sheetsync: Improve timing of main loop

Instead of always waiting 5 seconds between runs,
wait until 5 seconds after the previous run started.

This ensures we actually run every 5sec and not every 5sec + how long it takes to run

sheetsync/sheetsync/main.py

@@ -126,8 +126,12 @@ class SheetSync(object):
 		bustime = common.parse_bustime(value)
 		return common.bustime_to_dt(self.bustime_start, bustime)
 
-	def wait(self, interval):
+	def wait(self, base, interval):
-		self.stop.wait(common.jitter(interval))
+		"""Wait until INTERVAL seconds after BASE."""
+		now = monotonic()
+		to_wait = base + common.jitter(interval) - now
+		if to_wait > 0:
+			self.stop.wait(to_wait)
 
 	def run(self):
 		self.conn = self.dbmanager.get_conn()
@@ -150,7 +154,10 @@ class SheetSync(object):
 					worksheets = sorted(
 						self.worksheets.keys(), key=lambda k: self.worksheets[k], reverse=True,
 					)[:self.ACTIVE_SHEET_COUNT]
+
 				sync_count += 1
+				sync_start = monotonic()
+
 				for worksheet in worksheets:
 					rows = self.sheets.get_rows(self.sheet_id, worksheet)
 					for row_index, row in enumerate(rows):
@@ -173,11 +180,11 @@ class SheetSync(object):
 				# 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)
+				self.wait(sync_start, self.ERROR_RETRY_INTERVAL)
 			else:
 				logging.info("Successful sync")
 				sheets_synced.inc()
-				self.wait(self.RETRY_INTERVAL)
+				self.wait(sync_start, self.RETRY_INTERVAL)
 
 	def get_events(self):
 		"""Return the entire events table as a map {id: event namedtuple}"""