Introduce slot->BlockRef mapping for finalized chain (#3144)

* Introduce slot->BlockRef mapping for finalized chain

The finalized chain is linear, thus we can use a seq to lookup blocks by
slot number.

Here, we introduce such a seq, even though in the future, it should
likely be backed by a database structure instead, or, more likely, a
flat era file with a flat lookup index.

This dramatically speeds up requests by slot, such as those coming from
the REST interface or GetBlocksByRange, as these are currently served by
a linear iteration from head.

* fix REST block requests to not return blocks from an earlier slot when
the given slot is empty
* fix StateId interpretation such that it doesn't treat state roots as
block roots
* don't load full block from database just to return its root

AllTests-mainnet.md

@@ -92,7 +92,6 @@ OK: 5/5 Fail: 0/5 Skip: 0/5
 OK: 1/1 Fail: 0/1 Skip: 0/1
 ## BlockRef and helpers [Preset: mainnet]
 ```diff
-+ epochAncestor sanity [Preset: mainnet]                                                     OK
 + get_ancestor sanity [Preset: mainnet]                                                      OK
 + isAncestorOf sanity [Preset: mainnet]                                                      OK
 ```

beacon_chain/consensus_object_pools/block_pools_types.nim

@@ -89,6 +89,10 @@ type
     ## Directed acyclic graph of blocks pointing back to a finalized block on the chain we're
     ## interested in - we call that block the tail
 
+    finalizedBlocks*: seq[BlockRef] ##\
+    ## Slot -> BlockRef mapping for the canonical chain - use getBlockBySlot
+    ## to access, generally
+
     genesis*: BlockRef ##\
     ## The genesis block of the network
 

beacon_chain/consensus_object_pools/blockchain_dag.nim

@@ -137,6 +137,8 @@ func validatorKey*(
   ## non-head branch)!
   validatorKey(epochRef.dag, index)
 
+func epochAncestor*(dag: ChainDAGRef, blck: BlockRef, epoch: Epoch): EpochKey
+
 func init*(
     T: type EpochRef, dag: ChainDAGRef, state: StateData,
     cache: var StateCache): T =
@@ -144,7 +146,7 @@ func init*(
     epoch = state.data.get_current_epoch()
     epochRef = EpochRef(
       dag: dag, # This gives access to the validator pubkeys through an EpochRef
-      key: state.blck.epochAncestor(epoch),
+      key: epochAncestor(dag, state.blck, epoch),
       eth1_data: getStateField(state.data, eth1_data),
       eth1_deposit_index: getStateField(state.data, eth1_deposit_index),
       current_justified_checkpoint:
@@ -251,7 +253,21 @@ func atEpochStart*(blck: BlockRef, epoch: Epoch): BlockSlot =
   ## Return the BlockSlot corresponding to the first slot in the given epoch
   atSlot(blck, epoch.compute_start_slot_at_epoch)
 
-func epochAncestor*(blck: BlockRef, epoch: Epoch): EpochKey =
+func getBlockBySlot*(dag: ChainDAGRef, slot: Slot): BlockSlot =
+  ## Retrieve the canonical block at the given slot, or the last block that
+  ## comes before - similar to atSlot, but without the linear scan
+  if slot > dag.finalizedHead.slot:
+    return dag.head.atSlot(slot) # Linear iteration is the fastest we have
+
+  var tmp = slot.int
+  while true:
+    if dag.finalizedBlocks[tmp] != nil:
+      return dag.finalizedBlocks[tmp].atSlot(slot)
+    if tmp == 0:
+      raiseAssert "At least the genesis block should be available!"
+    tmp = tmp - 1
+
+func epochAncestor*(dag: ChainDAGRef, blck: BlockRef, epoch: Epoch): EpochKey =
   ## The state transition works by storing information from blocks in a
   ## "working" area until the epoch transition, then batching work collected
   ## during the epoch. Thus, last block in the ancestor epochs is the block
@@ -260,15 +276,17 @@ func epochAncestor*(blck: BlockRef, epoch: Epoch): EpochKey =
   ## This function returns a BlockSlot pointing to that epoch boundary, ie the
   ## boundary where the last block has been applied to the state and epoch
   ## processing has been done.
-  var blck = blck
+  let blck =
-  while blck.slot.epoch >= epoch and not blck.parent.isNil:
+    if epoch == GENESIS_EPOCH:
-    blck = blck.parent
+      dag.genesis
+    else:
+      dag.getBlockBySlot(compute_start_slot_at_epoch(epoch) - 1).blck
 
