verifier: don't try to request same chunk multiple times

electrum/network.py

@@ -751,14 +751,21 @@ class Network(PrintError):
             return False, "error: " + out
         return True, out
 
-    async def request_chunk(self, height, tip, session=None):
+    async def request_chunk(self, height, tip, session=None, can_return_early=False):
         if session is None: session = self.interface.session
         index = height // 2016
+        if can_return_early and index in self.requested_chunks:
+            return
         size = 2016
         if tip is not None:
             size = min(size, tip - index * 2016)
             size = max(size, 0)
-        res = await asyncio.wait_for(session.send_request('blockchain.block.headers', [index * 2016, size]), 20)
+        try:
+            self.requested_chunks.add(index)
+            res = await asyncio.wait_for(session.send_request('blockchain.block.headers', [index * 2016, size]), 20)
+        finally:
+            try: self.requested_chunks.remove(index)
+            except KeyError: pass
         conn = self.blockchain().connect_chunk(index, res['hex'])
         if not conn:
             return conn, 0

electrum/verifier.py

@@ -70,7 +70,7 @@ class SPV(ThreadJob):
             if header is None:
                 index = tx_height // 2016
                 if index < len(blockchain.checkpoints):
-                    await interface.group.spawn(self.network.request_chunk, tx_height, None)
+                    await interface.group.spawn(self.network.request_chunk(tx_height, None, can_return_early=True))
             elif (tx_hash not in self.requested_merkle
                     and tx_hash not in self.merkle_roots):
                 self.print_error('requested merkle', tx_hash)