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EIP 999 proposed to update parity's walletLibrary contract. Doesn't altering the bytecode also mean altering the blockhash, in which case, due to merkel chaining, don't all subsequent block-hashes need to be altered as well for the chain to be valid, or am I missing something?

https://github.com/ethereum/EIPs/blob/master/EIPS/eip-999.md

1

Normally, yes, changing what code is present at an address would require changing history in some way (and thus changing block hashes).

But the implementation of EIP 999 will/would be to do this in the node software, not in the blockchain data itself. To grossly over-simplify, geth and Parity (and any other client compatible with the hard fork) would have code like:

def getCode(address):
   if address == 0x123abc: # affected library
       return fixedLibraryCode
   else:
       # look up the code in the blockchain

So the blockchain data still shows that the code is 0x0 (self-destructed), but clients ignore that and instead return the fixed contract code.

Here's real-world code in a Parity branch that implements the fork: https://github.com/paritytech/parity/pull/8406/commits/8f5b89df27704d74e397f88bf053d74d306f38ed#diff-0b65bc4c84e99a0949fb761a499d0f32R223.

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0

Blockhash changes if there is a change in the transactions in the block. Or if we are trying to change the contents of a block.

But by EIP-999 it is being proposed to assign code to an address. And if implemented it will allow that address to have a code at its location which will take effect from a block in future. So that will not require changes in any block, hence no change in blockhash, hence not altering validity of the chain.

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