block_number genesis parameter removed

[Pet3ris]

• Version: latest
• Python: 3.9
• OS: osx

What was wrong?

I noticed that the block_number genesis parameter is removed to make London work (

eth-tester/eth_tester/backends/pyevm/main.py

Line 146 in 8587730

# "block_number": GENESIS_BLOCK_NUMBER,

). We are relying on it for forking - what would be the new preferred way to override block number in eth-tester?

How can it be fixed?

Uncomment block_number in supported genesis parameters?

[fselmo]

Hey @Pet3ris. This is due to an upstream set of changes in py-evm for London that made certain values unconfigurable, block_number being one of them. There's no reason we can't make it configurable but I don't see a great work-around that doesn't involve making this change in py-evm. That said, for now, you could get all the default values from the genesis header and unpack them into a dictionary, edit the block_number to your choosing while keeping all the other values the same, and set a new header with the updated parameters as follows:

>>> backend = PyEVMBackend(vm_configuration=(
...    (0, LondonVM),
... ))

>>> default_header_fields = vars(backend.chain.header).items()
>>> custom_header_fields = {}

>>> for key, val in default_header_fields:
...    normalized_key = key[1:]  # removes starting "_" from the property
...    if normalized_key == 'block_number':
...        custom_header_fields['block_number'] = 12345  # set block number here
...    else:
...        custom_header_fields[normalized_key] = val

>>> backend.chain.header = LondonBlockHeader(**custom_header_fields)
>>> backend.chain.header.block_number
12345
>>> tester = EthereumTester(backend=backend)

This is pretty hacky but I'm hoping it will at least get you unstuck for now. I will create an issue in py-evm to allow these values to be configurable and we can un-comment it out here in eth-tester once those changes are in.

Let me know if this workaround helps you get unstuck. Good luck!

[Pet3ris]

Wow thanks @fselmo - this is super cool, thanks for suggesting the work-around and adding the issue. Will give it a go!

[Pet3ris]

Alright - I was able to change the block number with this snippet but it looks like it changes the block number of the second block, not the first block.

I'm now getting the error:

Blocks must be numbered consecutively. Block number #12207202 has parent #0

which comes from here:

https://github.com/ethereum/py-evm/blob/cb6f44c3e43f3f56ff4082b10cc63ab90e6acf19/eth/vm/base.py#L608

Is there a way around this other than monkey patching out the parent check?

[fselmo]

@Pet3ris yeah... none of this is ideal. I can try to check out the py-evm customization issue soon. Hopefully until then you can work around this a bit.

[Pet3ris]

@fselmo no worries - unfortunate the cascading effects of the block number!

I'll try and think about what would be safe to monkey patch for now.