package bitcoindnotify import ( "errors" "fmt" "strings" "sync" "sync/atomic" "github.com/btcsuite/btcd/btcjson" "github.com/btcsuite/btcd/chaincfg/chainhash" "github.com/btcsuite/btcd/wire" "github.com/btcsuite/btcutil" "github.com/btcsuite/btcwallet/chain" "github.com/lightningnetwork/lnd/chainntnfs" "github.com/lightningnetwork/lnd/queue" ) const ( // notifierType uniquely identifies this concrete implementation of the // ChainNotifier interface. notifierType = "bitcoind" ) 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.") // ErrTransactionNotFound is an error returned when we attempt to find a // transaction by manually scanning the chain within a specific range // but it is not found. ErrTransactionNotFound = errors.New("transaction not found within range") ) // 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 } // TODO(roasbeef): generalize struct below: // * move chans to config // * extract common code // * allow outside callers to handle send conditions // BitcoindNotifier implements the ChainNotifier interface using a bitcoind // chain client. Multiple concurrent clients are supported. All notifications // are achieved via non-blocking sends on client channels. type BitcoindNotifier struct { confClientCounter uint64 // To be used atomically. 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 *chain.BitcoindClient notificationCancels chan interface{} notificationRegistry chan interface{} txNotifier *chainntnfs.TxNotifier blockEpochClients map[uint64]*blockEpochRegistration bestBlock chainntnfs.BlockEpoch // spendHintCache is a cache used to query and update the latest height // hints for an outpoint. Each height hint represents the earliest // height at which the outpoint could have been spent within the chain. spendHintCache chainntnfs.SpendHintCache // confirmHintCache is a cache used to query the latest height hints for // a transaction. Each height hint represents the earliest height at // which the transaction could have confirmed within the chain. confirmHintCache chainntnfs.ConfirmHintCache wg sync.WaitGroup quit chan struct{} } // Ensure BitcoindNotifier implements the ChainNotifier interface at compile // time. var _ chainntnfs.ChainNotifier = (*BitcoindNotifier)(nil) // New returns a new BitcoindNotifier instance. This function assumes the // bitcoind node detailed in the passed configuration is already running, and // willing to accept RPC requests and new zmq clients. func New(chainConn *chain.BitcoindConn, spendHintCache chainntnfs.SpendHintCache, confirmHintCache chainntnfs.ConfirmHintCache) *BitcoindNotifier { notifier := &BitcoindNotifier{ notificationCancels: make(chan interface{}), notificationRegistry: make(chan interface{}), blockEpochClients: make(map[uint64]*blockEpochRegistration), spendHintCache: spendHintCache, confirmHintCache: confirmHintCache, quit: make(chan struct{}), } notifier.chainConn = chainConn.NewBitcoindClient() return notifier } // Start connects to the running bitcoind node over websockets, registers for // block notifications, and finally launches all related helper goroutines. func (b *BitcoindNotifier) Start() error { // Already started? if atomic.AddInt32(&b.started, 1) != 1 { return nil } // Connect to bitcoind, and register for notifications on connected, // and disconnected blocks. if err := b.chainConn.Start(); err != nil { return err } if err := b.chainConn.NotifyBlocks(); err != nil { return err } currentHash, currentHeight, err := b.chainConn.GetBestBlock() if err != nil { return err } b.txNotifier = chainntnfs.NewTxNotifier( uint32(currentHeight), chainntnfs.ReorgSafetyLimit, b.confirmHintCache, b.spendHintCache, ) b.bestBlock = chainntnfs.BlockEpoch{ Height: currentHeight, Hash: currentHash, } b.wg.Add(1) go b.notificationDispatcher() return nil } // Stop shutsdown the BitcoindNotifier. func (b *BitcoindNotifier) Stop() error { // Already shutting down? if atomic.AddInt32(&b.stopped, 1) != 1 { return nil } // Shutdown the rpc client, this gracefully disconnects from bitcoind, // and cleans up all related resources. b.chainConn.Stop() close(b.quit) b.wg.Wait() // Notify all pending clients of our shutdown by closing the related // notification channels. for _, epochClient := range b.blockEpochClients { close(epochClient.cancelChan) epochClient.wg.Wait() close(epochClient.epochChan) } b.txNotifier.TearDown() return nil } // blockNtfn packages a notification of a connected/disconnected block along // with its height at the time. type blockNtfn struct { sha *chainhash.Hash height int32 } // notificationDispatcher is the primary goroutine which handles client // notification registrations, as well as notification dispatches. func (b *BitcoindNotifier) notificationDispatcher() { out: for { select { case cancelMsg := <-b.notificationCancels: switch msg := cancelMsg.