package btcdnotify import ( "errors" "fmt" "sync" "sync/atomic" "time" "github.com/lightningnetwork/lnd/chainntnfs" "github.com/roasbeef/btcd/btcjson" "github.com/roasbeef/btcd/chaincfg/chainhash" "github.com/roasbeef/btcd/rpcclient" "github.com/roasbeef/btcd/wire" "github.com/roasbeef/btcutil" ) const ( // notifierType uniquely identifies this concrete implementation of the // ChainNotifier interface. notifierType = "btcd" // reorgSafetyLimit is assumed maximum depth of a chain reorganization. // After this many confirmation, transaction confirmation info will be // pruned. reorgSafetyLimit = 100 ) var ( // ErrChainNotifierShuttingDown is used when we are trying to // measure a spend notification when notifier is already stopped. ErrChainNotifierShuttingDown = errors.New("chainntnfs: system interrupt " + "while attempting to register for spend notification.") ) // chainUpdate encapsulates an update to the current main chain. This struct is // used as an element within an unbounded queue in order to avoid blocking the // main rpc dispatch rule. type chainUpdate struct { blockHash *chainhash.Hash blockHeight int32 // connected is true if this update is a new block and false if it is a // disconnected block. connect bool } // txUpdate encapsulates a transaction related notification sent from btcd to // the registered RPC client. This struct is used as an element within an // unbounded queue in order to avoid blocking the main rpc dispatch rule. type txUpdate struct { tx *btcutil.Tx details *btcjson.BlockDetails } // TODO(roasbeef): generalize struct below: // * move chans to config, allow outside callers to handle send conditions // BtcdNotifier implements the ChainNotifier interface using btcd's websockets // notifications. Multiple concurrent clients are supported. All notifications // are achieved via non-blocking sends on client channels. type BtcdNotifier struct { spendClientCounter uint64 // To be used atomically. epochClientCounter uint64 // To be used atomically. started int32 // To be used atomically. stopped int32 // To be used atomically. chainConn *rpcclient.Client notificationCancels chan interface{} notificationRegistry chan interface{} spendNotifications map[wire.OutPoint]map[uint64]*spendNotification txConfNotifier *chainntnfs.TxConfNotifier blockEpochClients map[uint64]*blockEpochRegistration chainUpdates *chainntnfs.ConcurrentQueue txUpdates *chainntnfs.ConcurrentQueue wg sync.WaitGroup quit chan struct{} } // Ensure BtcdNotifier implements the ChainNotifier interface at compile time. var _ chainntnfs.ChainNotifier = (*BtcdNotifier)(nil) // New returns a new BtcdNotifier instance. This function assumes the btcd node // detailed in the passed configuration is already running, and willing to // accept new websockets clients. func New(config *rpcclient.ConnConfig) (*BtcdNotifier, error) { notifier := &BtcdNotifier{ notificationCancels: make(chan interface{}), notificationRegistry: make(chan interface{}), blockEpochClients: make(map[uint64]*blockEpochRegistration), spendNotifications: make(map[wire.OutPoint]map[uint64]*spendNotification), chainUpdates: chainntnfs.NewConcurrentQueue(10), txUpdates: chainntnfs.NewConcurrentQueue(10), quit: make(chan struct{}), } ntfnCallbacks := &rpcclient.NotificationHandlers{ OnBlockConnected: notifier.onBlockConnected, OnBlockDisconnected: notifier.onBlockDisconnected, OnRedeemingTx: notifier.onRedeemingTx, } // Disable connecting to btcd within the rpcclient.New method. We // defer establishing the connection to our .Start() method. config.DisableConnectOnNew = true config.DisableAutoReconnect = false chainConn, err := rpcclient.New(config, ntfnCallbacks) if err != nil { return nil, err } notifier.chainConn = chainConn return notifier, nil } // Start connects to the running btcd node over websockets, registers for block // notifications, and finally launches all related helper goroutines. func (b *BtcdNotifier) Start() error { // Already started? if atomic.AddInt32(&b.started, 1) != 1 { return nil } // Connect to btcd, and register for notifications on connected, and // disconnected blocks. if err := b.chainConn.Connect(20); err != nil { return err } if err := b.chainConn.NotifyBlocks(); err != nil { return err } _, currentHeight, err := b.chainConn.GetBestBlock() if err != nil { return err } b.txConfNotifier = chainntnfs.NewTxConfNotifier( uint32(currentHeight), reorgSafetyLimit) b.chainUpdates.Start() b.txUpdates.Start() b.wg.Add(1) go