avoid state copy in state transition (#930)

In BlockPool, we keep the head state around, so it's trivial to restore
the temporary state there and keep going as if nothing happened.

This solves 3 problems:
* stack space - the state copy on mainnet is huge
* GC scanning - using stack space for state slows down the GC
significantly
* reckless copying - the copy itself takes a long time

In state_sim, we'll do the same and allocate on heap - this helps a
little with GC - without it, the collection of the temporary strings
created with `toHex` while printing the json dominates the trace.

beacon_chain/block_pool.nim

@@ -398,7 +398,17 @@ proc add*(
     # TODO if the block is from the future, we should not be resolving it (yet),
     #      but maybe we should use it as a hint that our clock is wrong?
     updateStateData(pool, pool.tmpState, BlockSlot(blck: parent, slot: blck.slot - 1))
-    if not state_transition(pool.tmpState.data, signedBlock, {}):
+
+    let
+      poolPtr = unsafeAddr pool # safe because restore is short-lived
+    proc restore(v: var HashedBeaconState) =
+      # TODO address this ugly workaround - there should probably be a
+      #      `state_transition` that takes a `StateData` instead and updates
+      #      the block as well
+      doAssert v.addr == addr poolPtr.tmpState.data
+      poolPtr.tmpState = poolPtr.headState
+
+    if not state_transition(pool.tmpState.data, signedBlock, {}, restore):
       # TODO find a better way to log all this block data
       notice "Invalid block",
         blck = shortLog(blck),
@@ -561,26 +571,6 @@ func checkMissing*(pool: var BlockPool): seq[FetchRecord] =
     if v.tries.popcount() == 1:
       result.add(FetchRecord(root: k, historySlots: v.slots))
 
-proc skipAndUpdateState(
-    state: var HashedBeaconState, slot: Slot,
-    afterUpdate: proc (state: HashedBeaconState) {.gcsafe.}) =
-  while state.data.slot < slot:
-    # Process slots one at a time in case afterUpdate needs to see empty states
-    process_slots(state, state.data.slot + 1)
-    afterUpdate(state)
-
-proc skipAndUpdateState(
-    state: var HashedBeaconState, signedBlock: SignedBeaconBlock, flags: UpdateFlags,
-    afterUpdate: proc (state: HashedBeaconState) {.gcsafe.}): bool =
-
-  skipAndUpdateState(state, signedBlock.message.slot - 1, afterUpdate)
-
-  let ok = state_transition(state, signedBlock, flags)
-
-  afterUpdate(state)
-
-  ok
-
 proc putState(pool: BlockPool, state: HashedBeaconState, blck: BlockRef) =
   # TODO we save state at every epoch start but never remove them - we also
   #      potentially save multiple states per slot if reorgs happen, meaning
@@ -636,6 +626,31 @@ proc putState(pool: BlockPool, state: HashedBeaconState, blck: BlockRef) =
     # TODO this should be atomic with the above write..
     currentCache.putStateRoot(blck.root, state.data.slot, state.root)
 
+proc skipAndUpdateState(
+    pool: BlockPool,
+    state: var HashedBeaconState, blck: BlockRef, slot: Slot) =
+  while state.data.slot < slot:
+    # Process slots one at a time in case afterUpdate needs to see empty states
+    process_slots(state, state.data.slot + 1)
+    pool.putState(state, blck)
+
+proc skipAndUpdateState(
+    pool: BlockPool,
+    state: var StateData, blck: BlockData, flags: UpdateFlags): bool =
+
+  pool.skipAndUpdateState(state.data, blck.refs, blck.data.message.slot - 1)
+
+  var statePtr = unsafeAddr state # safe because `restore` is locally scoped
+  proc restore(v: var HashedBeaconState) =
+    doAssert (addr(statePtr.data) == addr v)
+    statePtr[] = pool.headState
+
+  let ok  = state_transition(state.data, blck.data, flags, restore)
+  if ok:
+    pool.putState(state.data, blck.refs)
+
+  ok
+
 proc rewindState(pool: BlockPool, state: var StateData, bs: BlockSlot):
     seq[BlockData] =
   logScope: pcs = "replay_state"
@@ -765,8 +780,7 @@ proc updateStateData*(pool: BlockPool, state: var StateData, bs: BlockSlot) =
   if state.blck.root == bs.blck.root and state.data.data.slot <= bs.slot:
     if state.data.data.slot != bs.slot:
       # Might be that we're moving to the same block but later slot
-      skipAndUpdateState(state.data, bs.slot) do(state: HashedBeaconState):
-        pool.putState(state, bs.blck)
+      pool.skipAndUpdateState(state.data, bs.blck, bs.slot)
 
     return # State already at the right spot
 
@@ -785,14 +799,12 @@ proc updateStateData*(pool: BlockPool, state: var StateData, bs: BlockSlot) =
   # applied. Pathologically quadratic in slot number, naïvely.
   for i in countdown(ancestors.len - 1, 0):
     let ok =
-      skipAndUpdateState(state.data,
-                         ancestors[i].data,
-                         {skipBlsValidation, skipMerkleValidation, skipStateRootValidation}) do (state: HashedBeaconState):
-        pool.putState(state, ancestors[i].refs)
+      pool.skipAndUpdateState(
+        state, ancestors[i],
+        {skipBlsValidation, skipMerkleValidation, skipStateRootValidation})
     doAssert ok, "Blocks in database should never fail to apply.."
 
