nimbus-eth2/beacon_chain/beacon_chain_db.nim

# beacon_chain
# Copyright (c) 2018-2021 Status Research & Development GmbH
# Licensed and distributed under either of
#   * MIT license (license terms in the root directory or at https://opensource.org/licenses/MIT).
#   * Apache v2 license (license terms in the root directory or at https://www.apache.org/licenses/LICENSE-2.0).
# at your option. This file may not be copied, modified, or distributed except according to those terms.

{.push raises: [Defect].}

import
  std/[typetraits, tables],
  stew/[arrayops, assign2, byteutils, endians2, io2, objects, results],
  serialization, chronicles, snappy,
  eth/db/[kvstore, kvstore_sqlite3],
  ./networking/network_metadata, ./beacon_chain_db_immutable,
  ./spec/[crypto, datatypes, digest, state_transition],
  ./ssz/[ssz_serialization, merkleization],
  ./filepath

type
  DbSeq*[T] = object
    insertStmt: SqliteStmt[openArray[byte], void]
    selectStmt: SqliteStmt[int64, openArray[byte]]
    recordCount: int64

  DepositsSeq = DbSeq[DepositData]
  ImmutableValidatorsSeq = DbSeq[ImmutableValidatorData]

  DepositContractSnapshot* = object
    eth1Block*: Eth2Digest
    depositContractState*: DepositContractState

  BeaconChainDBV0* = ref object
    ## BeaconChainDBV0 based on old kvstore table that sets the WITHOUT ROWID
    ## option which becomes unbearably slow with large blobs. It is used as a
    ## read-only store to support old versions - by freezing it at its current
    ## data set, downgrading remains possible since it's no longer touched -
    ## anyone downgrading will have to sync up whatever they missed.
    ##
    ## Newer versions read from the new tables first - if the data is not found,
    ## they turn to the old tables for reading. Writing is done only to the new
    ## tables.
    ##
    ## V0 stored most data in a single table, prefixing each key with a tag
    ## identifying the type of data.
    ##
    ## 1.1 introduced BeaconStateNoImmutableValidators storage where immutable
    ## validator data is stored in a separate table and only a partial
    ## BeaconState is written to kvstore
    ##
    ## 1.2 moved BeaconStateNoImmutableValidators to a separate table to
    ## alleviate some of the btree balancing issues - this doubled the speed but
    ## was still slow
    backend: KvStoreRef # kvstore
    stateStore: KvStoreRef # state_no_validators

  BeaconChainDB* = ref object
    ## Database storing resolved blocks and states - resolved blocks are such
    ## blocks that form a chain back to the tail block.
    ##
    ## We assume that the database backend is working / not corrupt - as such,
    ## we will raise a Defect any time there is an issue. This should be
    ## revisited in the future, when/if the calling code safely can handle
    ## corruption of this kind.
    ##
    ## We do however make an effort not to crash on invalid data inside the
    ## database - this may have a number of "natural" causes such as switching
    ## between different versions of the client and accidentally using an old
    ## database.
    db: SqStoreRef

    v0: BeaconChainDBV0
    preset*: RuntimePreset
    genesisDeposits*: DepositsSeq

    # ImmutableValidatorsSeq only stores the total count; it's a proxy for SQL
    # queries.
    immutableValidators*: ImmutableValidatorsSeq
    immutableValidatorsMem*: seq[ImmutableValidatorData]

    checkpoint*: proc() {.gcsafe, raises: [Defect].}

    keyValues: KvStoreRef # Random stuff using DbKeyKind - suitable for small values mainly!
    blocks: KvStoreRef # BlockRoot -> TrustedBeaconBlock
    stateRoots: KvStoreRef # (Slot, BlockRoot) -> StateRoot
    statesNoVal: KvStoreRef # StateRoot -> BeaconStateNoImmutableValidators
    stateDiffs: KvStoreRef ##\
      ## StateRoot -> BeaconStateDiff
      ## Instead of storing full BeaconStates, one can store only the diff from
      ## a different state. As 75% of a typical BeaconState's serialized form's
      ## the validators, which are mostly immutable and append-only, just using
      ## a simple append-diff representation helps significantly. Various roots
      ## are stored in a mod-increment pattern across fixed-sized arrays, which
      ## addresses most of the rest of the BeaconState sizes.

