Slightly cleaner semaphore fix

server/block_processor.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2016, Neil Booth
+# Copyright (c) 2016-2017, Neil Booth
 #
 # All rights reserved.
 #
@@ -88,9 +88,8 @@ class Prefetcher(LoggedClass):
 
         while True:
             try:
-                with await self.semaphore:
-                    fetched = await self._prefetch_blocks(caught_up_event.is_set())
-                if not fetched:
+                # Sleep a while if there is nothing to prefetch
+                if not await self._prefetch_blocks(caught_up_event.is_set()):
                     await asyncio.sleep(5)
                 await self.refill_event.wait()
             except DaemonError as e:
@@ -106,6 +105,7 @@ class Prefetcher(LoggedClass):
         sleep for a period of time before returning.
         '''
         daemon_height = await self.daemon.height(mempool)
+        with await self.semaphore:
             while self.cache_size < self.min_cache_size:
                 # Try and catch up all blocks but limit to room in cache.
                 # Constrain fetch count to between 0 and 2500 regardless.
@@ -123,6 +123,7 @@ class Prefetcher(LoggedClass):
                     self.logger.info('new block height {:,d} hash {}'
                                      .format(first + count-1, hex_hashes[-1]))
                 blocks = await self.daemon.raw_blocks(hex_hashes)
+
                 assert count == len(blocks)
 
                 # Strip the unspendable genesis coinbase