remove block fork guessing/inference from REST JSON decoding

[tersec]

Better, more sustainable alternative to #6551

Not for September release cycle; would be for October cycle.

[github-actions]

Unit Test Results

         9 files  ±0    1 355 suites  ±0   52m 40s ⏱️ - 2m 40s
  5 146 tests  - 1    4 798 ✔️  - 1  348 💤 ±0  0 ❌ ±0 
21 507 runs   - 3  21 103 ✔️  - 3  404 💤 ±0  0 ❌ ±0 

Results for commit 6e60261. ± Comparison against base commit 6ab3568.

♻️ This comment has been updated with latest results.

[etan-status]

getBlck(consensusFork.SignedBeaconBlock) avoids the switch as well and makes this maintenance free

[etan-status]

(withConsensusFork(consensusFork) to convert consensusFork from let to const)

[tersec]

Well, using the usual-fake-Nim-feature which has to be defined in forks and which Nim seems not to care much about supporting.

But sure, as long as that's maintained, then it can be used here.

[tersec]

nimbus-eth2/beacon_chain/spec/forks.nim

Lines 402 to 522 in bf4abf8

	template BeaconState*(kind: static ConsensusFork): auto =
	when kind == ConsensusFork.Electra:
	typedesc[electra.BeaconState]
	elif kind == ConsensusFork.Deneb:
	typedesc[deneb.BeaconState]
	elif kind == ConsensusFork.Capella:
	typedesc[capella.BeaconState]
	elif kind == ConsensusFork.Bellatrix:
	typedesc[bellatrix.BeaconState]
	elif kind == ConsensusFork.Altair:
	typedesc[altair.BeaconState]
	elif kind == ConsensusFork.Phase0:
	typedesc[phase0.BeaconState]
	else:
	static: raiseAssert "Unreachable"
	template BeaconBlock*(kind: static ConsensusFork): auto =
	when kind == ConsensusFork.Electra:
	typedesc[electra.BeaconBlock]
	elif kind == ConsensusFork.Deneb:
	typedesc[deneb.BeaconBlock]
	elif kind == ConsensusFork.Capella:
	typedesc[capella.BeaconBlock]
	elif kind == ConsensusFork.Bellatrix:
	typedesc[bellatrix.BeaconBlock]
	elif kind == ConsensusFork.Altair:
	typedesc[altair.BeaconBlock]
	elif kind == ConsensusFork.Phase0:
	typedesc[phase0.BeaconBlock]
	else:
	static: raiseAssert "Unreachable"
	template BeaconBlockBody*(kind: static ConsensusFork): auto =
	when kind == ConsensusFork.Electra:
	typedesc[electra.BeaconBlockBody]
	elif kind == ConsensusFork.Deneb:
	typedesc[deneb.BeaconBlockBody]
	elif kind == ConsensusFork.Capella:
	typedesc[capella.BeaconBlockBody]
	elif kind == ConsensusFork.Bellatrix:
	typedesc[bellatrix.BeaconBlockBody]
	elif kind == ConsensusFork.Altair:
	typedesc[altair.BeaconBlockBody]
	elif kind == ConsensusFork.Phase0:
	typedesc[phase0.BeaconBlockBody]
	else:
	static: raiseAssert "Unreachable"
	template SignedBeaconBlock*(kind: static ConsensusFork): auto =
	when kind == ConsensusFork.Electra:
	typedesc[electra.SignedBeaconBlock]
	elif kind == ConsensusFork.Deneb:
	typedesc[deneb.SignedBeaconBlock]
	elif kind == ConsensusFork.Capella:
	typedesc[capella.SignedBeaconBlock]
	elif kind == ConsensusFork.Bellatrix:
	typedesc[bellatrix.SignedBeaconBlock]
	elif kind == ConsensusFork.Altair:
	typedesc[altair.SignedBeaconBlock]
	elif kind == ConsensusFork.Phase0:
	typedesc[phase0.SignedBeaconBlock]
	else:
	static: raiseAssert "Unreachable"
	template TrustedSignedBeaconBlock*(kind: static ConsensusFork): auto =
	when kind == ConsensusFork.Electra:
	typedesc[electra.TrustedSignedBeaconBlock]
	elif kind == ConsensusFork.Deneb:
	typedesc[deneb.TrustedSignedBeaconBlock]
	elif kind == ConsensusFork.Capella:
	typedesc[capella.TrustedSignedBeaconBlock]
	elif kind == ConsensusFork.Bellatrix:
	typedesc[bellatrix.TrustedSignedBeaconBlock]
	elif kind == ConsensusFork.Altair:
	typedesc[altair.TrustedSignedBeaconBlock]
	elif kind == ConsensusFork.Phase0:
	typedesc[phase0.TrustedSignedBeaconBlock]
	else:
	static: raiseAssert "Unreachable"
	template ExecutionPayloadForSigning*(kind: static ConsensusFork): auto =
	when kind == ConsensusFork.Electra:
	typedesc[electra.ExecutionPayloadForSigning]
	elif kind == ConsensusFork.Deneb:
	typedesc[deneb.ExecutionPayloadForSigning]
	elif kind == ConsensusFork.Capella:
	typedesc[capella.ExecutionPayloadForSigning]
	elif kind == ConsensusFork.Bellatrix:
	typedesc[bellatrix.ExecutionPayloadForSigning]
	else:
	static: raiseAssert "Unreachable"
	template BlindedBeaconBlock*(kind: static ConsensusFork): auto =
	when kind == ConsensusFork.Electra:
	typedesc[electra_mev.BlindedBeaconBlock]
	elif kind == ConsensusFork.Deneb:
	typedesc[deneb_mev.BlindedBeaconBlock]
	elif kind == ConsensusFork.Capella or kind == ConsensusFork.Bellatrix:
	static: raiseAssert "Unsupported"
	else:
	static: raiseAssert "Unreachable"
	template MaybeBlindedBeaconBlock*(kind: static ConsensusFork): auto =
	when kind == ConsensusFork.Electra:
	typedesc[electra_mev.MaybeBlindedBeaconBlock]
	elif kind == ConsensusFork.Deneb:
	typedesc[deneb_mev.MaybeBlindedBeaconBlock]
	elif kind == ConsensusFork.Capella or kind == ConsensusFork.Bellatrix:
	static: raiseAssert "Unsupported"
	else:
	static: raiseAssert "Unreachable"
	template SignedBlindedBeaconBlock*(kind: static ConsensusFork): auto =
	when kind == ConsensusFork.Electra:
	typedesc[electra_mev.SignedBlindedBeaconBlock]
	elif kind == ConsensusFork.Deneb:
	typedesc[deneb_mev.SignedBlindedBeaconBlock]
	elif kind == ConsensusFork.Capella or kind == ConsensusFork.Bellatrix:
	static: raiseAssert "Unsupported"
	else:
	static: raiseAssert "Unreachable"

There's 120 lines of this

[tersec]

It also increases the coupling which makes for a higher risk/reward setup -- when withConsensusFork works automatically, it's nice, but when it doesn't, it's yet another thing which needs to be temporarily disabled/enabled during fork transitions.

The net effect of this kind of transformation is to centralize the case statements into fewer places, with more effect/impact each. That typically decreases maintenance, until it doesn't. Can't put some debugComment/debugAssert/etc on only the eth2 REST serialization, it has to be in the centralized forks module which affects that and a dozen other things.

But, sure. Maybe risk worth taking, since the point of this overall PR's refactoring is to have REST serialization not use entirely separate block reading for a few specific cases than anything else does, so it should follow other fixes more automatically regardless: 8644d89