   EpochKey(epoch: epoch, blck: blck)
 
 func findEpochRef*(
     dag: ChainDAGRef, blck: BlockRef, epoch: Epoch): EpochRef = # may return nil!
-  let ancestor = blck.epochAncestor(epoch)
+  let ancestor = epochAncestor(dag, blck, epoch)
   doAssert ancestor.blck != nil
   for i in 0..<dag.epochRefs.len:
     if dag.epochRefs[i] != nil and dag.epochRefs[i].key == ancestor:
@@ -540,6 +558,13 @@ proc init*(T: type ChainDAGRef, cfg: RuntimeConfig, db: BeaconChainDB,
                            tailRef.slot.epoch)
   dag.finalizedHead = headRef.atEpochStart(finalizedEpoch)
 
+  block:
+    dag.finalizedBlocks.setLen(dag.finalizedHead.slot.int + 1)
+    var tmp = dag.finalizedHead.blck
+    while not isNil(tmp):
+      dag.finalizedBlocks[tmp.slot.int] = tmp
+      tmp = tmp.parent
+
   dag.clearanceState = dag.headState
 
   # Pruning metadata
@@ -605,7 +630,7 @@ proc getEpochRef*(dag: ChainDAGRef, blck: BlockRef, epoch: Epoch): EpochRef =
   beacon_state_data_cache_misses.inc
 
   let
-    ancestor = blck.epochAncestor(epoch)
+    ancestor = epochAncestor(dag, blck, epoch)
 
   dag.withState(
       dag.epochRefState, ancestor.blck.atEpochStart(ancestor.epoch)):
@@ -724,7 +749,7 @@ func getBlockRange*(
     endSlot = startSlot + extraBlocks * skipStep
 
   var
-    b = dag.head.atSlot(endSlot)
+    b = dag.getBlockBySlot(endSlot)
     o = output.len
 
   # Process all blocks that follow the start block (may be zero blocks)
@@ -743,11 +768,6 @@ func getBlockRange*(
 
   o # Return the index of the first non-nil item in the output
 
-func getBlockBySlot*(dag: ChainDAGRef, slot: Slot): BlockSlot =
-  ## Retrieves the first block in the current canonical chain
-  ## with slot number less or equal to `slot`.
-  dag.head.atSlot(slot)
-
 proc getForkedBlock*(dag: ChainDAGRef, blck: BlockRef): ForkedTrustedSignedBeaconBlock =
   case dag.cfg.blockForkAtEpoch(blck.slot.epoch)
   of BeaconBlockFork.Phase0:
@@ -1318,6 +1338,16 @@ proc updateHead*(
       finalized = shortLog(getStateField(
         dag.headState.data, finalized_checkpoint))
 
+    block:
+      # Update `dag.finalizedBlocks` with all newly finalized blocks (those
+      # newer than the previous finalized head), then update `dag.finalizedHead`
+
+      dag.finalizedBlocks.setLen(finalizedHead.slot.int + 1)
+      var tmp = finalizedHead.blck
+      while not isNil(tmp) and tmp.slot >= dag.finalizedHead.slot:
+        dag.finalizedBlocks[tmp.slot.int] = tmp
+        tmp = tmp.parent
+
       dag.finalizedHead = finalizedHead
 
     beacon_finalized_epoch.set(getStateField(

beacon_chain/rpc/rest_beacon_api.nim

@@ -686,20 +686,16 @@ proc installBeaconApiHandlers*(router: var RestRouter, node: BeaconNode) =
                                          $rroot.error())
       return RestApiResponse.jsonError(Http500, NoImplementationError)
 
