prysm-pulse/beacon-chain/blockchain/process_attestation.go

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package blockchain
import (
"context"
"fmt"
"time"
"github.com/gogo/protobuf/proto"
"github.com/pkg/errors"
ethpb "github.com/prysmaticlabs/ethereumapis/eth/v1alpha1"
"github.com/prysmaticlabs/prysm/beacon-chain/core/helpers"
"github.com/prysmaticlabs/prysm/shared/bytesutil"
"go.opencensus.io/trace"
)
// ErrTargetRootNotInDB returns when the target block root of an attestation cannot be found in the
// beacon database.
var ErrTargetRootNotInDB = errors.New("target root does not exist in db")
// onAttestation is called whenever an attestation is received, verifies the attestation is valid and saves
/// it to the DB.
//
// Spec pseudocode definition:
// def on_attestation(store: Service, attestation: Attestation) -> None:
// """
// Run ``on_attestation`` upon receiving a new ``attestation`` from either within a block or directly on the wire.
//
// An ``attestation`` that is asserted as invalid may be valid at a later time,
// consider scheduling it for later processing in such case.
// """
// target = attestation.data.target
//
// # Attestations must be from the current or previous epoch
// current_epoch = compute_epoch_at_slot(get_current_slot(store))
// # Use GENESIS_EPOCH for previous when genesis to avoid underflow
// previous_epoch = current_epoch - 1 if current_epoch > GENESIS_EPOCH else GENESIS_EPOCH
// assert target.epoch in [current_epoch, previous_epoch]
// assert target.epoch == compute_epoch_at_slot(attestation.data.slot)
//
// # Attestations target be for a known block. If target block is unknown, delay consideration until the block is found
// assert target.root in store.blocks
// # Attestations cannot be from future epochs. If they are, delay consideration until the epoch arrives
// base_state = store.block_states[target.root].copy()
// assert store.time >= base_state.genesis_time + compute_start_slot_at_epoch(target.epoch) * SECONDS_PER_SLOT
//
// # Attestations must be for a known block. If block is unknown, delay consideration until the block is found
// assert attestation.data.beacon_block_root in store.blocks
// # Attestations must not be for blocks in the future. If not, the attestation should not be considered
// assert store.blocks[attestation.data.beacon_block_root].slot <= attestation.data.slot
//
// # Service target checkpoint state if not yet seen
// if target not in store.checkpoint_states:
// process_slots(base_state, compute_start_slot_at_epoch(target.epoch))
// store.checkpoint_states[target] = base_state
// target_state = store.checkpoint_states[target]
//
// # Attestations can only affect the fork choice of subsequent slots.
// # Delay consideration in the fork choice until their slot is in the past.
// assert store.time >= (attestation.data.slot + 1) * SECONDS_PER_SLOT
//
// # Get state at the `target` to validate attestation and calculate the committees
// indexed_attestation = get_indexed_attestation(target_state, attestation)
// assert is_valid_indexed_attestation(target_state, indexed_attestation)
//
// # Update latest messages
// for i in indexed_attestation.attesting_indices:
// if i not in store.latest_messages or target.epoch > store.latest_messages[i].epoch:
// store.latest_messages[i] = LatestMessage(epoch=target.epoch, root=attestation.data.beacon_block_root)
func (s *Service) onAttestation(ctx context.Context, a *ethpb.Attestation) ([]uint64, error) {
ctx, span := trace.StartSpan(ctx, "blockchain.onAttestation")
defer span.End()
tgt := proto.Clone(a.Data.Target).(*ethpb.Checkpoint)
tgtSlot := helpers.StartSlot(tgt.Epoch)
if helpers.SlotToEpoch(a.Data.Slot) != a.Data.Target.Epoch {
return nil, fmt.Errorf("data slot is not in the same epoch as target %d != %d", helpers.SlotToEpoch(a.Data.Slot), a.Data.Target.Epoch)
}
// Verify beacon node has seen the target block before.
if !s.beaconDB.HasBlock(ctx, bytesutil.ToBytes32(tgt.Root)) {
return nil, ErrTargetRootNotInDB
}
// Verify attestation target has had a valid pre state produced by the target block.
baseState, err := s.verifyAttPreState(ctx, tgt)
if err != nil {
return nil, err
}
// Verify attestation target is from current epoch or previous epoch.
if err := s.verifyAttTargetEpoch(ctx, baseState.GenesisTime, uint64(time.Now().Unix()), tgt); err != nil {
return nil, err
}
// Verify Attestations cannot be from future epochs.
if err := helpers.VerifySlotTime(baseState.GenesisTime, tgtSlot); err != nil {
return nil, errors.Wrap(err, "could not verify attestation target slot")
}
// Verify attestation beacon block is known and not from the future.
if err := s.verifyBeaconBlock(ctx, a.Data); err != nil {
return nil, errors.Wrap(err, "could not verify attestation beacon block")
}
// Service target checkpoint state if not yet seen.
baseState, err = s.saveCheckpointState(ctx, baseState, tgt)
if err != nil {
return nil, err
}
// Verify attestations can only affect the fork choice of subsequent slots.
if err := helpers.VerifySlotTime(baseState.GenesisTime, a.Data.Slot+1); err != nil {
return nil, err
}
// Use the target state to to validate attestation and calculate the committees.
indexedAtt, err := s.verifyAttestation(ctx, baseState, a)
if err != nil {
return nil, err
}
if err := s.beaconDB.SaveAttestation(ctx, a); err != nil {
return nil, err
}
return indexedAtt.AttestingIndices, nil
}