b.notificationDispatcher(currentHeight) return nil } // Stop shutsdown the BtcdNotifier. func (b *BtcdNotifier) Stop() error { // Already shutting down? if atomic.AddInt32(&b.stopped, 1) != 1 { return nil } // Shutdown the rpc client, this gracefully disconnects from btcd, and // cleans up all related resources. b.chainConn.Shutdown() close(b.quit) b.wg.Wait() b.chainUpdates.Stop() b.txUpdates.Stop() // Notify all pending clients of our shutdown by closing the related // notification channels. for _, spendClients := range b.spendNotifications { for _, spendClient := range spendClients { close(spendClient.spendChan) } } for _, epochClient := range b.blockEpochClients { close(epochClient.cancelChan) epochClient.wg.Wait() close(epochClient.epochChan) } b.txConfNotifier.TearDown() return nil } // onBlockConnected implements on OnBlockConnected callback for rpcclient. // Ingesting a block updates the wallet's internal utxo state based on the // outputs created and destroyed within each block. func (b *BtcdNotifier) onBlockConnected(hash *chainhash.Hash, height int32, t time.Time) { // Append this new chain update to the end of the queue of new chain // updates. b.chainUpdates.ChanIn() <- &chainUpdate{ blockHash: hash, blockHeight: height, connect: true, } } // filteredBlock represents a new block which has been connected to the main // chain. The slice of transactions will only be populated if the block // includes a transaction that confirmed one of our watched txids, or spends // one of the outputs currently being watched. // TODO(halseth): this is currently used for complete blocks. Change to use // onFilteredBlockConnected and onFilteredBlockDisconnected, making it easier // to unify with the Neutrino implementation. type filteredBlock struct { hash chainhash.Hash height uint32 txns []*btcutil.Tx // connected is true if this update is a new block and false if it is a // disconnected block. connect bool } // onBlockDisconnected implements on OnBlockDisconnected callback for rpcclient. func (b *BtcdNotifier) onBlockDisconnected(hash *chainhash.Hash, height int32, t time.Time) { // Append this new chain update to the end of the queue of new chain // updates. b.chainUpdates.ChanIn() <- &chainUpdate{ blockHash: hash, blockHeight: height, connect: false, } } // onRedeemingTx implements on OnRedeemingTx callback for rpcclient. func (b *BtcdNotifier) onRedeemingTx(tx *btcutil.Tx, details *btcjson.BlockDetails) { // Append this new transaction update to the end of the queue of new // chain updates. b.txUpdates.ChanIn() <- &txUpdate{tx, details} } // notificationDispatcher is the primary goroutine which handles client // notification registrations, as well as notification dispatches. func (b *BtcdNotifier) notificationDispatcher(currentHeight int32) { out: for { select { case cancelMsg := <-b.notificationCancels: switch msg := cancelMsg.(type) { case *spendCancel: chainntnfs.Log.Infof("Cancelling spend "+ "notification for out_point=%v, "+ "spend_id=%v", msg.op, msg.spendID) // Before we attempt to close the spendChan, // ensure that the notification hasn't already // yet been dispatched. if outPointClients, ok := b.spendNotifications[msg.op]; ok { close(outPointClients[msg.spendID].spendChan) delete(b.spendNotifications[msg.op], msg.spendID) } case *epochCancel: chainntnfs.Log.Infof("Cancelling epoch "+ "notification, epoch_id=%v", msg.epochID) // First, we'll lookup the original // registration in order to stop the active // queue goroutine. reg := b.blockEpochClients[msg.epochID] reg.epochQueue.Stop() // Next, close the cancel channel for this // specific client, and wait for the client to // exit. close(b.blockEpochClients[msg.epochID].cancelChan) b.blockEpochClients[msg.epochID].wg.Wait() // Once the client has exited, we can then // safely close the channel used to send epoch // notifications, in order to notify any // listeners that the intent has been // cancelled. close(b.blockEpochClients[msg.epochID].epochChan) delete(b.blockEpochClients, msg.epochID) } case registerMsg := <-b.notificationRegistry: switch msg := registerMsg.