(type) { 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 *chainntnfs.HistoricalConfDispatch: // Look up whether the transaction is already // included in the active chain. We'll do this // in a goroutine to prevent blocking // potentially long rescans. // // TODO(wilmer): add retry logic if rescan fails? b.wg.Add(1) go func() { defer b.wg.Done() confDetails, _, err := b.historicalConfDetails( msg.TxID, msg.StartHeight, msg.EndHeight, ) if err != nil { chainntnfs.Log.Error(err) return } // If the historical dispatch finished // without error, we will invoke // UpdateConfDetails even if none were // found. This allows the notifier to // begin safely updating the height hint // cache at tip, since any pending // rescans have now completed. err = b.txNotifier.UpdateConfDetails( *msg.TxID, confDetails, ) if err != nil { chainntnfs.Log.Error(err) } }() case *chainntnfs.HistoricalSpendDispatch: // In order to ensure we don't block the caller // on what may be a long rescan, we'll launch a // goroutine to do so in the background. // // TODO(wilmer): add retry logic if rescan fails? b.wg.Add(1) go func() { defer b.wg.Done() err := b.dispatchSpendDetailsManually(msg) if err != nil { chainntnfs.Log.Errorf("Rescan to "+ "determine the spend "+ "details of %v failed: %v", msg.OutPoint, err) } }() case *blockEpochRegistration: chainntnfs.Log.Infof("New block epoch subscription") b.blockEpochClients[msg.epochID] = msg if msg.bestBlock != nil { missedBlocks, err := chainntnfs.GetClientMissedBlocks( b.chainConn, msg.bestBlock, b.bestBlock.Height, true, ) if err != nil { msg.errorChan <- err continue } for _, block := range missedBlocks { b.notifyBlockEpochClient(msg, block.Height, block.Hash) } } msg.errorChan <- nil } case ntfn := <-b.chainConn.Notifications(): switch item := ntfn.(type) { case chain.BlockConnected: blockHeader, err := b.chainConn.GetBlockHeader(&item.Hash) if err != nil { chainntnfs.Log.Errorf("Unable to fetch "+ "block header: %v", err) continue } if blockHeader.PrevBlock != *b.bestBlock.Hash { // Handle the case where the notifier // missed some blocks from its chain // backend. chainntnfs.Log.Infof("Missed blocks, " + "attempting to catch up") newBestBlock, missedBlocks, err := chainntnfs.HandleMissedBlocks( b.chainConn, b.txNotifier, b.bestBlock, item.Height, true, ) if err != nil { // Set the bestBlock here in case // a catch up partially completed. b.bestBlock = newBestBlock chainntnfs.Log.Error(err) continue } for _, block := range missedBlocks { err := b.handleBlockConnected(block) if err != nil { chainntnfs.Log.Error(err) continue out } } } newBlock := chainntnfs.BlockEpoch{ Height: item.Height, Hash: &item.Hash, } if err := b.handleBlockConnected(newBlock); err != nil { chainntnfs.Log.Error(err) } continue case chain.BlockDisconnected: if item.Height != b.bestBlock.Height { chainntnfs.Log.Infof("Missed disconnected" + "blocks, attempting to catch up") } newBestBlock, err := chainntnfs.RewindChain( b.chainConn, b.txNotifier, b.bestBlock, item.Height-1, ) if err != nil { chainntnfs.Log.Errorf("Unable to rewind chain "+ "from height %d to height %d: %v", b.bestBlock.Height, item.Height-1, err) } // Set the bestBlock here in case a chain // rewind partially completed. b.bestBlock = newBestBlock case chain.RelevantTx: // We only care about notifying on confirmed // spends, so if this is a mempool spend, we can // ignore it and wait for the spend to appear in // on-chain. if item.Block == nil { continue } tx := &item.TxRecord.MsgTx err := b.txNotifier.ProcessRelevantSpendTx( tx, item.Block.Height, ) if err != nil { chainntnfs.Log.Errorf("Unable to "+ "process transaction %v: %v", tx.TxHash(), err) } } 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 *BitcoindNotifier) historicalConfDetails(txid *chainhash.Hash, startHeight, endHeight uint32) (*chainntnfs.TxConfirmation, chainntnfs.TxConfStatus, error) { // We'll