-  skipAndUpdateState(state.data, bs.slot) do(state: HashedBeaconState):
-    pool.putState(state, bs.blck)
+  pool.skipAndUpdateState(state.data, bs.blck, bs.slot)
 
   state.blck = bs.blck
 

beacon_chain/state_transition.nim

@@ -138,8 +138,15 @@ proc verifyStateRoot(state: BeaconState, blck: BeaconBlock): bool =
   else:
     true
 
+type
+  RollbackProc* = proc(v: var BeaconState) {.gcsafe.}
+
+proc noRollback*(state: var BeaconState) =
+  trace "Skipping rollback of broken state"
+
 proc state_transition*(
-    state: var BeaconState, signedBlock: SignedBeaconBlock, flags: UpdateFlags): bool {.nbench.}=
+    state: var BeaconState, signedBlock: SignedBeaconBlock, flags: UpdateFlags,
+    rollback: RollbackProc): bool {.nbench.} =
   ## Time in the beacon chain moves by slots. Every time (haha.) that happens,
   ## we will update the beacon state. Normally, the state updates will be driven
   ## by the contents of a new block, but it may happen that the block goes
@@ -151,6 +158,12 @@ proc state_transition*(
   ## The flags are used to specify that certain validations should be skipped
   ## for the new block. This is done during block proposal, to create a state
   ## whose hash can be included in the new block.
+  ##
+  ## `rollback` is called if the transition fails and the given state has been
+  ## partially changed. If a temporary state was given to `state_transition`,
+  ## it is safe to use `noRollback` and leave it broken, else the state
+  ## object should be rolled back to a consistent state. If the transition fails
+  ## before the state has been updated, `rollback` will not be called.
   #
   # TODO this function can be written with a loop inside to handle all empty
   #      slots up to the slot of the new_block - but then again, why not eagerly
@@ -167,11 +180,7 @@ proc state_transition*(
   #      many functions will mutate `state` partially without rolling back
   #      the changes in case of failure (look out for `var BeaconState` and
   #      bool return values...)
-
-  ## TODO, of cacheState/processEpoch/processSlot/processBloc, only the last
-  ## might fail, so should this bother capturing here, or?
-  var old_state = newClone(state)
-
+  doAssert not rollback.isNil, "use noRollback if it's ok to mess up state"
   # These should never fail.
   process_slots(state, signedBlock.message.slot)
 
@@ -194,7 +203,7 @@ proc state_transition*(
         return true
 
   # Block processing failed, roll back changes
-  state = old_state[]
+  rollback(state)
   false
 
 # Hashed-state transition functions
@@ -250,11 +259,16 @@ proc process_slots*(state: var HashedBeaconState, slot: Slot) =
         trace "Couldn't update metrics", msg = e.msg
     state.root = hash_tree_root(state.data)
 
-proc state_transition*(
-    state: var HashedBeaconState, signedBlock: SignedBeaconBlock, flags: UpdateFlags): bool =
-  # Save for rollback
-  var old_state = clone(state)
+type
+  RollbackHashedProc* = proc(state: var HashedBeaconState) {.gcsafe.}
 
+proc noRollback*(state: var HashedBeaconState) =
+  trace "Skipping rollback of broken state"
+
+proc state_transition*(
+    state: var HashedBeaconState, signedBlock: SignedBeaconBlock,
+    flags: UpdateFlags, rollback: RollbackHashedProc): bool =
+  doAssert not rollback.isNil, "use noRollback if it's ok to mess up state"
   process_slots(state, signedBlock.message.slot)
 
   if skipBLSValidation in flags or
@@ -275,6 +289,6 @@ proc state_transition*(
         return true
 
   # Block processing failed, roll back changes
-  state.data[] = old_state.data[]
-  state.root = old_state.root
+  rollback(state)
+
   false

nbench/scenarios.nim

@@ -149,7 +149,8 @@ proc runFullTransition*(dir, preState, blocksPrefix: string, blocksQty: int, ski
     let signedBlock = parseSSZ(blockPath, SignedBeaconBlock)
     let flags = if skipBLS: {skipBlsValidation}
                 else: {}
-    let success = state_transition(state[], signedBlock.message, flags)
+    let success = state_transition(
+      state[], signedBlock, flags, noRollback)
     echo "State transition status: ", if success: "SUCCESS ✓" else: "FAILURE ⚠️"
 
 proc runProcessSlots*(dir, preState: string, numSlots: uint64) =

ncli/ncli_transition.nim

@@ -10,7 +10,7 @@ cli do(pre: string, blck: string, post: string, verifyStateRoot = false):
     flags = if verifyStateRoot: {skipStateRootValidation} else: {}
 
