Update EIP-6110: add pubkey2index cache analysis by @naviechan

Merged by EIP-Bot.

EIPS/eip-6110.md

@@ -176,6 +176,8 @@ Detailed consensus layer specification can be found in following documents:
 
 Due to the large follow distance of `Eth1Data` poll an index of a new validator assigned during deposit processing remains the same across different branches of a block tree, i.e. with existing mechanism `(pubkey, index)` cache utilized by consensus layer clients is re-org resilient. The new deposit machinery breaks this invariant and consensus layer clients will have to deal with a fact that a validator index becomes fork dependent, i.e. a validator with the same `pubkey` can have different indexes in different block tree branches.
 
+Detailed [analysis](../assets/eip-6110/pubkey_to_index_cache_analysis.md) shows that `process_deposit` function is *the only* place requiring a fork dependent `(pubkey, index)` cache.
+
 #### `Eth1Data` poll deprecation
 
 Consensus layer clients will be able to remove `Eth1Data` poll mechanism in an uncoordinated fashion once transition period is finished. The transition period is considered as finished when a network reaches the point where `state.eth1_deposit_index == state.deposit_receipts_start_index`.

assets/eip-6110/pubkey_to_index_cache_analysis.md

@@ -0,0 +1,26 @@
+Here is a copy of the tl;dr section from an [extended analysis](https://hackmd.io/@adrninistrator1/SkHmz972n) of `pubkey2index` and `index2pubkey` cache usage provided by Navie Chan:
+
+tl;dr: `index2Pubkey` is used in a lot more scenarios than `pubkey2Index`. The use cases for `index2Pubkey` do not require unfinalized information. `process_deposit()` is the only place in consensus spec that needs unfinalized information and it utilizes `pubkey2Index`.
+In terms of the two-cache approach, `unfinalizedIndex2Pubkey` is not needed since there is not a place that utilizes it. `unfinalizedPubkey2Index`, however, is needed for `process_deposit()`.
+
+|  | unfinalized pubkey2Index? | unfinalized index2Pubkey? |
+|-|-|-|
+| onBlock - state_transition - verify_block_signature | N/A | No |
+| onBlock - state_transition - process_block - process_randao | N/A | No |
+| onBlock - state_transition - process_block - process_operations - process_proposer_slashing | N/A | No |
+| onBlock - state_transition - process_block - process_operations - process_attester_slashing - is_valid_indexed_attestation | N/A | No |
+| onBlock - state_transition - process_block - process_operations - process_attestation - is_valid_indexed_attestation | N/A | No |
+| onBlock - state_transition - process_block - process_operations - process_deposit - apply_deposit | Yes | N/A |
+| onBlock - state_transition - process_block - process_operations - process_sync_aggregate - eth_fast_aggregate_verify | No | No |
+| onBlock - state_transition - process_block - process_bls_to_execution_change | N/A | N/A |
+| onBlock - state_transition - process_block - process_voluntary_exit | N/A | No |
+| p2p - beacon_block ---- onBlock | N/A | No |
+| p2p - beacon_aggregate_and_proof ---- onAttestation | N/A | No |
+| p2p - voluntary_exit - process_voluntary_exit | N/A | No |
+| p2p - proposer_slashing - process_proposer_slashing | N/A | No |
+| p2p - attester_slashing - process_attester_slashing | N/A | No |
+| p2p - beacon_attestation_{subnet_id} ---- onAttestation | N/A | No |
+| p2p - sync_committee_contribution_and_proof | N/A | No |
+| p2p - sync_committee_{subnet_id} | N/A | No |
+| p2p - bls_to_execution_change - process_bls_to_execution_change | N/A | N/A |
+| p2p - blob_sidecar_{subnet_id} - verify_blob_sidecar_signature | N/A | No |