first attempt to retrieve the transaction using the node's // txindex. txConf, txStatus, err := b.confDetailsFromTxIndex(txid) // We'll then check the status of the transaction lookup returned to // determine whether we should proceed with any fallback methods. switch { // We failed querying the index for the transaction, fall back to // scanning manually. case err != nil: chainntnfs.Log.Debugf("Failed getting conf details from "+ "index (%v), scanning manually", err) return b.confDetailsManually(txid, startHeight, endHeight) // The transaction was found within the node's mempool. case txStatus == chainntnfs.TxFoundMempool: // The transaction was found within the node's txindex. case txStatus == chainntnfs.TxFoundIndex: // The transaction was not found within the node's mempool or txindex. case txStatus == chainntnfs.TxNotFoundIndex: // Unexpected txStatus returned. default: return nil, txStatus, fmt.Errorf("Got unexpected txConfStatus: %v", txStatus) } return txConf, txStatus, nil } // 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 TxConfStatus is returned. If it was found in // the mempool this will be TxFoundMempool, if it is found in a block this will // be TxFoundIndex. Otherwise TxNotFoundIndex is returned. If the tx is found // in a block its confirmation details are also returned. func (b *BitcoindNotifier) confDetailsFromTxIndex(txid *chainhash.Hash, ) (*chainntnfs.TxConfirmation, chainntnfs.TxConfStatus, 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 { // If the transaction lookup was successful, but it wasn't found // within the index itself, then we can exit early. We'll also // need to look at the error message returned as the error code // is used for multiple errors. txNotFoundErr := "No such mempool or blockchain transaction" jsonErr, ok := err.(*btcjson.RPCError) if ok && jsonErr.Code == btcjson.ErrRPCNoTxInfo && strings.Contains(jsonErr.Message, txNotFoundErr) { return nil, chainntnfs.TxNotFoundIndex, nil } return nil, chainntnfs.TxNotFoundIndex, fmt.Errorf("unable to query for txid %v: %v", txid, err) } // Make sure we actually retrieved a transaction that is included in a // block. If not, the transaction must be unconfirmed (in the mempool), // and we'll return TxFoundMempool together with a nil TxConfirmation. if tx.BlockHash == "" { return nil, chainntnfs.TxFoundMempool, 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, chainntnfs.TxNotFoundIndex, 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, chainntnfs.TxNotFoundIndex, 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 { details := &chainntnfs.TxConfirmation{ BlockHash: blockHash, BlockHeight: uint32(block.Height), TxIndex: uint32(txIndex), } return details, chainntnfs.TxFoundIndex, nil } } // We return an error because we should have found the transaction // within the block, but didn't. return nil, chainntnfs.TxNotFoundIndex, 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 *BitcoindNotifier) confDetailsManually(txid *chainhash.Hash, heightHint, currentHeight uint32) (*chainntnfs.TxConfirmation, chainntnfs.TxConfStatus, error) { targetTxidStr := txid.String() // Begin scanning blocks at every height to determine where the // transaction was included in. for height := currentHeight; height >= heightHint && height > 0; height-- { // Ensure we haven't been requested to shut down before // processing the next height. select { case <-b.quit: return nil, chainntnfs.TxNotFoundManually, ErrChainNotifierShuttingDown default: } blockHash, err := b.chainConn.GetBlockHash(int64(height)) if err != nil { return nil, chainntnfs.TxNotFoundManually, fmt.Errorf("unable to get hash from block "+ "with height %d", height) } block, err := b.chainConn.GetBlockVerbose(blockHash) if err != nil { return nil, chainntnfs.TxNotFoundManually, 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 { details := &chainntnfs.TxConfirmation{ BlockHash: blockHash, BlockHeight: height, TxIndex: uint32(txIndex), } return details, chainntnfs.TxFoundManually, 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, chainntnfs.TxNotFoundManually, nil } // handleBlockConnected 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. func (b *BitcoindNotifier) handleBlockConnected(block chainntnfs.BlockEpoch) error { // First, we'll fetch the raw block as we'll need to gather all the // transactions to