    summaries: KvStoreRef # BlockRoot -> BeaconBlockSummary

  DbKeyKind = enum
    kHashToState
    kHashToBlock
    kHeadBlock
      ## Pointer to the most recent block selected by the fork choice
    kTailBlock
      ## Pointer to the earliest finalized block - this is the genesis block when
      ## the chain starts, but might advance as the database gets pruned
      ## TODO: determine how aggressively the database should be pruned. For a
      ##       healthy network sync, we probably need to store blocks at least
      ##       past the weak subjectivity period.
    kBlockSlotStateRoot
      ## BlockSlot -> state_root mapping
    kGenesisBlockRoot
      ## Immutable reference to the network genesis state
      ## (needed for satisfying requests to the beacon node API).
    kEth1PersistedTo # Obsolete
    kDepositsFinalizedByEth1 # Obsolete
    kDepositsFinalizedByEth2
      ## A merkleizer checkpoint used for computing merkle proofs of
      ## deposits added to Eth2 blocks (it may lag behind the finalized
      ## eth1 deposits checkpoint).
    kHashToBlockSummary # Block summaries for fast startup
    kSpeculativeDeposits
      ## A merkelizer checkpoint created on the basis of deposit events
      ## that we were not able to verify against a `deposit_root` served
      ## by the web3 provider. This may happen on Geth nodes that serve
      ## only recent contract state data (i.e. only recent `deposit_roots`).
    kHashToStateDiff # Obsolete
    kHashToStateOnlyMutableValidators

  BeaconBlockSummary* = object
    ## Cache of beacon block summaries - during startup when we construct the
    ## chain dag, loading full blocks takes a lot of time - the block
    ## summary contains a minimal snapshot of what's needed to instanciate
    ## the BlockRef tree.
    slot*: Slot
    parent_root*: Eth2Digest

const
  # The largest object we're saving is the BeaconState, and by far, the largest
  # part of it is the validator - each validator takes up at least 129 bytes
  # in phase0,  which means 100k validators is >12mb - in addition to this,
  # there are several MB of hashes.
  maxDecompressedDbRecordSize = 64*1024*1024

# Subkeys essentially create "tables" within the key-value store by prefixing
# each entry with a table id

func subkey(kind: DbKeyKind): array[1, byte] =
  result[0] = byte ord(kind)

func subkey[N: static int](kind: DbKeyKind, key: array[N, byte]):
    array[N + 1, byte] =
  result[0] = byte ord(kind)
  result[1 .. ^1] = key

func subkey(kind: type BeaconState, key: Eth2Digest): auto =
  subkey(kHashToState, key.data)

func subkey(
    kind: type BeaconStateNoImmutableValidators, key: Eth2Digest): auto =
  subkey(kHashToStateOnlyMutableValidators, key.data)

func subkey(kind: type SignedBeaconBlock, key: Eth2Digest): auto =
  subkey(kHashToBlock, key.data)

func subkey(kind: type BeaconBlockSummary, key: Eth2Digest): auto =
  subkey(kHashToBlockSummary, key.data)

func subkey(root: Eth2Digest, slot: Slot): array[40, byte] =
  var ret: array[40, byte]
  # big endian to get a naturally ascending order on slots in sorted indices
  ret[0..<8] = toBytesBE(slot.uint64)
  # .. but 7 bytes should be enough for slots - in return, we get a nicely
  # rounded key length
  ret[0] = byte ord(kBlockSlotStateRoot)
  ret[8..<40] = root.data

  ret

template panic =
  # TODO(zah): Could we recover from a corrupted database?
  #            Review all usages.
  raiseAssert "The database should not be corrupted"

template expectDb(x: auto): untyped =
  # There's no meaningful error handling implemented for a corrupt database or
  # full disk - this requires manual intervention, so we'll panic for now
  x.expect("working database (disk broken/full?)")