-    let bdata =
+    let blck =
       block:
-        let head =
+        let res = node.getCurrentBlock(qslot)
-          block:
-            let res = node.getCurrentHead(qslot)
         if res.isErr():
-              return RestApiResponse.jsonError(Http404, SlotNotFoundError,
+          return RestApiResponse.jsonError(Http404, BlockNotFoundError,
                                             $res.error())
         res.get()
-        let blockSlot = head.atSlot(qslot)
-        if isNil(blockSlot.blck):
-          return RestApiResponse.jsonError(Http404, BlockNotFoundError)
-        node.dag.get(blockSlot.blck)
 
+
+    let bdata = node.dag.get(blck)
     return
       withBlck(bdata.data):
         RestApiResponse.jsonResponse(
@@ -871,18 +867,16 @@ proc installBeaconApiHandlers*(router: var RestRouter, node: BeaconNode) =
   # https://ethereum.github.io/beacon-APIs/#/Beacon/getBlockRoot
   router.api(MethodGet, "/api/eth/v1/beacon/blocks/{block_id}/root") do (
     block_id: BlockIdent) -> RestApiResponse:
-    let bdata =
+    let blck =
       block:
         if block_id.isErr():
           return RestApiResponse.jsonError(Http400, InvalidBlockIdValueError,
                                            $block_id.error())
-        let res = node.getBlockDataFromBlockIdent(block_id.get())
+        let res = node.getBlockRef(block_id.get())
         if res.isErr():
           return RestApiResponse.jsonError(Http404, BlockNotFoundError)
         res.get()
-    return
+    return RestApiResponse.jsonResponse((root: blck.root))
-      withBlck(bdata.data):
-        RestApiResponse.jsonResponse((root: blck.root))
 
   # https://ethereum.github.io/beacon-APIs/#/Beacon/getBlockAttestations
   router.api(MethodGet,

beacon_chain/rpc/rest_utils.nim

@@ -40,8 +40,22 @@ proc validate(key: string, value: string): int =
   else:
     1
 
-proc getCurrentHead*(node: BeaconNode,
+func getCurrentSlot*(node: BeaconNode, slot: Slot):
-                     slot: Slot): Result[BlockRef, cstring] =
+    Result[Slot, cstring] =
+  if slot <= (node.dag.head.slot + (SLOTS_PER_EPOCH * 2)):
+    ok(slot)
+  else:
+    err("Requesting slot too far ahead of the current head")
+
+func getCurrentBlock*(node: BeaconNode, slot: Slot):
+    Result[BlockRef, cstring] =
+  let bs = node.dag.getBlockBySlot(? node.getCurrentSlot(slot))
+  if bs.slot == bs.blck.slot:
+    ok(bs.blck)
+  else:
+    err("Block not found")
+
+proc getCurrentHead*(node: BeaconNode, slot: Slot): Result[BlockRef, cstring] =
   let res = node.dag.head
   # if not(node.isSynced(res)):
   #   return err("Cannot fulfill request until node is synced")
@@ -62,16 +76,13 @@ proc getBlockSlot*(node: BeaconNode,
                    stateIdent: StateIdent): Result[BlockSlot, cstring] =
   case stateIdent.kind
   of StateQueryKind.Slot:
-    let head = ? getCurrentHead(node, stateIdent.slot)
+    ok(node.dag.getBlockBySlot(? node.getCurrentSlot(stateIdent.slot)))
-    let bslot = head.atSlot(stateIdent.slot)
-    if isNil(bslot.blck):
-      return err("Block not found")
-    ok(bslot)
   of StateQueryKind.Root:
-    let blckRef = node.dag.getRef(stateIdent.root)
+    if stateIdent.root == getStateRoot(node.dag.headState.data):
-    if isNil(blckRef):
+      ok(node.dag.headState.blck.toBlockSlot())
-      return err("Block not found")
+    else:
-    ok(blckRef.toBlockSlot())
+      # We don't have a state root -> BlockSlot mapping
+      err("State not found")
   of StateQueryKind.Named:
     case stateIdent.value
     of StateIdentType.Head:
@@ -84,28 +95,29 @@ proc getBlockSlot*(node: BeaconNode,
       ok(node.dag.head.atEpochStart(getStateField(
         node.dag.headState.data, current_justified_checkpoint).epoch))
 
