cutter, sheetsync: Wait for DB to connect on startup

This is a nicer error than crashing in the depths of some error handler
(which is what will happen if the DB goes unavailable while they're running),
and it's a far more common case (eg. the DB is misconfigured) than having it fail
halfway through.

Neither of these services can do anything meaningful without the DB,
so crashing without it is acceptable behaviour.

cutter/cutter/main.py

@@ -565,13 +565,20 @@ def main(
 	# We want to error if either errors, and shut down if either exits.
 	dbmanager = None
 	stopping = gevent.event.Event()
-	while dbmanager is None:
+	dbmanager = DBManager(dsn=dbconnect)
+	while True:
 		try:
-			dbmanager = DBManager(dsn=dbconnect)
+			# Get a test connection so we know the database is up,
+			# this produces a clearer error in cases where there's a connection problem.
+			conn = dbmanager.get_conn()
 		except Exception:
 			delay = common.jitter(10)
 			logging.info('Cannot connect to database. Retrying in {:.0f} s'.format(delay))
 			stop.wait(delay)
+		else:
+			# put it back so it gets reused on next get_conn()
+			dbmanager.put_conn(conn)
+			break
 
 	with open(creds_file) as f:
 		credentials = json.load(f)

sheetsync/sheetsync/main.py

@@ -130,6 +130,8 @@ class SheetSync(object):
 				sync_errors.inc()
 				# 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 we can't re-connect, the program will crash from here,
+				# then restart and wait until it can connect again.
 				self.conn = self.dbmanager.get_conn()
 				self.wait(self.ERROR_RETRY_INTERVAL)
 			else:
@@ -293,14 +295,20 @@ def main(dbconnect, sheets_creds_file, edit_url, bustime_start, sheet_id, worksh
 
 	logging.info("Starting up")
 
-	dbmanager = None
-	while dbmanager is None:
+	dbmanager = DBManager(dsn=dbconnect)
+	while True:
 		try:
-			dbmanager = DBManager(dsn=dbconnect)
+			# Get a test connection so we know the database is up,
+			# this produces a clearer error in cases where there's a connection problem.
+			conn = dbmanager.get_conn()
 		except Exception:
 			delay = common.jitter(10)
 			logging.info('Cannot connect to database. Retrying in {:.0f} s'.format(delay))
 			stop.wait(delay)
+		else:
+			# put it back so it gets reused on next get_conn()
+			dbmanager.put_conn(conn)
+			break
 
 	sheets_creds = json.load(open(sheets_creds_file))