lndservices: add reorg aware block header cache; use in ChainBridge

lndservices/block_header_cache.go

@@ -0,0 +1,322 @@
+package lndservices
+
+import (
+	"fmt"
+	"sync"
+
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
+	"github.com/btcsuite/btcd/wire"
+)
+
+const (
+	// DefaultHeaderCacheSize is the default maximum number of block
+	// headers to cache.
+	DefaultHeaderCacheSize = 100_000
+
+	// DefaultPurgePercentage is the default percentage of entries to purge
+	// when the cache reaches capacity (from 1 to 100).
+	DefaultPurgePercentage = 10
+
+	// DefaultMinSettledBlockDepth is the default minimum block depth
+	// required before a block header is considered settled.
+	DefaultMinSettledBlockDepth = 6
+)
+
+// BlockHeaderCacheConfig holds configuration parameters for the block header
+// cache.
+type BlockHeaderCacheConfig struct {
+	// MaxSize is the maximum number of block headers to cache.
+	MaxSize uint32
+
+	// PurgePercentage is the percentage of entries to purge when the cache
+	// reaches capacity (from 1 to 100, inclusive).
+	PurgePercentage uint32
+
+	// MinSettledBlockDepth is the minimum block depth required before a
+	// block header is considered settled.
+	MinSettledBlockDepth uint32
+}
+
+// DefaultBlockHeaderCacheConfig returns a BlockHeaderCacheConfig with default
+// values.
+func DefaultBlockHeaderCacheConfig() BlockHeaderCacheConfig {
+	return BlockHeaderCacheConfig{
+		MaxSize:              DefaultHeaderCacheSize,
+		PurgePercentage:      DefaultPurgePercentage,
+		MinSettledBlockDepth: DefaultMinSettledBlockDepth,
+	}
+}
+
+// Validate checks that the configuration parameters are valid.
+func (c *BlockHeaderCacheConfig) Validate() error {
+	if c.PurgePercentage == 0 || c.PurgePercentage > 100 {
+		return fmt.Errorf("invalid PurgePercentage: %d, must "+
+			"be > 0 and <= 100", c.PurgePercentage)
+	}
+
+	return nil
+}
+
+// headerEntry represents a cached block header with metadata.
+type headerEntry struct {
+	// header is the cached block header.
+	header wire.BlockHeader
+
+	// hash is the cached block hash.
+	hash chainhash.Hash
+
+	// height is the block height of this header.
+	height uint32
+}
+
+// BlockHeaderCache is a reorg-aware cache of block headers.
+//
+// TODO(ffranr): Once this component is stable, consider moving btcd repo.
+type BlockHeaderCache struct {
+	// cfg is the cache configuration.
+	cfg BlockHeaderCacheConfig
+
+	// mu protects concurrent access to the cache.
+	mu sync.RWMutex
+
+	// byHeight maps block height to header entry.
+	byHeight map[uint32]*headerEntry
+
+	// byHash maps block hash to header entry.
+	byHash map[chainhash.Hash]*headerEntry
+
+	// maxHeight tracks the highest block height we've seen.
+	maxHeight uint32
+}
+
+// NewBlockHeaderCache creates a new block header cache with the given
+// configuration.
+func NewBlockHeaderCache(cfg BlockHeaderCacheConfig) (*BlockHeaderCache,
+	error) {
+
+	if err := cfg.Validate(); err != nil {
+		return nil, err
+	}
+
+	return &BlockHeaderCache{
+		cfg:      cfg,
+		byHeight: make(map[uint32]*headerEntry),
+		byHash:   make(map[chainhash.Hash]*headerEntry),
+	}, nil
+}
+
+// isSettled returns whether an entry is considered settled based on
+// block depth.
+func (c *BlockHeaderCache) isSettled(height uint32) bool {
+	settledHeight := height + c.cfg.MinSettledBlockDepth
+
+	// If the maximum height among all seen block headers meets or exceeds
+	// the settled height, this entry is considered settled.
+	return settledHeight <= c.maxHeight
+}
+
+// Put adds a block header to the cache at the given height.
+//
+// If the insertion exceeded capacity, entries are purged first. If a
+// conflicting header exists at this height, a reorg is detected and all headers
+// at or above this height are invalidated.
+func (c *BlockHeaderCache) Put(height uint32, header wire.BlockHeader) error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	hash := header.BlockHash()
+
+	// Check if there's already an entry at this height.
+	if existing, exists := c.byHeight[height]; exists {
+		existingHash := existing.hash
+
+		// If the hashes match, this is a duplicate insertion.
+		if existingHash == hash {
+			return nil
+		}
+
+		// The hashes do not match, indicating a reorg. Invalidate
+		// all known headers at or above this height.