(type) { case *spendNotification: chainntnfs.Log.Infof("New spend subscription: "+ "utxo=%v", msg.targetOutpoint) op := *msg.targetOutpoint if _, ok := b.spendNotifications[op]; !ok { b.spendNotifications[op] = make(map[uint64]*spendNotification) } b.spendNotifications[op][msg.spendID] = msg case *confirmationNotification: chainntnfs.Log.Infof("New confirmation "+ "subscription: txid=%v, numconfs=%v", msg.TxID, msg.NumConfirmations) // Lookup whether the transaction is already included in the // active chain. txConf, err := b.historicalConfDetails( msg.TxID, msg.heightHint, uint32(currentHeight), ) if err != nil { chainntnfs.Log.Error(err) } err = b.txConfNotifier.Register(&msg.ConfNtfn, txConf) if err != nil { chainntnfs.Log.Error(err) } case *blockEpochRegistration: chainntnfs.Log.Infof("New block epoch subscription") b.blockEpochClients[msg.epochID] = msg } case item := <-b.chainUpdates.ChanOut(): update := item.(*chainUpdate) if update.connect { if update.blockHeight != currentHeight+1 { chainntnfs.Log.Warnf("Received blocks out of order: "+ "current height=%d, new height=%d", currentHeight, update.blockHeight) continue } currentHeight = update.blockHeight rawBlock, err := b.chainConn.GetBlock(update.blockHash) if err != nil { chainntnfs.Log.Errorf("Unable to get block: %v", err) continue } chainntnfs.Log.Infof("New block: height=%v, sha=%v", update.blockHeight, update.blockHash) txns := btcutil.NewBlock(rawBlock).Transactions() block := &filteredBlock{ hash: *update.blockHash, height: uint32(update.blockHeight), txns: txns, connect: true, } if err := b.handleBlockConnected(block); err != nil { chainntnfs.Log.Error(err) } continue } if update.blockHeight != currentHeight { chainntnfs.Log.Warnf("Received blocks out of order: "+ "current height=%d, disconnected height=%d", currentHeight, update.blockHeight) continue } currentHeight = update.blockHeight - 1 chainntnfs.Log.Infof("Block disconnected from main chain: "+ "height=%v, sha=%v", update.blockHeight, update.blockHash) err := b.txConfNotifier.DisconnectTip(uint32(update.blockHeight)) if err != nil { chainntnfs.Log.Error(err) } // NOTE: we currently only use txUpdates for mempool spends and // rescan spends. It might get removed entirely in the future. case item := <-b.txUpdates.ChanOut(): newSpend := item.(*txUpdate) spendingTx := newSpend.tx // First, check if this transaction spends an output // that has an existing spend notification for it. for i, txIn := range spendingTx.MsgTx().TxIn { prevOut := txIn.PreviousOutPoint // If this transaction indeed does spend an // output which we have a registered // notification for, then create a spend // summary, finally sending off the details to // the notification subscriber. if clients, ok := b.spendNotifications[prevOut]; ok { spenderSha := newSpend.tx.Hash() spendDetails := &chainntnfs.SpendDetail{ SpentOutPoint: &prevOut, SpenderTxHash: spenderSha, SpendingTx: spendingTx.MsgTx(), SpenderInputIndex: uint32(i), } // TODO(roasbeef): after change to // loadfilter, only notify on block // inclusion? confirmedSpend := false if newSpend.details != nil { confirmedSpend = true spendDetails.SpendingHeight = newSpend.details.Height } else { spendDetails.SpendingHeight = currentHeight + 1 } // Keep spendNotifications that are // waiting for a confirmation around. // They will be notified when we find // the spend within a block. rem := make(map[uint64]*spendNotification) for c, ntfn := range clients { // If this is a mempool spend, // and this client didn't want // to be notified on mempool // spends, store it for later. if !confirmedSpend && !ntfn.mempool { rem[c] = ntfn continue } confStr := "unconfirmed" if confirmedSpend { confStr = "confirmed" } chainntnfs.Log.Infof("Dispatching %s "+ "spend notification for "+ "outpoint=%v at height %v", confStr, ntfn.targetOutpoint, spendDetails.SpendingHeight) ntfn.spendChan <- spendDetails // Close spendChan to ensure that any calls to Cancel will not // block. This is safe to do since the channel is buffered, and the // message can still be read by the receiver. close(ntfn.spendChan) } delete(b.spendNotifications, prevOut) // If we had any clients left, add them // back to the map. if len(rem) > 0 { b.spendNotifications[prevOut] = rem } } } case <-b.quit: break out } } b.wg.Done() } // historicalConfDetails looks up whether a transaction is already included in a // block in the active chain and, if so, returns details about the confirmation. func (b *BtcdNotifier) historicalConfDetails(txid *chainhash.Hash, heightHint, currentHeight uint32) (*chainntnfs.TxConfirmation, error) { // First, we'll attempt to retrieve the transaction details using the // backend node's transaction