proc init*[T](Seq: type DbSeq[T], db: SqStoreRef, name: string): Seq =
  db.exec("""
    CREATE TABLE IF NOT EXISTS """ & name & """(
       id INTEGER PRIMARY KEY,
       value BLOB
    );
  """).expectDb()

  let
    insertStmt = db.prepareStmt(
      "INSERT INTO " & name & "(value) VALUES (?);",
      openArray[byte], void, managed = false).expect("this is a valid statement")

    selectStmt = db.prepareStmt(
      "SELECT value FROM " & name & " WHERE id = ?;",
      int64, openArray[byte], managed = false).expect("this is a valid statement")

    countStmt = db.prepareStmt(
      "SELECT COUNT(1) FROM " & name & ";",
      NoParams, int64, managed = false).expect("this is a valid statement")

  var recordCount = int64 0
  let countQueryRes = countStmt.exec do (res: int64):
    recordCount = res

  let found = countQueryRes.expectDb()
  if not found: panic()
  countStmt.dispose()

  Seq(insertStmt: insertStmt,
      selectStmt: selectStmt,
      recordCount: recordCount)

proc close*(s: DbSeq) =
  s.insertStmt.dispose()
  s.selectStmt.dispose()

proc add*[T](s: var DbSeq[T], val: T) =
  var bytes = SSZ.encode(val)
  s.insertStmt.exec(bytes).expectDb()
  inc s.recordCount

template len*[T](s: DbSeq[T]): int64 =
  s.recordCount

proc get*[T](s: DbSeq[T], idx: int64): T =
  # This is used only locally
  let resultAddr = addr result

  let queryRes = s.selectStmt.exec(idx + 1) do (recordBytes: openArray[byte]):
    try:
      resultAddr[] = decode(SSZ, recordBytes, T)
    except SerializationError:
      panic()

  let found = queryRes.expectDb()
  if not found: panic()

proc loadImmutableValidators(db: BeaconChainDB): seq[ImmutableValidatorData] =
  # TODO not called, but build fails otherwise
  for i in 0 ..< db.immutableValidators.len:
    result.add db.immutableValidators.get(i)

proc new*(T: type BeaconChainDB,
          preset: RuntimePreset,
          dir: string,
          inMemory = false,
    ): BeaconChainDB =
  var db = if inMemory:
      SqStoreRef.init("", "test", inMemory = true).expect(
        "working database (out of memory?)")
    else:
      let s = secureCreatePath(dir)
      doAssert s.isOk # TODO(zah) Handle this in a better way

      SqStoreRef.init(
        dir, "nbc", manualCheckpoint = true).expectDb()

  # Remove the deposits table we used before we switched
  # to storing only deposit contract checkpoints
  if db.exec("DROP TABLE IF EXISTS deposits;").isErr:
    debug "Failed to drop the deposits table"

  # An old pubkey->index mapping that hasn't been used on any mainnet release
  if db.exec("DROP TABLE IF EXISTS validatorIndexFromPubKey;").isErr:
    debug "Failed to drop the validatorIndexFromPubKey table"

  var
    # V0 compatibility tables
    backend = kvStore db.openKvStore().expectDb()
    # state_no_validators is similar to state_no_validators2 but uses a
    # different key encoding and was created WITHOUT ROWID
    stateStore = kvStore db.openKvStore("state_no_validators").expectDb()

    genesisDepositsSeq =
      DbSeq[DepositData].init(db, "genesis_deposits")
    immutableValidatorsSeq =
      DbSeq[ImmutableValidatorData].init(db, "immutable_validators")

    # V1 - expected-to-be small rows get without rowid optimizations
    keyValues = kvStore db.openKvStore("key_values", true).expectDb()
    blocks = kvStore db.openKvStore("blocks").expectDb()
    stateRoots = kvStore db.openKvStore("state_roots", true).expectDb()
    statesNoVal = kvStore db.openKvStore("state_no_validators2").expectDb()
    stateDiffs = kvStore db.openKvStore("state_diffs").expectDb()
    summaries = kvStore db.openKvStore("beacon_block_summaries", true).expectDb()

