103 lines
4.5 KiB
Nim
103 lines
4.5 KiB
Nim
import
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sequtils, tables,
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spec/[datatypes, crypto, digest], ssz
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type
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# TODO is it better to only key on some subset (e.g., shard+slot+beacon state hash)
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# of AttestationData? Using whole AttestationData does defend against some attacks.
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#
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# Per Danny as of 2018-12-21:
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# Yeah, you can do any linear combination of signatures. but you have to
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# remember the linear combination of pubkeys that constructed
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# if you have two instances of a signature from pubkey p, then you need 2*p
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# in the group pubkey because the attestation bitfield is only 1 bit per
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# pubkey right now, attestations do not support this it could be extended to
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# support N overlaps up to N times per pubkey if we had N bits per validator
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# instead of 1
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# We are shying away from this for the time being. If there end up being
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# substantial difficulties in network layer aggregation, then adding bits to
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# aid in supporting overlaps is one potential solution
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# TODO replace array[32, byte] with Eth2Digest from hash_tree_root_final from
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# https://github.com/status-im/nim-beacon-chain/pull/47
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# It would be better to combine these incrementally, pending above.
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AttestationPool* = object
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attestations: Table[uint64, Table[array[32, byte], seq[Attestation]]]
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# TODO priority queue or similar to track most-voted-on-AttestationData
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# per shard
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proc init*(T: type AttestationPool): T =
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result.attestations = initTable[AttestationData, seq[Attestation]]()
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func getLookupKey(attestationData: AttestationData): array[0..31, byte] =
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hash_tree_root(attestationData)
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proc add*(pool: var AttestationPool,
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attestation: Attestation) =
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# Should be called for local and remote attestations.
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let key = getLookupKey(attestation.data)
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var attestations = pool.attestations.getOrDefault(attestation.data.shard).getOrDefault(key)
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# Basic sanity checks should already have been performed on this.
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# For example, if non-committee attestations shouldn't be included, this
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# doesn't separately check for that invariant.
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attestations.add(attestation)
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pool.attestations[attestation.data.shard][key] = attestations
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func findMostCovering(pool: AttestationPool, shard: uint64): AttestationData =
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# Just a simple linear scan for now; could use various acceleration
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# data structures later, depending on tradeoff of how often queried
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# Might not be perf-sensitive; mostly per-epoch
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var mostAttestedData: AttestationData
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var maxLen: int = 0
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for perShardAttestations in values(pool.attestations[shard]):
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let l = perShardAttestations.len
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if l > maxLen:
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# Guaranteed to have at least one element if > 0
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mostAttestedData = perShardAttestations[0].data
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maxLen = l
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mostAttestedData
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func getCombined(pool: AttestationPool, attestationData: AttestationData) : ValidatorSig =
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var signatures : seq[ValidatorSig] = @[]
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for perShardAttestation in pool.attestations.getOrDefault(attestationData.shard).getOrDefault(attestationData.getLookupKey):
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signatures.add(perShardAttestation.aggregate_signature)
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combine(signatures)
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proc bitfieldUnion(accum: var seq[byte], disjunct: seq[byte]) =
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# TODO replace with nim-ranges
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doAssert len(accum) == len(disjunct)
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for i in 0 ..< len(accum):
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accum[i] = accum[i] or disjunct[i]
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func getAggregatedAttestion*(pool: AttestationPool, shard: uint64) : Attestation =
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# TODO This might turn out to be a non-assertable condition, per, e.g.,
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# the recent discussion on error handling elsewhere in Nimbus, but it's
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# likelier that other code shouldn't be just randomly probing shards so
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# it's useful to start this way and catch logic errors early.
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assert shard in pool.attestations, "Attempt to query nonexistent shard"
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let mostCoveringAttestationData = findMostCovering(pool, shard)
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# TODO error handling where shard either doesn't exist or empty; needs
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# more holistic approach
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result.data = mostCoveringAttestationData
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let freqAttestations = pool.attestations[shard].getOrDefault(result.data.getLookupKey)
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# TODO probably this should not assert on failure
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# TODO Ugly, due to leaky seq[byte] non-abstraction. nim-ranges should help.
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result.participation_bitfield = repeat(0'u8, freqAttestations[0].participation_bitfield.len)
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for freqAttestation in freqAttestations:
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bitfieldUnion(result.participation_bitfield, freqAttestation.participation_bitfield)
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# TODO 2018-12-22 ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md
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# doesn't document semantics.
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# result.custody_bitfield = bitfieldUnion
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result.aggregate_signature = getCombined(pool, mostCoveringAttestationData)
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