index. txConf, err := b.confDetailsFromTxIndex(txid) if err != nil { return nil, err } if txConf != nil { return txConf, nil } // If the backend node's transaction index is not enabled, then we'll // fall back to manually scanning the chain's blocks, looking for the // block where the transaction was included in. return b.confDetailsManually(txid, heightHint, currentHeight) } // confDetailsFromTxIndex looks up whether a transaction is already included // in a block in the active chain by using the backend node's transaction index. // If the transaction is found, its confirmation details are returned. // Otherwise, nil is returned. func (b *BtcdNotifier) confDetailsFromTxIndex(txid *chainhash.Hash, ) (*chainntnfs.TxConfirmation, error) { // If the transaction has some or all of its confirmations required, // then we may be able to dispatch it immediately. tx, err := b.chainConn.GetRawTransactionVerbose(txid) if err != nil { // Avoid returning an error if the transaction index is not // enabled to proceed with fallback methods. jsonErr, ok := err.(*btcjson.RPCError) if !ok || jsonErr.Code != btcjson.ErrRPCNoTxInfo { return nil, fmt.Errorf("unable to query for txid "+ "%v: %v", txid, err) } } // Make sure we actually retrieved a transaction that is included in a // block. Without this, we won't be able to retrieve its confirmation // details. if tx == nil || tx.BlockHash == "" { return nil, nil } // As we need to fully populate the returned TxConfirmation struct, // grab the block in which the transaction was confirmed so we can // locate its exact index within the block. blockHash, err := chainhash.NewHashFromStr(tx.BlockHash) if err != nil { return nil, fmt.Errorf("unable to get block hash %v for "+ "historical dispatch: %v", tx.BlockHash, err) } block, err := b.chainConn.GetBlockVerbose(blockHash) if err != nil { return nil, fmt.Errorf("unable to get block with hash %v for "+ "historical dispatch: %v", blockHash, err) } // If the block was obtained, locate the transaction's index within the // block so we can give the subscriber full confirmation details. targetTxidStr := txid.String() for txIndex, txHash := range block.Tx { if txHash == targetTxidStr { return &chainntnfs.TxConfirmation{ BlockHash: blockHash, BlockHeight: uint32(block.Height), TxIndex: uint32(txIndex), }, nil } } // We return an error because we should have found the transaction // within the block, but didn't. return nil, fmt.Errorf("unable to locate tx %v in block %v", txid, blockHash) } // confDetailsManually looks up whether a transaction is already included in a // block in the active chain by scanning the chain's blocks, starting from the // earliest height the transaction could have been included in, to the current // height in the chain. If the transaction is found, its confirmation details // are returned. Otherwise, nil is returned. func (b *BtcdNotifier) confDetailsManually(txid *chainhash.Hash, heightHint, currentHeight uint32) (*chainntnfs.TxConfirmation, error) { targetTxidStr := txid.String() // Begin scanning blocks at every height to determine where the // transaction was included in. for height := heightHint; height <= currentHeight; height++ { blockHash, err := b.chainConn.GetBlockHash(int64(height)) if err != nil { return nil, fmt.Errorf("unable to get hash from block "+ "with height %d", height) } // TODO: fetch the neutrino filters instead. block, err := b.chainConn.GetBlockVerbose(blockHash) if err != nil { return nil, fmt.Errorf("unable to get block with hash "+ "%v: %v", blockHash, err) } for txIndex, txHash := range block.Tx { // If we're able to find the transaction in this block, // return its confirmation details. if txHash == targetTxidStr { return &chainntnfs.TxConfirmation{ BlockHash: blockHash, BlockHeight: height, TxIndex: uint32(txIndex), }, nil } } } // If we reach here, then we were not able to find the transaction // within a block, so we avoid returning an error. return nil, nil } // handleBlocksConnected applies a chain update for a new block. Any watched // transactions included this block will processed to either send notifications // now or after numConfirmations confs. // TODO(halseth): this is reusing the neutrino notifier implementation, unify // them. func (b *BtcdNotifier) handleBlockConnected(newBlock *filteredBlock) error { // First we'll notify any subscribed clients of the block. b.notifyBlockEpochs(int32(newBlock.height), &newBlock.hash) // Next, we'll