fix: race condition of x.signer in v1 consensus

[wjrjerome]

Proposed changes

Fix a small bug where race condition could happen when concurrently update x.signer

Types of changes

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• Bugfix (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)
• Documentation Update (if none of the other choices apply)
• Regular KTLO or any of the maintaince work. e.g code style
• CICD Improvement

Impacted Components

Which part of the codebase this PR will touch base on,

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• Consensus
• Account
• Network
• Geth
• Smart Contract
• External components
• Not sure (Please specify below)

Checklist

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• This PR has sufficient test coverage (unit/integration test) OR I have provided reason in the PR description for not having test coverage
• Provide an end-to-end test plan in the PR description on how to manually test it on the devnet/testnet.
• Tested the backwards compatibility.
• Tested with XDC nodes running this version co-exist with those running the previous version.
• Relevant documentation has been updated as part of this PR
• N/A

[gzliudan]

How about this version ?

	if number%x.config.Epoch != 0 {
		// Gather all the proposals that make sense voting on
		addresses := make([]common.Address, 0, len(x.proposals))
		for address, authorize := range x.proposals {
			if snap.validVote(address, authorize) {
				addresses = append(addresses, address)
			}
		}
		// If there's pending proposals, cast a vote on them
		if len(addresses) > 0 {
			header.Coinbase = addresses[rand.Intn(len(addresses))]
			if x.proposals[header.Coinbase] {
				copy(header.Nonce[:], utils.NonceAuthVote)
			} else {
				copy(header.Nonce[:], utils.NonceDropVote)
			}
		}
	}

	parent := chain.GetHeader(header.ParentHash, number-1)
	if parent == nil {
		return consensus.ErrUnknownAncestor
	}

	// Set the correct difficulty
	x.lock.RLock()
	signer := x.signer
	x.lock.RUnlock()
	header.Difficulty = x.calcDifficulty(chain, parent, signer)

[wjrjerome]

The locks are already in the code before, i'm not sure what values it's locking, probably x.proposals. I prefer not to change the original logic as it would involve unnecessary riskks

[gzliudan]

You can refer the struct and other codes:

lock   sync.RWMutex    // Protects the signer fields

[wjrjerome]

I'm not sure i understand what you mean above.
What's i'm referring to is that the lock is already within the original if statement.

I do not intend to change the behaviour of the original code hence move the signer := x.signer right before the Runlock in the original code. This would be the smallest change that would be required to achieve the same goal

[gzliudan]

The lock field protects signer field.