  T(
    db: db,
    v0: BeaconChainDBV0(
      backend: backend,
      stateStore: stateStore,
    ),
    preset: preset,
    genesisDeposits: genesisDepositsSeq,
    immutableValidators: immutableValidatorsSeq,
    immutableValidatorsMem: loadImmutableValidators(immutableValidatorsSeq),
    checkpoint: proc() = db.checkpoint(),
    keyValues: keyValues,
    blocks: blocks,
    stateRoots: stateRoots,
    statesNoVal: statesNoVal,
    stateDiffs: stateDiffs,
    summaries: summaries,
  )

proc decodeSSZ[T](data: openArray[byte], output: var T): bool =
  try:
    readSszBytes(data, output, updateRoot = false)
    true
  except SerializationError as e:
    # If the data can't be deserialized, it could be because it's from a
    # version of the software that uses a different SSZ encoding
    warn "Unable to deserialize data, old database?",
      err = e.msg, typ = name(T), dataLen = data.len
    false

proc decodeSnappySSZ[T](data: openArray[byte], output: var T): bool =
  try:
    let decompressed = snappy.decode(data, maxDecompressedDbRecordSize)
    readSszBytes(decompressed, output, updateRoot = false)
    true
  except SerializationError as e:
    # If the data can't be deserialized, it could be because it's from a
    # version of the software that uses a different SSZ encoding
    warn "Unable to deserialize data, old database?",
      err = e.msg, typ = name(T), dataLen = data.len
    false

proc encodeSSZ(v: auto): seq[byte] =
  try:
    SSZ.encode(v)
  except IOError as err:
    raiseAssert err.msg

proc encodeSnappySSZ(v: auto): seq[byte] =
  try:
    snappy.encode(SSZ.encode(v))
  except CatchableError as err:
    # In-memory encode shouldn't fail!
    raiseAssert err.msg

proc getRaw(db: KvStoreRef, key: openArray[byte], T: type Eth2Digest): Opt[T] =
  var res: Opt[T]
  proc decode(data: openArray[byte]) =
    if data.len == sizeof(Eth2Digest):
      res.ok Eth2Digest(data: toArray(sizeof(Eth2Digest), data))
    else:
      # If the data can't be deserialized, it could be because it's from a
      # version of the software that uses a different SSZ encoding
      warn "Unable to deserialize data, old database?",
       typ = name(T), dataLen = data.len
      discard

  discard db.get(key, decode).expectDb()

  res

proc putRaw(db: KvStoreRef, key: openArray[byte], v: Eth2Digest) =
  db.put(key, v.data).expectDb()

type GetResult = enum
  found = "Found"
  notFound = "Not found"
  corrupted = "Corrupted"

proc getSSZ[T](db: KvStoreRef, key: openArray[byte], output: var T): GetResult =
  var status = GetResult.notFound

  # TODO address is needed because there's no way to express lifetimes in nim
  #      we'll use unsafeAddr to find the code later
  var outputPtr = unsafeAddr output # callback is local, ptr wont escape
  proc decode(data: openArray[byte]) =
    status =
      if decodeSSZ(data, outputPtr[]): GetResult.found
      else: GetResult.corrupted

  discard db.get(key, decode).expectDb()

  status

proc putSSZ(db: KvStoreRef, key: openArray[byte], v: auto) =
  db.put(key, encodeSSZ(v)).expectDb()

proc getSnappySSZ[T](db: KvStoreRef, key: openArray[byte], output: var T): GetResult =
  var status = GetResult.notFound

