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efa6ba37bb
This change makes the `total_difficulty` field in `ExecutionBlock` an `Option<Uint256>` since newer clients are no longer including the `totalDifficulty` field.
I think this will fix https://github.com/sigp/lighthouse/issues/6937 but I was actually more focused on the builder registration case described below.
In our [builder-playground](https://github.com/flashbots/builder-playground) we setup a local devnet using lighthouse, reth, and mev-boost-relay. After upgrading to reth 1.2.0 and lighthouse v7.0.0.beta.0 for Pectra, we noticed that the validator registration process was _sometimes_ failing with:
```
Feb 25 23:35:25.038 ERRO Unable to publish proposer preparation to all beacon nodes, error: Some endpoints failed, num_failed: 1 http://localhost:3500/ => RequestFailed(ServerMessage(ErrorMessage { code: 400, message: "BAD_REQUEST: error updating proposer preparations: ForkchoiceUpdate(EngineError(Api { error: Json(Error(\"missing field `totalDifficulty`\", line: 0, column: 0)) }))", stacktraces: [] })), service: preparation
Feb 25 23:35:25.099 WARN Unable to publish validator registrations to the builder network, error: Some endpoints failed, num_failed: 1 http://localhost:3500/ => RequestFailed(ServerMessage(ErrorMessage { code: 400, message: "BAD_REQUEST: error updating proposer preparations: ForkchoiceUpdate(EngineError(Api { error: Json(Error(\"missing field `totalDifficulty`\", line: 0, column: 0)) }))", stacktraces: [] })), service: preparation
```
What was even more confusing, was that it was sometimes working, which actually led to a wild goose chase thinking it was a networking issue. However, when tracing through the LH code, I came across this comment:
70194dfc6a/beacon_node/beacon_chain/src/beacon_chain.rs (L6048-L6049)
This explained why it sometimes worked, in our playground we run lighthouse with `--prepare-payload-lookahead 8000` thus there was always a 4-second window where the call wasn't made.
But, if the call was made, then this code would 100% fail with updated reth:
https://github.com/sigp/lighthouse/blob/unstable/beacon_node/execution_layer/src/lib.rs#L1688-L1692
Which would then mapped to a `Error::ForkchoiceUpdate` in `update_execution_engine_forkchoice`.
Anyways, the fix was to make `total_difficulty` Optional, and then to update any code paths where it was used. In doing so, I assume that if the EL doesn't include total difficulty then the chain is already post-merge.