Add attestation gossip pre-verification (#983)

* Add PH & MS slot clock changes

* Account for genesis time

* Add progress on duties refactor

* Add simple is_aggregator bool to val subscription

* Start work on attestation_verification.rs

* Add progress on ObservedAttestations

* Progress with ObservedAttestations

* Fix tests

* Add observed attestations to the beacon chain

* Add attestation observation to processing code

* Add progress on attestation verification

* Add first draft of ObservedAttesters

* Add more tests

* Add observed attesters to beacon chain

* Add observers to attestation processing

* Add more attestation verification

* Create ObservedAggregators map

* Remove commented-out code

* Add observed aggregators into chain

* Add progress

* Finish adding features to attestation verification

* Ensure beacon chain compiles

* Link attn verification into chain

* Integrate new attn verification in chain

* Remove old attestation processing code

* Start trying to fix beacon_chain tests

* Split adding into pools into two functions

* Add aggregation to harness

* Get test harness working again

* Adjust the number of aggregators for test harness

* Fix edge-case in harness

* Integrate new attn processing in network

* Fix compile bug in validator_client

* Update validator API endpoints

* Fix aggreagation in test harness

* Fix enum thing

* Fix attestation observation bug:

* Patch failing API tests

* Start adding comments to attestation verification

* Remove unused attestation field

* Unify "is block known" logic

* Update comments

* Supress fork choice errors for network processing

* Add todos

* Tidy

* Add gossip attn tests

* Disallow test harness to produce old attns

* Comment out in-progress tests

* Partially address pruning tests

* Fix failing store test

* Add aggregate tests

* Add comments about which spec conditions we check

* Dont re-aggregate

* Split apart test harness attn production

* Fix compile error in network

* Make progress on commented-out test

* Fix skipping attestation test

* Add fork choice verification tests

* Tidy attn tests, remove dead code

* Remove some accidentally added code

* Fix clippy lint

* Rename test file

* Add block tests, add cheap block proposer check

* Rename block testing file

* Add observed_block_producers

* Tidy

* Switch around block signature verification

* Finish block testing

* Remove gossip from signature tests

* First pass of self review

* Fix deviation in spec

* Update test spec tags

* Start moving over to hashset

* Finish moving observed attesters to hashmap

* Move aggregation pool over to hashmap

* Make fc attn borrow again

* Fix rest_api compile error

* Fix missing comments

* Fix monster test

* Uncomment increasing slots test

* Address remaining comments

* Remove unsafe, use cfg test

* Remove cfg test flag

* Fix dodgy comment

* Ignore aggregates that are already known.

* Unify aggregator modulo logic

* Fix typo in logs

* Refactor validator subscription logic

* Avoid reproducing selection proof

* Skip HTTP call if no subscriptions

* Rename DutyAndState -> DutyAndProof

* Tidy logs

* Print root as dbg

* Fix compile errors in tests

* Fix compile error in test

beacon_node/beacon_chain/src/block_verification.rs

@@ -104,10 +104,18 @@ pub enum BlockError {
     },
     /// Block is already known, no need to re-import.
     BlockIsAlreadyKnown,
+    /// A block for this proposer and slot has already been observed.
+    RepeatProposal { proposer: u64, slot: Slot },
     /// The block slot exceeds the MAXIMUM_BLOCK_SLOT_NUMBER.
     BlockSlotLimitReached,
+    /// The `BeaconBlock` has a `proposer_index` that does not match the index we computed locally.
+    ///
+    /// The block is invalid.
+    IncorrectBlockProposer { block: u64, local_shuffling: u64 },
     /// The proposal signature in invalid.
     ProposalSignatureInvalid,
+    /// The `block.proposal_index` is not known.
+    UnknownValidator(u64),
     /// A signature in the block is invalid (exactly which is unknown).
     InvalidSignature,
     /// The provided block is from an earlier slot than its parent.
@@ -126,7 +134,18 @@ pub enum BlockError {
 
 impl From<BlockSignatureVerifierError> for BlockError {
     fn from(e: BlockSignatureVerifierError) -> Self {
-        BlockError::BeaconChainError(BeaconChainError::BlockSignatureVerifierError(e))
+        match e {
+            // Make a special distinction for `IncorrectBlockProposer` since it indicates an
+            // invalid block, not an internal error.
+            BlockSignatureVerifierError::IncorrectBlockProposer {
+                block,
+                local_shuffling,
+            } => BlockError::IncorrectBlockProposer {
+                block,
+                local_shuffling,
+            },
+            e => BlockError::BeaconChainError(BeaconChainError::BlockSignatureVerifierError(e)),
+        }
     }
 }
 
@@ -286,7 +305,17 @@ impl<T: BeaconChainTypes> GossipVerifiedBlock<T> {
         // Do not gossip a block from a finalized slot.
         check_block_against_finalized_slot(&block.message, chain)?;
 
-        // TODO: add check for the `(block.proposer_index, block.slot)` tuple once we have v0.11.0
+        // Check that we have not already received a block with a valid signature for this slot.
+        if chain
+            .observed_block_producers
+            .proposer_has_been_observed(&block.message)
+            .map_err(|e| BlockError::BeaconChainError(e.into()))?
+        {
+            return Err(BlockError::RepeatProposal {
+                proposer: block.message.proposer_index,
+                slot: block.message.slot,
+            });
+        }
 
         let mut parent = load_parent(&block.message, chain)?;
         let block_root = get_block_root(&block);
@@ -297,21 +326,54 @@ impl<T: BeaconChainTypes> GossipVerifiedBlock<T> {
             &chain.spec,
         )?;
 
-        let pubkey_cache = get_validator_pubkey_cache(chain)?;
+        let signature_is_valid = {
+            let pubkey_cache = get_validator_pubkey_cache(chain)?;
+            let pubkey = pubkey_cache
+                .get(block.message.proposer_index as usize)
+                .ok_or_else(|| BlockError::UnknownValidator(block.message.proposer_index))?;
+            block.verify_signature(
+                Some(block_root),
+                pubkey,
+                &state.fork,
+                chain.genesis_validators_root,
+                &chain.spec,
+            )
+        };
 
-        let mut signature_verifier = get_signature_verifier(&state, &pubkey_cache, &chain.spec);
-
-        signature_verifier.include_block_proposal(&block, Some(block_root))?;
-
-        if signature_verifier.verify().is_ok() {
-            Ok(Self {
-                block,
-                block_root,
-                parent,
-            })
-        } else {
-            Err(BlockError::ProposalSignatureInvalid)
+        if !signature_is_valid {
+            return Err(BlockError::ProposalSignatureInvalid);
         }
+
+        // Now the signature is valid, store the proposal so we don't accept another from this
+        // validator and slot.
+        //
+        // It's important to double-check that the proposer still hasn't been observed so we don't
+        // have a race-condition when verifying two blocks simultaneously.
+        if chain
+            .observed_block_producers
+            .observe_proposer(&block.message)
+            .map_err(|e| BlockError::BeaconChainError(e.into()))?
+        {
+            return Err(BlockError::RepeatProposal {
+                proposer: block.message.proposer_index,
+                slot: block.message.slot,
+            });
+        }
+
+        let expected_proposer =
+            state.get_beacon_proposer_index(block.message.slot, &chain.spec)? as u64;
+        if block.message.proposer_index != expected_proposer {
+            return Err(BlockError::IncorrectBlockProposer {
+                block: block.message.proposer_index,
+                local_shuffling: expected_proposer,
+            });
+        }
+
+        Ok(Self {
+            block,
+            block_root,
+            parent,
+        })
     }
 
     pub fn block_root(&self) -> Hash256 {