wallet/wallet: add new recovery method

In this commit, we add a new recovery method to the wallet. This method
attempts to recover any unspent outputs which pay to any of the wallet's
addresses. Most of the logic found within it is heavily borrowed from
the existing syncWithChain method. This method is currently unused, but
it will end up replacing some of the existing sync logic in a later
commit.

wallet/wallet.go

@@ -855,6 +855,130 @@ func (w *Wallet) syncToBirthday() (*waddrmgr.BlockStamp, error) {
 	return birthdayStamp, nil
 }
 
+// recovery attempts to recover any unspent outputs that pay to any of our
+// addresses starting from the specified height.
+//
+// NOTE: The starting height must be at least the height of the wallet's
+// birthday or later.
+func (w *Wallet) recovery(startHeight int32) error {
+	log.Infof("RECOVERY MODE ENABLED -- rescanning for used addresses "+
+		"with recovery_window=%d", w.recoveryWindow)
+
+	// We'll initialize the recovery manager with a default batch size of
+	// 2000.
+	recoveryMgr := NewRecoveryManager(
+		w.recoveryWindow, recoveryBatchSize, w.chainParams,
+	)
+
+	// In the event that this recovery is being resumed, we will need to
+	// repopulate all found addresses from the database. For basic recovery,
+	// we will only do so for the default scopes.
+	scopedMgrs, err := w.defaultScopeManagers()
+	if err != nil {
+		return err
+	}
+	tx, err := w.db.BeginReadWriteTx()
+	if err != nil {
+		return err
+	}
+	txMgrNS := tx.ReadBucket(wtxmgrNamespaceKey)
+	credits, err := w.TxStore.UnspentOutputs(txMgrNS)
+	if err != nil {
+		tx.Rollback()
+		return err
+	}
+	addrMgrNS := tx.ReadWriteBucket(waddrmgrNamespaceKey)
+	err = recoveryMgr.Resurrect(addrMgrNS, scopedMgrs, credits)
+	if err != nil {
+		tx.Rollback()
+		return err
+	}
+
+	// We'll also retrieve our chain backend client in order to filter the
+	// blocks as we go.
+	chainClient, err := w.requireChainClient()
+	if err != nil {
+		tx.Rollback()
+		return err
+	}
+
+	// We'll begin scanning the chain from the specified starting height.
+	// Since we assume that the lowest height we start with will at least be
+	// that of our birthday, we can just add every block we process from
+	// this point forward to the recovery batch.
+	err = w.scanChain(startHeight, func(height int32,
+		hash *chainhash.Hash, header *wire.BlockHeader) error {
+
+		recoveryMgr.AddToBlockBatch(hash, height, header.Timestamp)
+
+		// We'll checkpoint our current batch every 2K blocks, so we'll
+		// need to start a new database transaction. If our current
+		// batch is empty, then this will act as a NOP.
+		if height%recoveryBatchSize == 0 {
+			blockBatch := recoveryMgr.BlockBatch()
+			err := w.recoverDefaultScopes(
+				chainClient, tx, addrMgrNS, blockBatch,
+				recoveryMgr.State(),
+			)
+			if err != nil {
+				return err
+			}
+
+			// Clear the batch of all processed blocks.
+			recoveryMgr.ResetBlockBatch()
+
+			if err := tx.Commit(); err != nil {
+				return err
+			}
+
+			log.Infof("Recovered addresses from blocks %d-%d",
+				blockBatch[0].Height,
+				blockBatch[len(blockBatch)-1].Height)
+
+			tx, err = w.db.BeginReadWriteTx()
+			if err != nil {
+				return err
+			}
+			addrMgrNS = tx.ReadWriteBucket(waddrmgrNamespaceKey)
+		}
+
+		// Since the recovery in a way acts as a rescan, we'll update
+		// the wallet's tip to point to the current block so that we
+		// don't unnecessarily rescan the same block again later on.
+		return w.Manager.SetSyncedTo(addrMgrNS, &waddrmgr.BlockStamp{
+			Hash:      *hash,
+			Height:    height,
+			Timestamp: header.Timestamp,
+		})
+	})
+	if err != nil {
+		tx.Rollback()
+		return err
+	}
+
+	// Now that we've reached the chain tip, we can process our final batch
+	// with the remaining blocks if it did not reach its maximum size.
+	blockBatch := recoveryMgr.BlockBatch()
+	err = w.recoverDefaultScopes(
+		chainClient, tx, addrMgrNS, blockBatch, recoveryMgr.State(),
+	)
+	if err != nil {
+		tx.Rollback()
+		return err
+	}
+
+	// With the recovery complete, we can persist our new state and exit.
+	if err := tx.Commit(); err != nil {
+		tx.Rollback()
+		return err
+	}
+
+	log.Infof("Recovered addresses from blocks %d-%d", blockBatch[0].Height,
+		blockBatch[len(blockBatch)-1].Height)
+
+	return nil
+}
+
 // defaultScopeManagers fetches the ScopedKeyManagers from the wallet using the
 // default set of key scopes.
 func (w *Wallet) defaultScopeManagers() (