backfiller: Don't crash on DB errors

We move all connection handling into get_nodes().
This means that problems connecting won't cause further errors
and cause the application to completely crash.

In turn, this means that the behaviour if the database goes down becomes
"continue backfilling from the nodes we know about" instead of crashing.

backfiller/backfiller/main.py

@@ -189,9 +189,8 @@ class BackfillerManager(object):
 		self.start = start
 		self.run_once = run_once
 		self.node_file = node_file
-		self.node_database = node_database
-		if self.node_database is not None:
-			self.db_manager = database.DBManager(dsn=self.node_database)
+		self.db_manager = None if node_database is None else database.DBManager(dsn=node_database)
+		self.connection = None
 		self.localhost = localhost
 		self.download_concurrency = download_concurrency
 		self.recent_cutoff = recent_cutoff
@@ -228,8 +227,6 @@ class BackfillerManager(object):
 		get_nodes are stopped. If self.run_once, only call nodes once. Calling
 		stop will exit the loop."""
 		self.logger.info('Starting')
-		if self.node_database is not None:
-			self.connection = self.db_manager.get_conn()
 		failures = 0
 
 		while not self.stopping.is_set():
@@ -238,17 +235,12 @@ class BackfillerManager(object):
 			except Exception:
 				# To ensure a fresh slate and clear any DB-related errors, get a new conn on error.
 				# This is heavy-handed but simple and effective.
-				if self.node_database is not None:
-					self.connection = self.db_manager.get_conn()
+				self.connection = None
 				if failures < MAX_BACKOFF:
 					failures += 1
 				delay = common.jitter(TIMEOUT * 2**failures)
 				self.logger.exception('Getting nodes failed. Retrying in {:.0f} s'.format(delay))
-				try:
-					host = [s.split('=')[-1] for s in self.connection.dsn.split() if 'host' in s][0]
-				except Exception:
-					host = ''
-				node_list_errors.labels(filename=self.node_file, database=host).inc()
+				node_list_errors.labels(filename=self.node_file).inc()
 				self.stopping.wait(delay)
 				continue
 			exisiting_nodes = set(self.workers.keys())
@@ -299,7 +291,9 @@ class BackfillerManager(object):
 					else:
 						nodes[substrs[0]] = substrs[1]
 
-		if self.node_database is not None:
+		if self.db_manager is not None:
+			if self.connection is None:
+				self.connection = self.db_manager.get_conn()
 			host = [s.split('=')[-1] for s in self.connection.dsn.split() if 'host' in s][0]
 			self.logger.info('Fetching list of nodes from {}'.format(host))
 			results = database.query(self.connection, """