+		c.invalidateFromHeight(height)
+	}
+
+	// Check capacity and purge if needed.
+	if uint32(len(c.byHeight)) >= c.cfg.MaxSize {
+		c.purge()
+	}
+
+	// Create the new entry and store in the cache.
+	entry := &headerEntry{
+		header: header,
+		hash:   hash,
+		height: height,
+	}
+
+	c.byHeight[height] = entry
+	c.byHash[hash] = entry
+
+	// Update max height seen.
+	if height > c.maxHeight {
+		c.maxHeight = height
+	}
+
+	return nil
+}
+
+// GetByHeight retrieves a block header by height. Returns ok=false if not found
+// or if the entry is unsettled (to force external lookup).
+func (c *BlockHeaderCache) GetByHeight(height uint32) (wire.BlockHeader, bool) {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	var zero wire.BlockHeader
+
+	entry, exists := c.byHeight[height]
+	if !exists || !c.isSettled(height) {
+		return zero, false
+	}
+
+	return entry.header, true
+}
+
+// GetByHash retrieves a block header by hash. Returns ok=false if not found or
+// if the entry is unsettled (to force external lookup).
+func (c *BlockHeaderCache) GetByHash(hash chainhash.Hash) (wire.BlockHeader,
+	bool) {
+
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	var zero wire.BlockHeader
+
+	entry, exists := c.byHash[hash]
+	if !exists || !c.isSettled(entry.height) {
+		return zero, false
+	}
+
+	return entry.header, true
+}
+
+// invalidateFromHeight removes all entries at or above the given height,
+// effectively invalidating the orphaned chain.
+func (c *BlockHeaderCache) invalidateFromHeight(heightLowerBound uint32) {
+	// Track new max height after entries are removed.
+	var newMaxHeight uint32
+
+	// Iterate over all entries and remove those at or above the lower
+	// bound.
+	for height, entry := range c.byHeight {
+		// Skip entries below the lower bound.
+		if height < heightLowerBound {
+			// Update new max height if needed.
+			if height > newMaxHeight {
+				newMaxHeight = height
+			}
+
+			continue
+		}
+
+		// Remove the entry which is at or above the lower bound.
+		hash := entry.hash
+		delete(c.byHeight, height)
+		delete(c.byHash, hash)
+	}
+
+	c.maxHeight = newMaxHeight
+}
+
+// purge removes a random set of entries from the cache at the configured
+// purge percentage.
+func (c *BlockHeaderCache) purge() {
+	numToPurge := len(c.byHeight) * int(c.cfg.PurgePercentage) / 100
+	if numToPurge == 0 {
+		numToPurge = 1
+	}
+
+	// Remove entries directly from the map iteration (already random
+	// order).
+	maxHeightDeleted := false
+	count := 0
+	for height, entry := range c.byHeight {
+		if count >= numToPurge {
+			break
+		}
+
+		if height == c.maxHeight {
+			maxHeightDeleted = true
+		}
+
+		hash := entry.hash
+		delete(c.byHeight, height)
+		delete(c.byHash, hash)
+		count++
+	}
+
+	if !maxHeightDeleted {
+		return
+	}
+
+	// Recalculate max height only if it was deleted.
+	c.maxHeight = 0
+	for height := range c.byHeight {
+		if height > c.maxHeight {
+			c.maxHeight = height
+		}
+	}
+}
+
+// Size returns the current number of entries in the cache.
+func (c *BlockHeaderCache) Size() int {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	return len(c.byHeight)
+}
+
+// Clear removes all entries from the cache.
+func (c *BlockHeaderCache) Clear() {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	c.byHeight = make(map[uint32]*headerEntry)
+	c.byHash = make(map[chainhash.Hash]*headerEntry)
+	c.maxHeight = 0
+}
+
+// Stats returns statistics about the cache.
+func (c *BlockHeaderCache) Stats() CacheStats {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	settled := 0
+	for height := range c.byHeight {
+		if c.isSettled(height) {
+			settled++
+		}
+	}
+
+	return CacheStats{
+		TotalEntries:   len(c.byHeight),
+		SettledEntries: settled,
+		MaxHeight:      c.maxHeight,
+	}
+}
+
+// CacheStats holds statistics about the block header cache.
+type CacheStats struct {
+	// TotalEntries is the total number of entries in the cache.
+	TotalEntries int
+
+	// SettledEntries is the number of settled entries in the cache.
+	SettledEntries int
+
+	// MaxHeight is the highest block height seen.
+	MaxHeight uint32
+}
+
+// String returns a string representation of the cache stats.
+func (s CacheStats) String() string {
+	return fmt.Sprintf("BlockHeaderCacheStats(total=%d, settled=%d, "+
+		"max_height=%d)", s.TotalEntries, s.SettledEntries,
+		s.MaxHeight)
+}