   var stateY = HashedBeaconState(data: stateX, root: hash_tree_root(stateX))
-  if not state_transition(stateY, blckX, flags):
+  if not state_transition(stateY, blckX, flags, noRollback):
     error "State transition failed"
-
+  else:
     SSZ.saveFile(post, stateY.data)

nfuzz/libnfuzz.nim

@@ -106,7 +106,7 @@ proc nfuzz_attester_slashing(input: openArray[byte], output: ptr byte,
 proc nfuzz_block(input: openArray[byte], output: ptr byte,
     output_size: ptr uint, disable_bls: bool): bool {.exportc, raises: [FuzzCrashError, Defect].} =
   decodeAndProcess(BlockInput):
-    state_transition(data.state[], data.beaconBlock, flags)
+    state_transition(data.state[], data.beaconBlock, flags, noRollback)
 
 proc nfuzz_block_header(input: openArray[byte], output: ptr byte,
     output_size: ptr uint, disable_bls: bool): bool {.exportc, raises: [FuzzCrashError, Defect].} =

research/state_sim.nim

@@ -78,15 +78,13 @@ cli do(slots = SLOTS_PER_EPOCH * 6,
   echo "Generating Genesis..."
 
   let
-    genesisState =
-      initialize_beacon_state_from_eth1(Eth2Digest(), 0, deposits, flags)
-    genesisBlock = get_initial_beacon_block(genesisState[])
+    state = initialize_beacon_state_from_eth1(Eth2Digest(), 0, deposits, flags)
+  let genesisBlock = get_initial_beacon_block(state[])
 
   echo "Starting simulation..."
 
   var
     attestations = initTable[Slot, seq[Attestation]]()
-    state = genesisState
     latest_block_root = hash_tree_root(genesisBlock.message)
     timers: array[Timers, RunningStat]
     attesters: RunningStat
@@ -102,10 +100,10 @@ cli do(slots = SLOTS_PER_EPOCH * 6,
         write(stdout, ".")
 
       if last:
-        writeJson("state.json", state)
+        writeJson("state.json", state[])
     else:
-      if state.slot mod json_interval.uint64 == 0:
-        writeJson(jsonName(prefix, state.slot), state)
+      if state[].slot mod json_interval.uint64 == 0:
+        writeJson(jsonName(prefix, state.slot), state[])
         write(stdout, ":")
       else:
         write(stdout, ".")

tests/mocking/mock_attestations.nim

@@ -131,4 +131,4 @@ proc add*(state: var BeaconState, attestation: Attestation, slot: Slot) =
   signMockBlock(state, signedBlock)
 
   doAssert state_transition(
-    state, signedBlock, flags = {skipStateRootValidation})
+    state, signedBlock, flags = {skipStateRootValidation}, noRollback)

tests/mocking/mock_blocks.nim

@@ -65,4 +65,5 @@ proc applyEmptyBlock*(state: var BeaconState) =
   ## Do a state transition with an empty signed block
   ## on the current slot
   let signedBlock = mockBlock(state, state.slot, flags = {})
-  doAssert state_transition(state, signedBlock, {skipStateRootValidation})
+  doAssert state_transition(
+    state, signedBlock, {skipStateRootValidation}, noRollback)

tests/official/test_fixture_sanity_blocks.nim

@@ -44,10 +44,12 @@ proc runTest(identifier: string) =
 
         if hasPostState:
           # TODO: The EF is using invalid BLS keys so we can't verify them
-          let success = state_transition(preState[], blck, flags = {skipBlsValidation})
+          let success = state_transition(
+            preState[], blck, flags = {skipBlsValidation}, noRollback)
           doAssert success, "Failure when applying block " & $i
         else:
-          let success = state_transition(preState[], blck, flags = {})
+          let success = state_transition(
+            preState[], blck, flags = {}, noRollback)
           doAssert not success, "We didn't expect this invalid block to be processed"
 
       # check: preState.hash_tree_root() == postState.hash_tree_root()

tests/test_state_transition.nim

@@ -38,7 +38,7 @@ suiteReport "Block processing" & preset():
       previous_block_root = hash_tree_root(genesisBlock.message)
       new_block = makeTestBlock(state[], previous_block_root)
 
-    let block_ok = state_transition(state[], new_block, {})
+    let block_ok = state_transition(state[], new_block, {}, noRollback)
 
     check:
       block_ok
@@ -58,7 +58,7 @@ suiteReport "Block processing" & preset():
     for i in 1..SLOTS_PER_EPOCH.int:
       let new_block = makeTestBlock(state[], previous_block_root)
 
-      let block_ok = state_transition(state[], new_block, {})
+      let block_ok = state_transition(state[], new_block, {}, noRollback)
 
       check:
         block_ok
@@ -91,7 +91,7 @@ suiteReport "Block processing" & preset():
       new_block = makeTestBlock(state[], previous_block_root,
         attestations = @[attestation]
       )
-    discard state_transition(state[], new_block, {})
+    check state_transition(state[], new_block, {}, noRollback)
 
     check:
       # TODO epoch attestations can get multiplied now; clean up paths to