RegularChainSyncer crashes when block importing fails

So, validate_rlp_equal() is raising a TypeError even though the diff between the imported block and the original one is empty and they seem to be instances of the same class, and that is causing RegularChainSyncer to crash:

   DEBUG  09-25 17:02:51           ByzantiumVM  BLOCK REWARD: 3000000000000000000 -> b'\x1b\xc1\xeet\x1b= GT?Yq\xadu4\x074\xc1D\x13'
   DEBUG  09-25 17:02:51    RegularChainSyncer  <coroutine object RegularChainSyncer._import_ready_blocks at 0x7fd3147e72b0> finished while <trinity.sync.full.chain.           RegularChainSyncer object at 0x7fd3179ac940> is still running, terminating as well
 WARNING  09-25 17:02:51    RegularChainSyncer  Task <coroutine object RegularChainSyncer._import_ready_blocks at 0x7fd3147e7308> finished unexpectedly: block                  (<ByzantiumBlock(#Block #4111023)>) != imported block (<ByzantiumBlock(#Block #4111023)>) but got an empty diff
   DEBUG  09-25 17:02:51    RegularChainSyncer  Task failure traceback
trinity.chains.RemoteTraceback:
"""
Traceback (most recent call last):
  File "/home/salgado/src/py-evm/trinity/chains/__init__.py", line 184, in wrapper
    return attr(*args, **kwargs)
  File "/home/salgado/src/py-evm/eth/chains/base.py", line 646, in import_block
    validate_imported_block_unchanged(imported_block, block)
  File "cytoolz/functoolz.pyx", line 236, in cytoolz.functoolz.curry.__call__
  File "cytoolz/functoolz.pyx", line 232, in cytoolz.functoolz.curry.__call__
  File "/home/salgado/src/py-evm/eth/utils/rlp.py", line 65, in validate_rlp_equal
    obj_b,
TypeError: block (<ByzantiumBlock(#Block #4111023)>) != imported block (<ByzantiumBlock(#Block #4111023)>) but got an empty diff
"""

[gsalgado]

And the above happens because the original block (the one we want to import) is pickled (with its ._hash_cache) and sent to a different process, but python’s hash() builtin is not stable across processes.

This wasn’t a problem in the past because it was only recently that validate_rlp_equal() was modified to raise a TypeError when the objects don’t compare equally but the diff is empty

I can see a few alternatives to fix this:

1. Get rid of ._hash_cache from rlp.Serializable
2. Add a __getstate__() method to rlp.Serializable so we don’t pickle the ._hash_cache attribute
3. Use a stable hashing implementation (e.g. hashlib) instead of the builtin hash()

I’d be in favour of 1 as I’m not sure caching the hash of rlp.Serializables provides a significant performance improvement anywhere

[pipermerriam]

This issue can be closed once the rlp dependency has been bumped to >=1.0.3