  # TODO address is needed because there's no way to express lifetimes in nim
  #      we'll use unsafeAddr to find the code later
  var outputPtr = unsafeAddr output # callback is local, ptr wont escape
  proc decode(data: openArray[byte]) =
    status =
      if decodeSnappySSZ(data, outputPtr[]): GetResult.found
      else: GetResult.corrupted

  discard db.get(key, decode).expectDb()

  status

proc putSnappySSZ(db: KvStoreRef, key: openArray[byte], v: auto) =
  db.put(key, encodeSnappySSZ(v)).expectDb()

proc close*(db: BeaconChainDBV0) =
  discard db.stateStore.close()
  discard db.backend.close()

proc close*(db: BeaconchainDB) =
  if db.db == nil: return

  # Close things in reverse order
  discard db.summaries.close()
  discard db.stateDiffs.close()
  discard db.statesNoVal.close()
  discard db.stateRoots.close()
  discard db.blocks.close()
  discard db.keyValues.close()
  db.immutableValidators.close()
  db.genesisDeposits.close()
  db.v0.close()
  db.db.close()

  db.db = nil

func toBeaconBlockSummary(v: SomeBeaconBlock): BeaconBlockSummary =
  BeaconBlockSummary(
    slot: v.slot,
    parent_root: v.parent_root,
  )

proc putBeaconBlockSummary(
    db: BeaconChainDB, root: Eth2Digest, value: BeaconBlockSummary) =
  # Summaries are too simple / small to compress, store them as plain SSZ
  db.summaries.putSSZ(root.data, value)

proc putBlock*(db: BeaconChainDB, value: TrustedSignedBeaconBlock) =
  db.blocks.putSnappySSZ(value.root.data, value)
  db.putBeaconBlockSummary(value.root, value.message.toBeaconBlockSummary())

proc updateImmutableValidators(
    db: BeaconChainDB, immutableValidators: var seq[ImmutableValidatorData],
    validators: auto) =
  let
    numValidators = validators.lenu64
    origNumImmutableValidators = immutableValidators.lenu64

  doAssert immutableValidators.len == db.immutableValidators.len

  if numValidators <= origNumImmutableValidators:
    return

  for validatorIndex in origNumImmutableValidators ..< numValidators:
    # This precedes state storage
    let immutableValidator =
      getImmutableValidatorData(validators[validatorIndex])
    db.immutableValidators.add immutableValidator
    immutableValidators.add immutableValidator

proc putState*(db: BeaconChainDB, key: Eth2Digest, value: var BeaconState) =
  db.updateImmutableValidators(db.immutableValidatorsMem, value.validators)
  db.statesNoVal.putSnappySSZ(
    key.data,
    isomorphicCast[BeaconStateNoImmutableValidators](value))

proc putState*(db: BeaconChainDB, value: var BeaconState) =
  db.putState(hash_tree_root(value), value)

func stateRootKey(root: Eth2Digest, slot: Slot): array[40, byte] =
  var ret: array[40, byte]
  # big endian to get a naturally ascending order on slots in sorted indices
  ret[0..<8] = toBytesBE(slot.uint64)
  ret[8..<40] = root.data

  ret

proc putStateRoot*(db: BeaconChainDB, root: Eth2Digest, slot: Slot,
    value: Eth2Digest) =
  db.stateRoots.putRaw(stateRootKey(root, slot), value)

proc putStateDiff*(db: BeaconChainDB, root: Eth2Digest, value: BeaconStateDiff) =
  db.stateDiffs.putSnappySSZ(root.data, value)

proc delBlock*(db: BeaconChainDB, key: Eth2Digest) =
  db.blocks.del(key.data).expectDb()
  db.summaries.del(key.data).expectDb()

proc delState*(db: BeaconChainDB, key: Eth2Digest) =
  db.statesNoVal.del(key.data).expectDb()

proc delStateRoot*(db: BeaconChainDB, root: Eth2Digest, slot: Slot) =
  db.stateRoots.del(stateRootKey(root, slot)).expectDb()

proc delStateDiff*(db: BeaconChainDB, root: Eth2Digest) =
  db.stateDiffs.del(root.data).expectDb()

proc putHeadBlock*(db: BeaconChainDB, key: Eth2Digest) =
  db.keyValues.putRaw(subkey(kHeadBlock), key)