determine whether any are relevant to our registered // clients. rawBlock, err := b.chainConn.GetBlock(block.Hash) if err != nil { return fmt.Errorf("unable to get block: %v", err) } txns := btcutil.NewBlock(rawBlock).Transactions() // We'll then extend the txNotifier's height with the information of // this new block, which will handle all of the notification logic for // us. err = b.txNotifier.ConnectTip(block.Hash, uint32(block.Height), txns) if err != nil { return fmt.Errorf("unable to connect tip: %v", err) } chainntnfs.Log.Infof("New block: height=%v, sha=%v", block.Height, block.Hash) // Now that we've guaranteed the new block extends the txNotifier's // current tip, we'll proceed to dispatch notifications to all of our // registered clients whom have had notifications fulfilled. Before // doing so, we'll make sure update our in memory state in order to // satisfy any client requests based upon the new block. b.bestBlock = block b.notifyBlockEpochs(block.Height, block.Hash) return b.txNotifier.NotifyHeight(uint32(block.Height)) } // notifyBlockEpochs notifies all registered block epoch clients of the newly // connected block to the main chain. func (b *BitcoindNotifier) notifyBlockEpochs(newHeight int32, newSha *chainhash.Hash) { for _, client := range b.blockEpochClients { b.notifyBlockEpochClient(client, newHeight, newSha) } } // notifyBlockEpochClient sends a registered block epoch client a notification // about a specific block. func (b *BitcoindNotifier) notifyBlockEpochClient(epochClient *blockEpochRegistration, height int32, sha *chainhash.Hash) { epoch := &chainntnfs.BlockEpoch{ Height: height, Hash: sha, } select { case epochClient.epochQueue.ChanIn() <- epoch: case <-epochClient.cancelChan: case <-b.quit: } } // 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 *BitcoindNotifier) RegisterSpendNtfn(outpoint *wire.OutPoint, pkScript []byte, heightHint uint32) (*chainntnfs.SpendEvent, error) { // First, we'll construct a spend notification request and hand it off // to the txNotifier. spendID := atomic.AddUint64(&b.spendClientCounter, 1) cancel := func() { b.txNotifier.CancelSpend(*outpoint, spendID) } ntfn := &chainntnfs.SpendNtfn{ SpendID: spendID, OutPoint: *outpoint, PkScript: pkScript, Event: chainntnfs.NewSpendEvent(cancel), HeightHint: heightHint, } historicalDispatch, err := b.txNotifier.RegisterSpend(ntfn) if err != nil { return nil, err } // If the txNotifier didn't return any details to perform a historical // scan of the chain, then we can return early as there's nothing left // for us to do. if historicalDispatch == nil { return ntfn.Event, nil } // We'll then request the backend to notify us when it has detected the // outpoint as spent. if err := b.chainConn.NotifySpent([]*wire.OutPoint{outpoint}); err != nil { return nil, err } // In addition to the check above, we'll also check the backend's UTXO // set to determine whether the outpoint has been spent. If it hasn't, // we can return to the caller as well. txOut, err := b.chainConn.GetTxOut(&outpoint.Hash, outpoint.Index, true) if err != nil { return nil, err } if txOut != nil { // We'll let the txNotifier know the outpoint is still unspent // in order to begin updating its spend hint. err := b.txNotifier.UpdateSpendDetails(*outpoint, nil) if err != nil { return nil, err } return ntfn.Event, nil } // Otherwise, we'll determine when the output was spent by scanning the // chain. We'll begin by determining where to start our historical // rescan. // // As a minimal optimization, we'll query the backend's transaction // index (if enabled) to determine if we have a better rescan starting // height. We can do this as the GetRawTransaction call will return the // hash of the block it was included in within the chain. 