Extension into branch

[miha-stopar]

Description

Light client mainnet test with block 18363441 revealed some issues:

• getDriftedPosition in branch.go issue
• Calling getProof/getStorageProof in modified_extension_node.go when the short extension node is a branch and there is no trie node at this address/key
• Wrongly prepared storage leaf

Furthermore, the test revealed a case which was not envisioned before - when proving that the leaf doesn't exist one might get a "wrong" extension node (so there are three scenarios for does-not-exist proofs: nil object in a branch, wrong leaf, wrong extension node), so WrongGadget needs to be extended to cover this case.

Issue Link

#1738

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Test

TestExtensionIntoBranch, TestAccountWrongExtensionNode in gen_witness_from_local_blockchain_test.go.
TestStorageWrongExtensionNode, TestStorageWrongExtensionNode1 in gen_witness_from_infura_blockchain_test.go.

[KimiWu123]

A question about the keyLen.
In this if condition, leafKeyRow is not 248, let's say it's 249, then the length of the payload should be leafKeyRow[1]*256 + leafKeyRow[2]. I don't get why the keyLen here is int(leafKeyRow[1] - 128). Besides, leafKeyRow[2] in below (L160) should be also part of the length.

I also found that the keyLen in the else case in L178 is using leafKeyRow[2] instead of leafKeyRow[1]. I guess the implementation in if and else should be swithced?

---

I made this fix in my PR bcs the drifted position is incorrect in my case.

[miha-stopar]

The reason this might seem confusing is because for the account and storage leaves I think the length is never big enough for the first byte to be 249, it's at most 248.

It would be more clear if the first condition would say leafKeyRow[0] < 248 instead of leafKeyRow[0] != 248 (and the second block would need to say leafKeyRow[0] == 248).

The first if block handles the cases when leafKeyRow[0] < 248, the second block handles the cases for leafKeyRow[0] == 248. It works ok for account and storage leaves, but not for the transaction leaves which can be very long and can have the first byte 249. Another block is needed for these cases.

[KimiWu123]

First round (witness generation) pass, only left some minor suggestions.

[KimiWu123]

LGTM!