proc putTailBlock*(db: BeaconChainDB, key: Eth2Digest) =
  db.keyValues.putRaw(subkey(kTailBlock), key)

proc putGenesisBlockRoot*(db: BeaconChainDB, key: Eth2Digest) =
  db.keyValues.putRaw(subkey(kGenesisBlockRoot), key)

proc putEth2FinalizedTo*(db: BeaconChainDB,
                         eth1Checkpoint: DepositContractSnapshot) =
  db.keyValues.putSnappySSZ(subkey(kDepositsFinalizedByEth2), eth1Checkpoint)

proc getBlock(db: BeaconChainDBV0, key: Eth2Digest): Opt[TrustedSignedBeaconBlock] =
  # We only store blocks that we trust in the database
  result.ok(TrustedSignedBeaconBlock())
  if db.backend.getSnappySSZ(subkey(SignedBeaconBlock, key), result.get) != GetResult.found:
    result.err()
  else:
    # set root after deserializing (so it doesn't get zeroed)
    result.get().root = key

proc getBlock*(db: BeaconChainDB, key: Eth2Digest): Opt[TrustedSignedBeaconBlock] =
  # We only store blocks that we trust in the database
  result.ok(TrustedSignedBeaconBlock())
  if db.blocks.getSnappySSZ(key.data, result.get) != GetResult.found:
    result = db.v0.getBlock(key)
  else:
    # set root after deserializing (so it doesn't get zeroed)
    result.get().root = key

proc getStateOnlyMutableValidators(
    immutableValidatorsMem: openArray[ImmutableValidatorData],
    store: KvStoreRef, key: openArray[byte], output: var BeaconState,
    rollback: RollbackProc): bool =
  ## Load state into `output` - BeaconState is large so we want to avoid
  ## re-allocating it if possible
  ## Return `true` iff the entry was found in the database and `output` was
  ## overwritten.
  ## Rollback will be called only if output was partially written - if it was
  ## not found at all, rollback will not be called
  # TODO rollback is needed to deal with bug - use `noRollback` to ignore:
  #      https://github.com/nim-lang/Nim/issues/14126
  # TODO RVO is inefficient for large objects:
  #      https://github.com/nim-lang/Nim/issues/13879

  case store.getSnappySSZ(
    key, isomorphicCast[BeaconStateNoImmutableValidators](output))
  of GetResult.found:
    let numValidators = output.validators.len
    doAssert immutableValidatorsMem.len >= numValidators

    for i in 0 ..< numValidators:
      let
        # Bypass hash cache invalidation
        dstValidator = addr output.validators.data[i]

      assign(dstValidator.pubkey, immutableValidatorsMem[i].pubkey)
      assign(dstValidator.withdrawal_credentials,
        immutableValidatorsMem[i].withdrawal_credentials)

    output.validators.resetCache()

    true
  of GetResult.notFound:
    false
  of GetResult.corrupted:
    rollback(output)
    false

proc getState(
    db: BeaconChainDBV0,
    immutableValidatorsMem: openArray[ImmutableValidatorData],
    key: Eth2Digest, output: var BeaconState,
    rollback: RollbackProc): bool =
  # Nimbus 1.0 reads and writes writes genesis BeaconState to `backend`
  # Nimbus 1.1 writes a genesis BeaconStateNoImmutableValidators to `backend` and
  # reads both BeaconState and BeaconStateNoImmutableValidators from `backend`
  # Nimbus 1.2 writes a genesis BeaconStateNoImmutableValidators to `stateStore`
  # and reads BeaconState from `backend` and BeaconStateNoImmutableValidators
  # from `stateStore`. We will try to read the state from all these locations.
  if getStateOnlyMutableValidators(
      immutableValidatorsMem, db.stateStore,
      subkey(BeaconStateNoImmutableValidators, key), output, rollback):
    return true
  if getStateOnlyMutableValidators(
      immutableValidatorsMem, db.backend,
      subkey(BeaconStateNoImmutableValidators, key), output, rollback):
    return true

