state.go

// Copyright (C) 2019-2021, Ava Labs, Inc. All rights reserved.
// See the file LICENSE for licensing terms.

package chain

import (
	"fmt"

	"github.com/prometheus/client_golang/prometheus"

	"github.com/ava-labs/avalanchego/cache"
	"github.com/ava-labs/avalanchego/cache/metercacher"
	"github.com/ava-labs/avalanchego/database"
	"github.com/ava-labs/avalanchego/ids"
	"github.com/ava-labs/avalanchego/snow/choices"
	"github.com/ava-labs/avalanchego/snow/consensus/snowman"
)

// State implements an efficient caching layer used to wrap a VM
// implementation.
type State struct {
	// getBlock retrieves a block from the VM's storage. If getBlock returns
	// a nil error, then the returned block must not have the status Unknown
	getBlock func(ids.ID) (snowman.Block, error)
	// unmarshals [b] into a block
	unmarshalBlock func([]byte) (snowman.Block, error)
	// buildBlock attempts to build a block on top of the currently preferred block
	// buildBlock should always return a block with status Processing since it should never
	// create an unknown block, and building on top of the preferred block should never yield
	// a block that has already been decided.
	buildBlock func() (snowman.Block, error)

	// getStatus returns the status of the block
	getStatus func(snowman.Block) (choices.Status, error)

	// verifiedBlocks is a map of blocks that have been verified and are
	// therefore currently in consensus.
	verifiedBlocks map[ids.ID]*BlockWrapper
	// decidedBlocks is an LRU cache of decided blocks.
	// Every value in [decidedBlocks] is a (*BlockWrapper)
	decidedBlocks cache.Cacher
	// unverifiedBlocks is an LRU cache of blocks with status processing
	// that have not yet passed verification.
	// Every value in [unverifiedBlocks] is a (*BlockWrapper)
	unverifiedBlocks cache.Cacher
	// missingBlocks is an LRU cache of missing blocks
	// Every value in [missingBlocks] is an empty struct.
	missingBlocks cache.Cacher
	// string([byte repr. of block]) --> the block's ID
	bytesToIDCache    cache.Cacher
	lastAcceptedBlock *BlockWrapper
}

// Config defines all of the parameters necessary to initialize State
type Config struct {
	// Cache configuration:
	DecidedCacheSize, MissingCacheSize, UnverifiedCacheSize, BytesToIDCacheSize int

	LastAcceptedBlock  snowman.Block
	GetBlock           func(ids.ID) (snowman.Block, error)
	UnmarshalBlock     func([]byte) (snowman.Block, error)
	BuildBlock         func() (snowman.Block, error)
	GetBlockIDAtHeight func(uint64) (ids.ID, error)
}

// Block is an interface wrapping the normal snowman.Block interface to be used in
// association with passing in a non-nil function to GetBlockIDAtHeight
type Block interface {
	snowman.Block

	SetStatus(choices.Status)
}

// produceGetStatus creates a getStatus function that infers the status of a block by using a function
// passed in from the VM that gets the block ID at a specific height. It is assumed that for any height
// less than or equal to the last accepted block, getBlockIDAtHeight returns the accepted blockID at
// the requested height.
func produceGetStatus(s *State, getBlockIDAtHeight func(uint64) (ids.ID, error)) func(snowman.Block) (choices.Status, error) {
	return func(blk snowman.Block) (choices.Status, error) {
		internalBlk, ok := blk.(Block)
		if !ok {
			return choices.Unknown, fmt.Errorf("expected block to match chain Block interface but found block of type %T", blk)
		}
		lastAcceptedHeight := s.lastAcceptedBlock.Height()
		blkHeight := internalBlk.Height()
		if blkHeight > lastAcceptedHeight {
			internalBlk.SetStatus(choices.Processing)
			return choices.Processing, nil
		}

		acceptedID, err := getBlockIDAtHeight(blkHeight)
		switch err {
		case nil:
			if acceptedID == blk.ID() {
				internalBlk.SetStatus(choices.Accepted)
				return choices.Accepted, nil
			}
			internalBlk.SetStatus(choices.Rejected)
			return choices.Rejected, nil
		case database.ErrNotFound:
			// Not found can happen if chain history is missing. In this case,
			// the block may have been accepted or rejected, it isn't possible
			// to know here.
			internalBlk.SetStatus(choices.Processing)
			return choices.Processing, nil
		default:
			return choices.Unknown, fmt.Errorf("failed to get accepted blkID at height %d", blkHeight)
		}
	}
}

