Only serve chunks in forward direction

This disconnects bad clients that are on the wrong network
for some reason, before they consume enormous bandwidth.

Closes #132

server/controller.py

@@ -81,7 +81,7 @@ class Controller(util.LoggedClass):
             ('blockchain',
              'address.get_balance address.get_history address.get_mempool '
              'address.get_proof address.listunspent '
-             'block.get_header block.get_chunk estimatefee relayfee '
+             'block.get_header estimatefee relayfee '
              'transaction.get transaction.get_merkle utxo.get_address'),
             ('server', 'donation_address'),
         ]
@@ -794,13 +794,6 @@ class Controller(util.LoggedClass):
                  'height': utxo.height, 'value': utxo.value}
                 for utxo in sorted(await self.get_utxos(hashX))]
 
-    def block_get_chunk(self, index):
-        '''Return a chunk of block headers.
-
-        index: the chunk index'''
-        index = self.non_negative_integer(index)
-        return self.get_chunk(index)
-
     def block_get_header(self, height):
         '''The deserialized header at a given height.
 

server/session.py

@@ -112,8 +112,10 @@ class ElectrumX(SessionBase):
         self.max_subs = self.env.max_session_subs
         self.hashX_subs = {}
         self.mempool_statuses = {}
+        self.chunk_indices = []
         self.electrumx_handlers = {
             'blockchain.address.subscribe': self.address_subscribe,
+            'blockchain.block.get_chunk': self.block_get_chunk,
             'blockchain.headers.subscribe': self.headers_subscribe,
             'blockchain.numblocks.subscribe': self.numblocks_subscribe,
             'blockchain.script_hash.subscribe': self.script_hash_subscribe,
@@ -261,6 +263,21 @@ class ElectrumX(SessionBase):
         '''Returns a dictionary of server features.'''
         return self.controller.peer_mgr.myself.features
 
+    def block_get_chunk(self, index):
+        '''Return a chunk of block headers as a hexadecimal string.
+
+        index: the chunk index'''
+        index = self.controller.non_negative_integer(index)
+        self.chunk_indices.append(index)
+        self.chunk_indices = self.chunk_indices[-5:]
+        # -2 allows backing up a single chunk but no more.
+        if index <= max(self.chunk_indices[:-2], default=-1):
+            msg = ('chunk indices not advancing (wrong network?): {}'
+                   .format(self.chunk_indices))
+            # INVALID_REQUEST triggers a disconnect
+            raise RPCError(mrg, JSONRPC.INVALID_REQUEST)
+        return self.controller.get_chunk(index)
+
     def is_tor(self):
         '''Try to detect if the connection is to a tor hidden service we are
         running.'''