-proc getBlockDataFromBlockIdent*(node: BeaconNode,
+proc getBlockRef*(node: BeaconNode,
-                                 id: BlockIdent): Result[BlockData, cstring] =
+                   id: BlockIdent): Result[BlockRef, cstring] =
   case id.kind
   of BlockQueryKind.Named:
     case id.value
     of BlockIdentType.Head:
-      ok(node.dag.get(node.dag.head))
+      ok(node.dag.head)
     of BlockIdentType.Genesis:
-      ok(node.dag.getGenesisBlockData())
+      ok(node.dag.genesis)
     of BlockIdentType.Finalized:
-      ok(node.dag.get(node.dag.finalizedHead.blck))
+      ok(node.dag.finalizedHead.blck)
   of BlockQueryKind.Root:
-    let res = node.dag.get(id.root)
+    let res = node.dag.getRef(id.root)
-    if res.isNone():
+    if isNil(res):
-      return err("Block not found")
+      err("Block not found")
-    ok(res.get())
+    else:
+      ok(res)
   of BlockQueryKind.Slot:
-    let head = ? node.getCurrentHead(id.slot)
+    node.getCurrentBlock(id.slot)
-    let blockSlot = head.atSlot(id.slot)
+
-    if isNil(blockSlot.blck):
+proc getBlockDataFromBlockIdent*(node: BeaconNode,
-      return err("Block not found")
+                                 id: BlockIdent): Result[BlockData, cstring] =
-    ok(node.dag.get(blockSlot.blck))
+  ok(node.dag.get(? node.getBlockRef(id)))
 
 template withStateForBlockSlot*(node: BeaconNode,
                                 blockSlot: BlockSlot, body: untyped): untyped =

beacon_chain/rpc/rest_validator_api.nim

@@ -259,7 +259,7 @@ proc installValidatorApiHandlers*(router: var RestRouter, node: BeaconNode) =
       # in order to compute the sync committee for the epoch. See the following
       # discussion for more details:
       # https://github.com/status-im/nimbus-eth2/pull/3133#pullrequestreview-817184693
-      node.withStateForBlockSlot(node.dag.head.atSlot(earliestSlotInQSyncPeriod)):
+      node.withStateForBlockSlot(node.dag.getBlockBySlot(earliestSlotInQSyncPeriod)):
         let res = withState(stateData().data):
           when stateFork >= BeaconStateFork.Altair:
             produceResponse(indexList,

tests/test_blockchain_dag.nim

@@ -68,22 +68,6 @@ suite "BlockRef and helpers" & preset():
       s4.get_ancestor(Slot(3)) == s2
       s4.get_ancestor(Slot(4)) == s4
 
-  test "epochAncestor sanity" & preset():
-    let
-      s0 = BlockRef(slot: Slot(0))
-    var cur = s0
-    for i in 1..SLOTS_PER_EPOCH * 2:
-      cur = BlockRef(slot: Slot(i), parent: cur)
-
-    let ancestor = cur.epochAncestor(cur.slot.epoch)
-
-    check:
-      ancestor.epoch == cur.slot.epoch
-      ancestor.blck != cur # should have selected a parent
-
-      ancestor.blck.epochAncestor(cur.slot.epoch) == ancestor
-      ancestor.blck.epochAncestor(ancestor.blck.slot.epoch) != ancestor
-
 suite "BlockSlot and helpers" & preset():
   test "atSlot sanity" & preset():
     let