blockcache: add blockcache package

This commit adds a new blockcache package along with the GetBlock method
to be used along with the blockcache.

blockcache/blockcache.go

@@ -0,0 +1,70 @@
+package blockcache
+
+import (
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
+	"github.com/btcsuite/btcd/wire"
+	"github.com/btcsuite/btcutil"
+	"github.com/lightninglabs/neutrino/cache"
+	"github.com/lightninglabs/neutrino/cache/lru"
+	"github.com/lightningnetwork/lnd/lntypes"
+	"github.com/lightningnetwork/lnd/multimutex"
+)
+
+// BlockCache is an lru cache for blocks.
+type BlockCache struct {
+	Cache     *lru.Cache
+	HashMutex *multimutex.HashMutex
+}
+
+// NewBlockCache creates a new BlockCache with the given maximum capacity.
+func NewBlockCache(capacity uint64) *BlockCache {
+	return &BlockCache{
+		Cache:     lru.NewCache(capacity),
+		HashMutex: multimutex.NewHashMutex(),
+	}
+}
+
+// GetBlock first checks to see if the BlockCache already contains the block
+// with the given hash. If it does then the block is fetched from the cache and
+// returned. Otherwise the getBlockImpl function is used in order to fetch the
+// new block and then it is stored in the block cache and returned.
+func (bc *BlockCache) GetBlock(hash *chainhash.Hash,
+	getBlockImpl func(hash *chainhash.Hash) (*wire.MsgBlock,
+		error)) (*wire.MsgBlock, error) {
+
+	bc.HashMutex.Lock(lntypes.Hash(*hash))
+	defer bc.HashMutex.Unlock(lntypes.Hash(*hash))
+
+	// Create an inv vector for getting the block.
+	inv := wire.NewInvVect(wire.InvTypeWitnessBlock, hash)
+
+	// Check if the block corresponding to the given hash is already
+	// stored in the blockCache and return it if it is.
+	cacheBlock, err := bc.Cache.Get(*inv)
+	if err != nil && err != cache.ErrElementNotFound {
+		return nil, err
+	}
+	if cacheBlock != nil {
+		return cacheBlock.(*cache.CacheableBlock).MsgBlock(), nil
+	}
+
+	// Fetch the block from the chain backends.
+	block, err := getBlockImpl(hash)
+	if err != nil {
+		return nil, err
+	}
+
+	// Add the new block to blockCache. If the Cache is at its maximum
+	// capacity then the LFU item will be evicted in favour of this new
+	// block.
+	_, err = bc.Cache.Put(
+		*inv, &cache.CacheableBlock{
+			Block: btcutil.NewBlock(block),
+		},
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	return block, nil
+}