func (s *State) initialize(config *Config) {
	s.verifiedBlocks = make(map[ids.ID]*BlockWrapper)
	s.getBlock = config.GetBlock
	s.buildBlock = config.BuildBlock
	s.unmarshalBlock = config.UnmarshalBlock
	if config.GetBlockIDAtHeight == nil {
		s.getStatus = func(blk snowman.Block) (choices.Status, error) { return blk.Status(), nil }
	} else {
		s.getStatus = produceGetStatus(s, config.GetBlockIDAtHeight)
	}
	s.lastAcceptedBlock = &BlockWrapper{
		Block: config.LastAcceptedBlock,
		state: s,
	}
	s.decidedBlocks.Put(config.LastAcceptedBlock.ID(), s.lastAcceptedBlock)
}

func NewState(config *Config) *State {
	c := &State{
		verifiedBlocks:   make(map[ids.ID]*BlockWrapper),
		decidedBlocks:    &cache.LRU{Size: config.DecidedCacheSize},
		missingBlocks:    &cache.LRU{Size: config.MissingCacheSize},
		unverifiedBlocks: &cache.LRU{Size: config.UnverifiedCacheSize},
		bytesToIDCache:   &cache.LRU{Size: config.BytesToIDCacheSize},
	}
	c.initialize(config)
	return c
}

func NewMeteredState(
	registerer prometheus.Registerer,
	config *Config,
) (*State, error) {
	decidedCache, err := metercacher.New(
		"decided_cache",
		registerer,
		&cache.LRU{Size: config.DecidedCacheSize},
	)
	if err != nil {
		return nil, err
	}
	missingCache, err := metercacher.New(
		"missing_cache",
		registerer,
		&cache.LRU{Size: config.MissingCacheSize},
	)
	if err != nil {
		return nil, err
	}
	unverifiedCache, err := metercacher.New(
		"unverified_cache",
		registerer,
		&cache.LRU{Size: config.UnverifiedCacheSize},
	)
	if err != nil {
		return nil, err
	}
	bytesToIDCache, err := metercacher.New(
		"bytes_to_id_cache",
		registerer,
		&cache.LRU{Size: config.BytesToIDCacheSize},
	)
	if err != nil {
		return nil, err
	}
	c := &State{
		verifiedBlocks:   make(map[ids.ID]*BlockWrapper),
		decidedBlocks:    decidedCache,
		missingBlocks:    missingCache,
		unverifiedBlocks: unverifiedCache,
		bytesToIDCache:   bytesToIDCache,
	}
	c.initialize(config)
	return c, nil
}

// SetLastAcceptedBlock sets the last accepted block to [lastAcceptedBlock]. This should be called
// with an internal block - not a wrapped block returned from state.
//
// This also flushes [lastAcceptedBlock] from missingBlocks and unverifiedBlocks to
// ensure that their contents stay valid.
func (s *State) SetLastAcceptedBlock(lastAcceptedBlock snowman.Block) error {
	if len(s.verifiedBlocks) != 0 {
		return fmt.Errorf("cannot set chain state last accepted block with non-zero number of verified blocks in processing: %d", len(s.verifiedBlocks))
	}

	// [lastAcceptedBlock] is no longer missing or unverified, so we evict it from the corresponding
	// caches.
	//
	// Note: there's no need to evict from the decided blocks cache or bytesToIDCache since their
	// contents will still be valid.
	lastAcceptedBlockID := lastAcceptedBlock.ID()
	s.missingBlocks.Evict(lastAcceptedBlockID)
	s.unverifiedBlocks.Evict(lastAcceptedBlockID)
	s.lastAcceptedBlock = &BlockWrapper{
		Block: lastAcceptedBlock,
		state: s,
	}
	s.decidedBlocks.Put(lastAcceptedBlockID, s.lastAcceptedBlock)

	return nil
}

// Flush each block cache
func (s *State) Flush() {
	s.decidedBlocks.Flush()
	s.missingBlocks.Flush()
	s.unverifiedBlocks.Flush()
	s.bytesToIDCache.Flush()
}

// GetBlock returns the BlockWrapper as snowman.Block corresponding to [blkID]
func (s *State) GetBlock(blkID ids.ID) (snowman.Block, error) {
	if blk, ok := s.getCachedBlock(blkID); ok {
		return blk, nil
	}

	if _, ok := s.missingBlocks.Get(blkID); ok {
		return nil, database.ErrNotFound
	}

	blk, err := s.getBlock(blkID)
	// If getBlock returns [database.ErrNotFound], State considers
	// this a cacheable miss.
	if err == database.ErrNotFound {
		s.missingBlocks.Put(blkID, struct{}{})
		return nil, err
	} else if err != nil {
		return nil, err
	}