  case db.backend.getSnappySSZ(subkey(BeaconState, key), output)
  of GetResult.found:
    true
  of GetResult.notFound:
    false
  of GetResult.corrupted:
    rollback(output)
    false

proc getState*(
    db: BeaconChainDB, key: Eth2Digest, output: var BeaconState,
    rollback: RollbackProc): bool =
  ## Load state into `output` - BeaconState is large so we want to avoid
  ## re-allocating it if possible
  ## Return `true` iff the entry was found in the database and `output` was
  ## overwritten.
  ## Rollback will be called only if output was partially written - if it was
  ## not found at all, rollback will not be called
  # TODO rollback is needed to deal with bug - use `noRollback` to ignore:
  #      https://github.com/nim-lang/Nim/issues/14126
  # TODO RVO is inefficient for large objects:
  #      https://github.com/nim-lang/Nim/issues/13879
  if not getStateOnlyMutableValidators(
      db.immutableValidatorsMem, db.statesNoVal, key.data, output, rollback):
    db.v0.getState(db.immutableValidatorsMem, key, output, rollback)
  else:
    true

proc getStateRoot(db: BeaconChainDBV0,
                   root: Eth2Digest,
                   slot: Slot): Opt[Eth2Digest] =
  db.backend.getRaw(subkey(root, slot), Eth2Digest)

proc getStateRoot*(db: BeaconChainDB,
                   root: Eth2Digest,
                   slot: Slot): Opt[Eth2Digest] =
  db.stateRoots.getRaw(stateRootKey(root, slot), Eth2Digest) or
    db.v0.getStateRoot(root, slot)

proc getStateDiff*(db: BeaconChainDB,
                   root: Eth2Digest): Opt[BeaconStateDiff] =
  result.ok(BeaconStateDiff())
  if db.stateDiffs.getSnappySSZ(root.data, result.get) != GetResult.found:
    result.err

proc getHeadBlock(db: BeaconChainDBV0): Opt[Eth2Digest] =
  db.backend.getRaw(subkey(kHeadBlock), Eth2Digest)

proc getHeadBlock*(db: BeaconChainDB): Opt[Eth2Digest] =
  db.keyValues.getRaw(subkey(kHeadBlock), Eth2Digest) or
    db.v0.getHeadBlock()

proc getTailBlock(db: BeaconChainDBV0): Opt[Eth2Digest] =
  db.backend.getRaw(subkey(kTailBlock), Eth2Digest)

proc getTailBlock*(db: BeaconChainDB): Opt[Eth2Digest] =
  db.keyValues.getRaw(subkey(kTailBlock), Eth2Digest) or
    db.v0.getTailBlock()

proc getGenesisBlockRoot(db: BeaconChainDBV0): Eth2Digest =
  db.backend.getRaw(subkey(kGenesisBlockRoot), Eth2Digest).expectDb()

proc getGenesisBlockRoot*(db: BeaconChainDB): Eth2Digest =
  db.keyValues.getRaw(subkey(kGenesisBlockRoot), Eth2Digest).expect(
    "The database must be seeded with the genesis state")

proc getEth2FinalizedTo(db: BeaconChainDBV0): Opt[DepositContractSnapshot] =
  result.ok(DepositContractSnapshot())
  let r = db.backend.getSnappySSZ(subkey(kDepositsFinalizedByEth2), result.get)
  if r != found: result.err()

proc getEth2FinalizedTo*(db: BeaconChainDB): Opt[DepositContractSnapshot] =
  result.ok(DepositContractSnapshot())
  let r = db.keyValues.getSnappySSZ(subkey(kDepositsFinalizedByEth2), result.get)
  if r != found: return db.v0.getEth2FinalizedTo()

proc containsBlock*(db: BeaconChainDBV0, key: Eth2Digest): bool =
  db.backend.contains(subkey(SignedBeaconBlock, key)).expectDb()

