Cache the database time for a longer time

Cache the database timestamp for up to 10 minutes and keep it up to date
during that time by local clock. This helps limiting the number of
database queries on the communication thread.

src/master/database.py

@@ -22,8 +22,12 @@ import logging
 import threading
 import traceback
 import time
+import datetime
 from master_config import get
 
+CACHE_TIME = 600 # We cache the database calibration for 10 minutes
+LONG_TRANSACTION_TRESHOLD = 10 # Transactions here should not take longer than 10 seconds, we warn for longer
+
 logger = logging.getLogger(name='database')
 
 class __CursorContext:
@@ -51,7 +55,7 @@ class __CursorContext:
 		if self.__depth:
 			return # Didn't exit all the contexts yet
 		duration = time.time() - self.__start_time
-		if duration > 10:
+		if duration > LONG_TRANSACTION_TRESHOLD:
 			logger.warn('The transaction took a long time (%s seconds): %s', duration, traceback.format_stack())
 		self.__start_time = None
 		if exc_type:
@@ -118,12 +122,22 @@ __time_update = 0
 __time_db = 0
 
 def now():
+	"""
+	Return the current database timestamp.
+
+	To minimise the number of accesses to the database (because it can be blocking
+	and it is on a different server), we cache the result for some time and adjust
+	it by local clock. We re-request the database timestamp from time to time, so
+	the database stays the authoritative source of time.
+	"""
 	global __time_update
 	global __time_db
 	t = time.time()
-	if __time_update + 2 < t:
+	diff = t - __time_update
+	if diff > CACHE_TIME: # More than 10 minutes since the last update, request a new one
 		__time_update = t
+		diff = 0 # We request it now, so it is in sync
 		with transaction() as t:
 			t.execute("SELECT CURRENT_TIMESTAMP AT TIME ZONE 'UTC'");
 			(__time_db,) = t.fetchone()
-	return __time_db
+	return __time_db + datetime.timedelta(seconds=diff)