	// Since this block is not in consensus, addBlockOutsideConsensus
	// is called to add [blk] to the correct cache.
	return s.addBlockOutsideConsensus(blk)
}

// getCachedBlock checks the caches for [blkID] by priority. Returning
// true if [blkID] is found in one of the caches.
func (s *State) getCachedBlock(blkID ids.ID) (snowman.Block, bool) {
	if blk, ok := s.verifiedBlocks[blkID]; ok {
		return blk, true
	}

	if blk, ok := s.decidedBlocks.Get(blkID); ok {
		return blk.(snowman.Block), true
	}

	if blk, ok := s.unverifiedBlocks.Get(blkID); ok {
		return blk.(snowman.Block), true
	}

	return nil, false
}

// GetBlockInternal returns the internal representation of [blkID]
func (s *State) GetBlockInternal(blkID ids.ID) (snowman.Block, error) {
	wrappedBlk, err := s.GetBlock(blkID)
	if err != nil {
		return nil, err
	}

	return wrappedBlk.(*BlockWrapper).Block, nil
}

// ParseBlock attempts to parse [b] into an internal Block and adds it to the appropriate
// caching layer if successful.
func (s *State) ParseBlock(b []byte) (snowman.Block, error) {
	// See if we've cached this block's ID by its byte repr.
	blkIDIntf, blkIDCached := s.bytesToIDCache.Get(string(b))
	if blkIDCached {
		blkID := blkIDIntf.(ids.ID)
		// See if we have this block cached
		if cachedBlk, ok := s.getCachedBlock(blkID); ok {
			return cachedBlk, nil
		}
	}

	// We don't have this block cached by its byte repr.
	// Parse the block from bytes
	blk, err := s.unmarshalBlock(b)
	if err != nil {
		return nil, err
	}
	blkID := blk.ID()
	s.bytesToIDCache.Put(string(b), blkID)

	// Only check the caches if we didn't do so above
	if !blkIDCached {
		// Check for an existing block, so we can return a unique block
		// if processing or simply allow this block to be immediately
		// garbage collected if it is already cached.
		if cachedBlk, ok := s.getCachedBlock(blkID); ok {
			return cachedBlk, nil
		}
	}

	s.missingBlocks.Evict(blkID)

	// Since this block is not in consensus, addBlockOutsideConsensus
	// is called to add [blk] to the correct cache.
	return s.addBlockOutsideConsensus(blk)
}

// BuildBlock attempts to build a new internal Block, wraps it, and adds it
// to the appropriate caching layer if successful.
func (s *State) BuildBlock() (snowman.Block, error) {
	blk, err := s.buildBlock()
	if err != nil {
		return nil, err
	}

	blkID := blk.ID()
	// Defensive: buildBlock should not return a block that has already been verified.
	// If it does, make sure to return the existing reference to the block.
	if existingBlk, ok := s.getCachedBlock(blkID); ok {
		return existingBlk, nil
	}
	// Evict the produced block from missing blocks in case it was previously
	// marked as missing.
	s.missingBlocks.Evict(blkID)

	// wrap the returned block and add it to the correct cache
	return s.addBlockOutsideConsensus(blk)
}

// addBlockOutsideConsensus adds [blk] to the correct cache and returns
// a wrapped version of [blk]
// assumes [blk] is a known, non-wrapped block that is not currently
// in consensus. [blk] could be either decided or a block that has not yet
// been verified and added to consensus.
func (s *State) addBlockOutsideConsensus(blk snowman.Block) (snowman.Block, error) {
	wrappedBlk := &BlockWrapper{
		Block: blk,
		state: s,
	}

	blkID := blk.ID()
	status, err := s.getStatus(blk)
	if err != nil {
		return nil, fmt.Errorf("could not get block status for %s due to %w", blkID, err)
	}
	switch status {
	case choices.Accepted, choices.Rejected:
		s.decidedBlocks.Put(blkID, wrappedBlk)
	case choices.Processing:
		s.unverifiedBlocks.Put(blkID, wrappedBlk)
	default:
		return nil, fmt.Errorf("found unexpected status for blk %s: %s", blkID, status)
	}

	return wrappedBlk, nil
}

func (s *State) LastAccepted() (ids.ID, error) {
	return s.lastAcceptedBlock.ID(), nil
}

// LastAcceptedBlock returns the last accepted wrapped block
func (s *State) LastAcceptedBlock() *BlockWrapper {
	return s.lastAcceptedBlock
}

// LastAcceptedBlockInternal returns the internal snowman.Block that was last accepted
func (s *State) LastAcceptedBlockInternal() snowman.Block {
	return s.LastAcceptedBlock().Block
}