interface: workaround electrs erroring on 'blockchain.estimatefee'

15.00 | E | i/interface.[electrum.blockstream.info:50002] | Exception in run: ProtocolError(-32600, 'ill-formed response error object: cannot estimate fee for 25 blocks')
Traceback (most recent call last):
  File "...\electrum\electrum\util.py", line 1261, in wrapper
    return await func(*args, **kwargs)
  File "...\electrum\electrum\interface.py", line 516, in wrapper_func
    return await func(self, *args, **kwargs)
  File "...\electrum\electrum\interface.py", line 539, in run
    await self.open_session(ssl_context)
  File "...\electrum\electrum\interface.py", line 689, in open_session
    async with self.taskgroup as group:
  File "...\aiorpcX\aiorpcx\curio.py", line 304, in __aexit__
    await self.join()
  File "...\electrum\electrum\util.py", line 1423, in join
    task.result()
  File "...\electrum\electrum\interface.py", line 726, in request_fee_estimates
    async with OldTaskGroup() as group:
  File "...\aiorpcX\aiorpcx\curio.py", line 304, in __aexit__
    await self.join()
  File "...\electrum\electrum\util.py", line 1423, in join
    task.result()
  File "...\electrum\electrum\interface.py", line 1128, in get_estimatefee
    res = await self.session.send_request('blockchain.estimatefee', [num_blocks])
  File "...\electrum\electrum\interface.py", line 171, in send_request
    response = await asyncio.wait_for(
  File "...\Python310\lib\asyncio\tasks.py", line 408, in wait_for
    return await fut
  File "...\aiorpcX\aiorpcx\session.py", line 540, in send_request
    return await self._send_concurrent(message, future, 1)
  File "...\aiorpcX\aiorpcx\session.py", line 512, in _send_concurrent
    return await future
  File "...\aiorpcX\aiorpcx\jsonrpc.py", line 721, in receive_message
    item, request_id = self._protocol.message_to_item(message)
  File "...\aiorpcX\aiorpcx\jsonrpc.py", line 273, in message_to_item
    return cls._process_response(payload)
  File "...\aiorpcX\aiorpcx\jsonrpc.py", line 220, in _process_response
    raise cls._error(code, message, False, request_id)
aiorpcx.jsonrpc.ProtocolError: (-32600, 'ill-formed response error object: cannot estimate fee for 25 blocks')

electrum/interface.py

@@ -1121,11 +1121,21 @@ class Interface(Logger):
     async def get_estimatefee(self, num_blocks: int) -> int:
         """Returns a feerate estimate for getting confirmed within
         num_blocks blocks, in sat/kbyte.
+        Returns -1 if the server could not provide an estimate.
         """
         if not is_non_negative_integer(num_blocks):
             raise Exception(f"{repr(num_blocks)} is not a num_blocks")
         # do request
+        try:
             res = await self.session.send_request('blockchain.estimatefee', [num_blocks])
+        except aiorpcx.jsonrpc.ProtocolError as e:
+            # The protocol spec says the server itself should already have returned -1
+            # if it cannot provide an estimate, however apparently electrs does not conform
+            # and sends an error instead. Convert it here:
+            if "cannot estimate fee" in e.message:
+                res = -1
+            else:
+                raise
         # check response
         if res != -1:
             assert_non_negative_int_or_float(res)