blockchain: Add pruning support

This change is part of the effort to add pruning support to btcd.

A field for pruning is added and the BlockChain struct is now able to be
configured with pruning.  Prune is called on every block connect and
the prune target field is passed to PruneBlocks func.  There's no check
to keep the latest 288 blocks to abide by the NODE_NETWORK_LIMITED rule.
That'll have to be enforced by the caller creating the BlockChain
struct.

blockchain/chain.go

@@ -115,6 +115,10 @@ type BlockChain struct {
 	// fields in this struct below this point.
 	chainLock sync.RWMutex
 
+	// pruneTarget is the size in bytes the database targets for when the node
+	// is pruned.
+	pruneTarget uint64
+
 	// These fields are related to the memory block index.  They both have
 	// their own locks, however they are often also protected by the chain
 	// lock to help prevent logic races when blocks are being processed.
@@ -600,6 +604,26 @@ func (b *BlockChain) connectBlock(node *blockNode, block *btcutil.Block,
 
 	// Atomically insert info into the database.
 	err = b.db.Update(func(dbTx database.Tx) error {
+		// If the pruneTarget isn't 0, we should attempt to delete older blocks
+		// from the database.
+		if b.pruneTarget != 0 {
+			// When the total block size is under the prune target, prune blocks is
+			// a no-op and the deleted hashes are nil.
+			deletedHashes, err := dbTx.PruneBlocks(b.pruneTarget)
+			if err != nil {
+				return err
+			}
+
+			// Only attempt to delete if we have any deleted blocks.
+			if len(deletedHashes) != 0 {
+				// Delete the spend journals of the pruned blocks.
+				err = dbPruneSpendJournalEntry(dbTx, deletedHashes)
+				if err != nil {
+					return err
+				}
+			}
+		}
+
 		// Update best block state.
 		err := dbPutBestState(dbTx, state, node.workSum)
 		if err != nil {
@@ -1702,6 +1726,11 @@ type Config struct {
 	// This field can be nil if the caller is not interested in using a
 	// signature cache.
 	HashCache *txscript.HashCache
+
+	// Prune specifies the target database usage (in bytes) the database
+	// will target for with block files.  Prune at 0 specifies that no
+	// blocks will be deleted.
+	Prune uint64
 }
 
 // New returns a BlockChain instance using the provided configuration details.
@@ -1757,6 +1786,7 @@ func New(config *Config) (*BlockChain, error) {
 		prevOrphans:         make(map[chainhash.Hash][]*orphanBlock),
 		warningCaches:       newThresholdCaches(vbNumBits),
 		deploymentCaches:    newThresholdCaches(chaincfg.DefinedDeployments),
+		pruneTarget:         config.Prune,
 	}
 
 	// Ensure all the deployments are synchronized with our clock if

blockchain/chainio.go

@@ -494,6 +494,21 @@ func dbRemoveSpendJournalEntry(dbTx database.Tx, blockHash *chainhash.Hash) erro
 	return spendBucket.Delete(blockHash[:])
 }
 
+// dbPruneSpendJournalEntry uses an existing database transaction to remove all
+// the spend journal entries for the pruned blocks.
+func dbPruneSpendJournalEntry(dbTx database.Tx, blockHashes []chainhash.Hash) error {
+	spendBucket := dbTx.Metadata().Bucket(spendJournalBucketName)
+
+	for _, blockHash := range blockHashes {
+		err := spendBucket.Delete(blockHash[:])
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
 // -----------------------------------------------------------------------------
 // The unspent transaction output (utxo) set consists of an entry for each
 // unspent output using a format that is optimized to reduce space using domain