proc containsBlock*(db: BeaconChainDB, key: Eth2Digest): bool =
  db.blocks.contains(key.data).expectDb() or db.v0.containsBlock(key)

proc containsState*(db: BeaconChainDBV0, key: Eth2Digest): bool =
  let sk = subkey(BeaconStateNoImmutableValidators, key)
  db.stateStore.contains(sk).expectDb() or
    db.backend.contains(sk).expectDb() or
    db.backend.contains(subkey(BeaconState, key)).expectDb

proc containsState*(db: BeaconChainDB, key: Eth2Digest): bool =
  db.statesNoVal.contains(key.data).expectDb or db.v0.containsState(key)

iterator getAncestors*(db: BeaconChainDB, root: Eth2Digest):
    TrustedSignedBeaconBlock =
  ## Load a chain of ancestors for blck - returns a list of blocks with the
  ## oldest block last (blck will be at result[0]).
  ##
  ## The search will go on until the ancestor cannot be found.

  var
    res: TrustedSignedBeaconBlock
    root = root
  while db.blocks.getSnappySSZ(root.data, res) == GetResult.found or
        db.v0.backend.getSnappySSZ(
          subkey(SignedBeaconBlock, root), res) == GetResult.found:
    res.root = root
    yield res
    root = res.message.parent_root

proc loadSummaries(db: BeaconChainDB): Table[Eth2Digest, BeaconBlockSummary] =
  # Load summaries into table - there's no telling what order they're in so we
  # load them all - bugs in nim prevent this code from living in the iterator.
  var summaries = initTable[Eth2Digest, BeaconBlockSummary](1024*1024)

  discard db.summaries.find([], proc(k, v: openArray[byte]) =
    var output: BeaconBlockSummary

    if k.len() == sizeof(Eth2Digest) and decodeSSz(v, output):
      summaries[Eth2Digest(data: toArray(sizeof(Eth2Digest), k))] = output
    else:
      warn "Invalid summary in database", klen = k.len(), vlen = v.len()
  )

  summaries

iterator getAncestorSummaries*(db: BeaconChainDB, root: Eth2Digest):
    tuple[root: Eth2Digest, summary: BeaconBlockSummary] =
  ## Load a chain of ancestors for blck - returns a list of blocks with the
  ## oldest block last (blck will be at result[0]).
  ##
  ## The search will go on until the ancestor cannot be found.

  # Summaries are loaded from the dedicated summaries table. For backwards
  # compatibility, we also load from `kvstore` and finally, if no summaries
  # can be found, by loading the blocks instead.

  # First, load the full summary table into memory in one query - this makes
  # initial startup very fast.
  var
    summaries = db.loadSummaries()
    res: tuple[root: Eth2Digest, summary: BeaconBlockSummary]
    blck: TrustedSignedBeaconBlock
    foundOldSummary = false

  res.root = root

  # Yield summaries in reverse chain order by walking the parent references.
  # If a summary is missing, try loading it from the older version or create one
  # from block data.
  while true:
    summaries.withValue(res.root, summary) do:
      res.summary = summary[]
      yield res
    do: # Summary was not found in summary table, look elsewhere
      if db.v0.backend.getSnappySSZ(subkey(BeaconBlockSummary, res.root), res.summary) == GetResult.found:
        yield res
      elif db.v0.backend.getSnappySSZ(subkey(SignedBeaconBlock, res.root), blck) == GetResult.found:
        res.summary = blck.message.toBeaconBlockSummary()
        yield res
      else:
        break
      # Next time, load them from the right place
      db.putBeaconBlockSummary(res.root, res.summary)

    res.root = res.summary.parent_root

  if false:
    # When the current version has been online for a bit, we can safely remove
    # summaries from kvstore by enabling this little snippet - if users were
    # to downgrade after the summaries have been purged, the old versions that
    # use summaries can also recreate them on the fly from blocks.
    db.db.exec(
      "DELETE FROM kvstore WHERE key >= ? and key < ?",
      ([byte ord(kHashToBlockSummary)], [byte ord(kHashToBlockSummary) + 1])).expectDb()