check succeeded hereafter).
+						pass
 					self.start()
 				if not self.executeCmd(checkCmd, self.timeout):
 					self._logSys.critical("Unable to restore environment")

fail2ban/tests/actiontestcase.py

@@ -335,7 +335,16 @@ class CommandActionTest(LogCaptureTestCase):
 		self.__action.actionban = "rm /tmp/fail2ban.test"
 		self.__action.actioncheck = "[ -e /tmp/fail2ban.test ]"
 		self.assertRaises(RuntimeError, self.__action.ban, {'ip': None})
-		self.assertLogged('Unable to restore environment')
+		self.assertLogged('Invariant check failed', 'Unable to restore environment', all=True)
+		# 2nd time, try to restore with producing error in stop, but succeeded start hereafter:
+		self.pruneLog('[phase 2]')
+		self.__action.actionstart = "touch /tmp/fail2ban.test"
+		self.__action.actionstop = "rm /tmp/fail2ban.test"
+		self.__action.actionban = 'printf "%%b\n" <ip> >> /tmp/fail2ban.test'
+		self.__action.actioncheck = "[ -e /tmp/fail2ban.test ]"
+		self.__action.ban({'ip': None})
+		self.assertLogged('Invariant check failed')
+		self.assertNotLogged('Unable to restore environment')
 
 	def testExecuteActionCheckRepairEnvironment(self):
 		self.__action.actionstart = ""