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) } } // If the transaction index was enabled, we'll use the block's hash to // retrieve its height and check whether it provides a better starting // point for our rescan. if tx != nil { // If the transaction containing the outpoint hasn't confirmed // on-chain, then there's no need to perform a rescan. if tx.BlockHash == "" { return ntfn.Event, nil } blockHash, err := chainhash.NewHashFromStr(tx.BlockHash) if err != nil { return nil, err } blockHeight, err := b.chainConn.GetBlockHeight(blockHash) if err != nil { return nil, err } if uint32(blockHeight) > historicalDispatch.StartHeight { historicalDispatch.StartHeight = uint32(blockHeight) } } // Now that we've determined the starting point of our rescan, we can // dispatch it. select { case b.notificationRegistry <- historicalDispatch: return ntfn.Event, nil case <-b.quit: return nil, ErrChainNotifierShuttingDown } } // disaptchSpendDetailsManually attempts to manually scan the chain within the // given height range for a transaction that spends the given outpoint. If one // is found, it's spending details are sent to the notifier dispatcher, which // will then dispatch the notification to all of its clients. func (b *BitcoindNotifier) dispatchSpendDetailsManually( historicalDispatchDetails *chainntnfs.HistoricalSpendDispatch) error { op := historicalDispatchDetails.OutPoint startHeight := historicalDispatchDetails.StartHeight endHeight := historicalDispatchDetails.EndHeight // Begin scanning blocks at every height to determine if the outpoint // was spent. for height := endHeight; height >= startHeight && height > 0; height-- { // Ensure we haven't been requested to shut down before // processing the next height. select { case <-b.quit: return ErrChainNotifierShuttingDown default: } // First, we'll fetch the block for the current height. blockHash, err := b.chainConn.GetBlockHash(int64(height)) if err != nil { return fmt.Errorf("unable to retrieve hash for block "+ "with height %d: %v", height, err) } block, err := b.chainConn.GetBlock(blockHash) if err != nil { return fmt.Errorf("unable to retrieve block with hash "+ "%v: %v", blockHash, err) } // Then, we'll manually go over every transaction in it and // determine whether it spends the outpoint in question. for _, tx := range block.Transactions { for i, txIn := range tx.TxIn { if txIn.PreviousOutPoint != op { continue } // If it does, we'll construct its spend details // and hand them over to the TxNotifier so that // it can properly notify its registered // clients. txHash := tx.TxHash() details := &chainntnfs.SpendDetail{ SpentOutPoint: &op, SpenderTxHash: &txHash, SpendingTx: tx, SpenderInputIndex: uint32(i), SpendingHeight: int32(height), } return b.txNotifier.UpdateSpendDetails( op, details, ) } } } return ErrTransactionNotFound } // RegisterConfirmationsNtfn registers a notification with BitcoindNotifier // which will be triggered once the txid reaches numConfs number of // confirmations. func (b *BitcoindNotifier) RegisterConfirmationsNtfn(txid *chainhash.Hash, _ []byte, numConfs, heightHint uint32) (*chainntnfs.ConfirmationEvent, error) { // Construct a notification request for the transaction and send it to // the main event loop. ntfn := &chainntnfs.ConfNtfn{ ConfID: atomic.AddUint64(&b.confClientCounter, 1), TxID: txid, NumConfirmations: numConfs, Event: chainntnfs.NewConfirmationEvent(numConfs), HeightHint: heightHint, } chainntnfs.Log.Infof("New confirmation subscription: "+ "txid=%v, numconfs=%v", txid, numConfs) // Register the conf notification with the TxNotifier. A non-nil value // for `dispatch` will be returned if we are required to perform a // manual scan for the confirmation. Otherwise the notifier will begin // watching at tip for the transaction to confirm. dispatch, err := b.txNotifier.RegisterConf(ntfn) if err != nil { return nil, err } if dispatch == nil { return ntfn.Event, nil } select { case b.notificationRegistry <- dispatch: return ntfn.Event, nil case <-b.quit: return nil, ErrChainNotifierShuttingDown } } // 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 *queue.ConcurrentQueue bestBlock *chainntnfs.BlockEpoch errorChan chan error cancelChan chan struct{} wg sync.WaitGroup } // epochCancel is a message sent to the BitcoindNotifier 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. Clients have the option of passing in their best known block, which // the notifier uses to check if they are behind on blocks and catch them up. func (b *BitcoindNotifier) RegisterBlockEpochNtfn( bestBlock *chainntnfs.BlockEpoch) (*chainntnfs.BlockEpochEvent, error) { reg := &blockEpochRegistration{ epochQueue: queue.NewConcurrentQueue(20), epochChan: make(chan *chainntnfs.BlockEpoch, 20), cancelChan: make(chan struct{}), epochID: atomic.AddUint64(&b.epochClientCounter, 1), bestBlock: bestBlock, errorChan: make(chan error, 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 } }