scan over the list of relevant transactions and possibly // dispatch notifications for confirmations and spends. for _, tx := range newBlock.txns { mtx := tx.MsgTx() txSha := mtx.TxHash() for i, txIn := range mtx.TxIn { prevOut := txIn.PreviousOutPoint // If this transaction indeed does spend an output which we have a // registered notification for, then create a spend summary, finally // sending off the details to the notification subscriber. clients, ok := b.spendNotifications[prevOut] if !ok { continue } // TODO(roasbeef): many integration tests expect spend to be // notified within the mempool. spendDetails := &chainntnfs.SpendDetail{ SpentOutPoint: &prevOut, SpenderTxHash: &txSha, SpendingTx: mtx, SpenderInputIndex: uint32(i), SpendingHeight: int32(newBlock.height), } for _, ntfn := range clients { chainntnfs.Log.Infof("Dispatching spend notification for "+ "outpoint=%v", ntfn.targetOutpoint) ntfn.spendChan <- spendDetails // Close spendChan to ensure that any calls to Cancel will not // block. This is safe to do since the channel is buffered, and // the message can still be read by the receiver. close(ntfn.spendChan) } delete(b.spendNotifications, prevOut) } } // A new block has been connected to the main chain. // Send out any N confirmation notifications which may // have been triggered by this new block. b.txConfNotifier.ConnectTip(&newBlock.hash, newBlock.height, newBlock.txns) return nil } // notifyBlockEpochs notifies all registered block epoch clients of the newly // connected block to the main chain. func (b *BtcdNotifier) notifyBlockEpochs(newHeight int32, newSha *chainhash.Hash) { epoch := &chainntnfs.BlockEpoch{ Height: newHeight, Hash: newSha, } for _, epochClient := range b.blockEpochClients { select { case epochClient.epochQueue.ChanIn() <- epoch: case <-epochClient.cancelChan: case <-b.quit: } } } // spendNotification couples a target outpoint along with the channel used for // notifications once a spend of the outpoint has been detected. type spendNotification struct { targetOutpoint *wire.OutPoint spendChan chan *chainntnfs.SpendDetail spendID uint64 mempool bool heightHint uint32 } // spendCancel is a message sent to the BtcdNotifier when a client wishes to // cancel an outstanding spend notification that has yet to be dispatched. type spendCancel struct { // op is the target outpoint of the notification to be cancelled. op wire.OutPoint // spendID the ID of the notification to cancel. spendID uint64 } // RegisterSpendNtfn registers an intent to be notified once the target // outpoint has been spent by a transaction on-chain. Once a spend of the target // outpoint has been detected, the details of the spending event will be sent // across the 'Spend' channel. The heightHint should represent the earliest // height in the chain where the transaction could have been spent in. func (b *BtcdNotifier) RegisterSpendNtfn(outpoint *wire.OutPoint, heightHint uint32, mempool bool) (*chainntnfs.SpendEvent, error) { ntfn := &spendNotification{ targetOutpoint: outpoint, spendChan: make(chan *chainntnfs.SpendDetail, 1), spendID: atomic.AddUint64(&b.spendClientCounter, 1), heightHint: heightHint, mempool: mempool, } select { case <-b.quit: return nil, ErrChainNotifierShuttingDown case b.notificationRegistry <- ntfn: } if err := b.chainConn.NotifySpent([]*wire.OutPoint{outpoint}); err != nil { return nil, err } // The following conditional checks to ensure that when a spend // notification is registered, the output hasn't already been spent. If // the output is no longer in the UTXO set, the chain will be rescanned // from the point where the output was added. The rescan will dispatch // the notification. txOut, err := b.chainConn.GetTxOut(&outpoint.Hash, outpoint.Index, true) if err != nil { return nil, err } if txOut == nil { // First, we'll attempt to retrieve the transaction's block hash // using the backend's transaction index. tx, err := b.chainConn.GetRawTransactionVerbose(&outpoint.Hash) if err != nil { // Avoid returning an error if the transaction was not // found to proceed with fallback methods. jsonErr, ok := err.