indexers: flush utreexo state on prune

For the utreexo state to be recoverable on unexpected crashes, there
must be blocks available for it to reindex on crashes. If there aren't,
the utreexo state is irrecoverable. To prevent this from happening, we
check what the last stored block on the disk is after a prune and flush
the utreexo state if the last flush happened before the last kept block.

blockchain/indexers/addrindex.go

@@ -816,7 +816,7 @@ func (idx *AddrIndex) DisconnectBlock(dbTx database.Tx, block *btcutil.Block,
 // supported with pruning.
 //
 // This is part of the Indexer interface.
-func (idx *AddrIndex) PruneBlock(dbTx database.Tx, blockHash *chainhash.Hash) error {
+func (idx *AddrIndex) PruneBlock(_ database.Tx, _ *chainhash.Hash, _ int32) error {
 	return nil
 }
 

blockchain/indexers/cfindex.go

@@ -261,7 +261,7 @@ func (idx *CfIndex) DisconnectBlock(dbTx database.Tx, block *btcutil.Block,
 // reindexing as a pruned node.
 //
 // This is part of the Indexer interface.
-func (idx *CfIndex) PruneBlock(dbTx database.Tx, blockHash *chainhash.Hash) error {
+func (idx *CfIndex) PruneBlock(dbTx database.Tx, blockHash *chainhash.Hash, _ int32) error {
 	for _, key := range cfIndexKeys {
 		err := dbDeleteFilterIdxEntry(dbTx, key, blockHash)
 		if err != nil {

blockchain/indexers/common.go

@@ -65,7 +65,7 @@ type Indexer interface {
 
 	// PruneBlock is invoked when an older block is deleted after it's been
 	// processed.
-	PruneBlock(database.Tx, *chainhash.Hash) error
+	PruneBlock(dbTx database.Tx, deletedBlock *chainhash.Hash, lastKeptHeight int32) error
 
 	// Flush flushes the index.
 	Flush(*chainhash.Hash, blockchain.FlushMode, bool) error

blockchain/indexers/flatutreexoproofindex.go

@@ -888,8 +888,29 @@ func (idx *FlatUtreexoProofIndex) DisconnectBlock(dbTx database.Tx, block *btcut
 // processed.
 //
 // This is part of the Indexer interface.
-func (idx *FlatUtreexoProofIndex) PruneBlock(dbTx database.Tx, blockHash *chainhash.Hash) error {
-	return nil
+func (idx *FlatUtreexoProofIndex) PruneBlock(_ database.Tx, _ *chainhash.Hash, lastKeptHeight int32) error {
+	hash, _, err := dbFetchUtreexoStateConsistency(idx.utreexoState.utreexoStateDB)
+	if err != nil {
+		return err
+	}
+
+	// It's ok to call block by hash here as the utreexo state consistency hash is always
+	// included in the best chain.
+	lastFlushHeight, err := idx.chain.BlockHeightByHash(hash)
+	if err != nil {
+		return err
+	}
+
+	// If the last flushed utreexo state is the last or greater than the kept block,
+	// we can sync up to the tip so a flush is not required.
+	if lastKeptHeight <= lastFlushHeight {
+		return nil
+	}
+
+	// It's ok to fetch the best snapshot here as the block called on pruneblock has not
+	// been yet connected yet on the utreexo state. So this is indeed the correct hash.
+	bestHash := idx.chain.BestSnapshot().Hash
+	return idx.Flush(&bestHash, blockchain.FlushRequired, true)
 }
 
 // FetchUtreexoProof returns the Utreexo proof data for the given block height.

blockchain/indexers/manager.go

@@ -652,7 +652,7 @@ func (m *Manager) PruneBlocks(dbTx database.Tx, lastKeptHeight int32,
 			}
 
 			// Notify the indexer with the connected block so it can prune it.
-			err = index.PruneBlock(dbTx, blockHash)
+			err = index.PruneBlock(dbTx, blockHash, lastKeptHeight)
 			if err != nil {
 				return err
 			}

blockchain/indexers/ttlindex.go

@@ -97,7 +97,7 @@ func (idx *TTLIndex) DisconnectBlock(dbTx database.Tx, block *btcutil.Block,
 // supported with pruning.
 //
 // This is part of the Indexer interface.
-func (idx *TTLIndex) PruneBlock(dbTx database.Tx, blockHash *chainhash.Hash) error {
+func (idx *TTLIndex) PruneBlock(_ database.Tx, _ *chainhash.Hash, _ int32) error {
 	return nil
 }
 

blockchain/indexers/txindex.go

@@ -437,7 +437,7 @@ func (idx *TxIndex) DisconnectBlock(dbTx database.Tx, block *btcutil.Block,
 // supported with pruning.
 //
 // This is part of the Indexer interface.
-func (idx *TxIndex) PruneBlock(dbTx database.Tx, blockHash *chainhash.Hash) error {
+func (idx *TxIndex) PruneBlock(_ database.Tx, _ *chainhash.Hash, _ int32) error {
 	return nil
 }
 

blockchain/indexers/utreexoproofindex.go

@@ -575,8 +575,29 @@ func (idx *UtreexoProofIndex) VerifyAccProof(toProve []utreexo.Hash,
 // processed.
 //
 // This is part of the Indexer interface.
-func (idx *UtreexoProofIndex) PruneBlock(dbTx database.Tx, blockHash *chainhash.Hash) error {
-	return nil
+func (idx *UtreexoProofIndex) PruneBlock(_ database.Tx, _ *chainhash.Hash, lastKeptHeight int32) error {
+	hash, _, err := dbFetchUtreexoStateConsistency(idx.utreexoState.utreexoStateDB)
+	if err != nil {
+		return err
+	}
+
+	// It's ok to call block by hash here as the utreexo state consistency hash is always
+	// included in the best chain.
+	lastFlushHeight, err := idx.chain.BlockHeightByHash(hash)
+	if err != nil {
+		return err
+	}
+
+	// If the last flushed utreexo state is the last or greater than the kept block,
+	// we can sync up to the tip so a flush is not required.
+	if lastKeptHeight <= lastFlushHeight {
+		return nil
+	}
+
+	// It's ok to fetch the best snapshot here as the block called on pruneblock has not
+	// been yet connected yet on the utreexo state. So this is indeed the correct hash.
+	bestHash := idx.chain.BestSnapshot().Hash
+	return idx.Flush(&bestHash, blockchain.FlushRequired, true)
 }
 
 // NewUtreexoProofIndex returns a new instance of an indexer that is used to create a utreexo