(*btcjson.RPCError) if !ok || jsonErr.Code != btcjson.ErrRPCNoTxInfo { return nil, fmt.Errorf("unable to query for "+ "txid %v: %v", outpoint.Hash, err) } } var blockHash *chainhash.Hash if tx != nil && tx.BlockHash != "" { // If we're able to retrieve a valid block hash from the // transaction, then we'll use it as our rescan starting // point. blockHash, err = chainhash.NewHashFromStr(tx.BlockHash) if err != nil { return nil, err } } else { // Otherwise, we'll attempt to retrieve the hash for the // block at the heightHint. blockHash, err = b.chainConn.GetBlockHash( int64(heightHint), ) if err != nil { return nil, err } } // We'll only request a rescan if the transaction has actually // been included within a block. Otherwise, we'll encounter an // error when scanning for blocks. This can happen in the case // of a race condition, wherein the output itself is unspent, // and only arrives in the mempool after the getxout call. if blockHash != nil { ops := []*wire.OutPoint{outpoint} err = b.chainConn.Rescan(blockHash, nil, ops) if err != nil { chainntnfs.Log.Errorf("Rescan for spend "+ "notification txout failed: %v", err) return nil, err } } } return &chainntnfs.SpendEvent{ Spend: ntfn.spendChan, Cancel: func() { cancel := &spendCancel{ op: *outpoint, spendID: ntfn.spendID, } // Submit spend cancellation to notification dispatcher. select { case b.notificationCancels <- cancel: // Cancellation is being handled, drain the spend chan until it is // closed before yielding to the caller. for { select { case _, ok := <-ntfn.spendChan: if !ok { return } case <-b.quit: return } } case <-b.quit: } }, }, nil } // confirmationNotification represents a client's intent to receive a // notification once the target txid reaches numConfirmations confirmations. type confirmationNotification struct { chainntnfs.ConfNtfn heightHint uint32 } // RegisterConfirmationsNtfn registers a notification with BtcdNotifier // which will be triggered once the txid reaches numConfs number of // confirmations. func (b *BtcdNotifier) RegisterConfirmationsNtfn(txid *chainhash.Hash, numConfs, heightHint uint32) (*chainntnfs.ConfirmationEvent, error) { ntfn := &confirmationNotification{ ConfNtfn: chainntnfs.ConfNtfn{ TxID: txid, NumConfirmations: numConfs, Event: chainntnfs.NewConfirmationEvent(numConfs), }, heightHint: heightHint, } select { case <-b.quit: return nil, ErrChainNotifierShuttingDown case b.notificationRegistry <- ntfn: return ntfn.Event, nil } } // blockEpochRegistration represents a client's intent to receive a // notification with each newly connected block. type blockEpochRegistration struct { epochID uint64 epochChan chan *chainntnfs.BlockEpoch epochQueue *chainntnfs.ConcurrentQueue cancelChan chan struct{} wg sync.WaitGroup } // epochCancel is a message sent to the BtcdNotifier when a client wishes to // cancel an outstanding epoch notification that has yet to be dispatched. type epochCancel struct { epochID uint64 } // RegisterBlockEpochNtfn returns a BlockEpochEvent which subscribes the // caller to receive notifications, of each new block connected to the main // chain. func (b *BtcdNotifier) RegisterBlockEpochNtfn() (*chainntnfs.BlockEpochEvent, error) { reg := &blockEpochRegistration{ epochQueue: chainntnfs.NewConcurrentQueue(20), epochChan: make(chan *chainntnfs.BlockEpoch, 20), cancelChan: make(chan struct{}), epochID: atomic.AddUint64(&b.epochClientCounter, 1), } reg.epochQueue.Start() // Before we send the request to the main goroutine, we'll launch a new // goroutine to proxy items added to our queue to the client itself. // This ensures that all notifications are received *in order*. reg.wg.Add(1) go func() { defer reg.wg.Done() for { select { case ntfn := <-reg.epochQueue.ChanOut(): blockNtfn := ntfn.(*chainntnfs.BlockEpoch) select { case reg.epochChan <- blockNtfn: case <-reg.cancelChan: return case <-b.quit: return } case <-reg.cancelChan: return case <-b.quit: return } } }() select { case <-b.quit: // As we're exiting before the registration could be sent, // we'll stop the queue now ourselves. reg.epochQueue.Stop() return nil, errors.New("chainntnfs: system interrupt while " + "attempting to register for block epoch notification.") case b.notificationRegistry <- reg: return &chainntnfs.BlockEpochEvent{ Epochs: reg.epochChan, Cancel: func() { cancel := &epochCancel{ epochID: reg.epochID, } // Submit epoch cancellation to notification dispatcher. select { case b.notificationCancels <- cancel: // Cancellation is being handled, drain // the epoch channel until it is closed // before yielding to caller. for { select { case _, ok := <-reg.epochChan: if !ok { return } case <-b.quit: return } } case <-b.quit: } }, }, nil } }