package bitcoindnotify import ( "errors" "fmt" "sync" "sync/atomic" "github.com/btcsuite/btcd/btcjson" "github.com/btcsuite/btcd/chaincfg" "github.com/btcsuite/btcd/chaincfg/chainhash" "github.com/btcsuite/btcd/txscript" "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" ) // 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 { epochClientCounter uint64 // To be used atomically. started int32 // To be used atomically. stopped int32 // To be used atomically. chainConn *chain.BitcoindClient chainParams *chaincfg.Params 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, chainParams *chaincfg.Params, spendHintCache chainntnfs.SpendHintCache, confirmHintCache chainntnfs.ConfirmHintCache) *BitcoindNotifier { notifier := &BitcoindNotifier{ chainParams: chainParams, 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.ConfRequest, msg.StartHeight, msg.EndHeight, ) if err != nil { chainntnfs.Log.Errorf("Rescan to "+ "determine the conf "+ "details of %v within "+ "range %d-%d failed: %v", msg.ConfRequest, msg.StartHeight, msg.EndHeight, 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.ConfRequest, confDetails, ) if err != nil { chainntnfs.Log.Errorf("Unable "+ "to update conf "+ "details of %v: %v", msg.ConfRequest, 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() spendDetails, err := b.historicalSpendDetails( msg.SpendRequest, msg.StartHeight, msg.EndHeight, ) if err != nil { chainntnfs.Log.Errorf("Rescan to "+ "determine the spend "+ "details of %v within "+ "range %d-%d failed: %v", msg.SpendRequest, msg.StartHeight, msg.EndHeight, err) return } // If the historical dispatch finished // without error, we will invoke // UpdateSpendDetails 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.UpdateSpendDetails( msg.SpendRequest, spendDetails, ) if err != nil { chainntnfs.Log.Errorf("Unable "+ "to update spend "+ "details of %v: %v", msg.SpendRequest, err) } }() case *blockEpochRegistration: chainntnfs.Log.Infof("New block epoch subscription") b.blockEpochClients[msg.epochID] = msg // If the client did not provide their best // known block, then we'll immediately dispatch // a notification for the current tip. if msg.bestBlock == nil { b.notifyBlockEpochClient( msg, b.bestBlock.Height, b.bestBlock.Hash, ) msg.errorChan <- nil continue } // Otherwise, we'll attempt to deliver the // backlog of notifications from their best // known block. 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 := btcutil.NewTx(&item.TxRecord.MsgTx) err := b.txNotifier.ProcessRelevantSpendTx( tx, uint32(item.Block.Height), ) if err != nil { chainntnfs.Log.Errorf("Unable to "+ "process transaction %v: %v", tx.Hash(), err) } } case <-b.quit: break out } } b.wg.Done() } // historicalConfDetails looks up whether a confirmation request (txid/output // script) has already been included in a block in the active chain and, if so, // returns details about said block. func (b *BitcoindNotifier) historicalConfDetails(confRequest chainntnfs.ConfRequest, startHeight, endHeight uint32) (*chainntnfs.TxConfirmation, chainntnfs.TxConfStatus, error) { // If a txid was not provided, then we should dispatch upon seeing the // script on-chain, so we'll short-circuit straight to scanning manually // as there doesn't exist a script index to query. if confRequest.TxID == chainntnfs.ZeroHash { return b.confDetailsManually( confRequest, startHeight, endHeight, ) } // Otherwise, we'll dispatch upon seeing a transaction on-chain with the // given hash. // // We'll first attempt to retrieve the transaction using the node's // txindex. txNotFoundErr := "No such mempool or blockchain transaction" txConf, txStatus, err := chainntnfs.ConfDetailsFromTxIndex( b.chainConn, confRequest, txNotFoundErr, ) // 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(confRequest, 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 } // confDetailsManually looks up whether a transaction/output script has already // been included in a block in the active chain by scanning the chain's blocks // within the given range. If the transaction/output script is found, its // confirmation details are returned. Otherwise, nil is returned. func (b *BitcoindNotifier) confDetailsManually(confRequest chainntnfs.ConfRequest, heightHint, currentHeight uint32) (*chainntnfs.TxConfirmation, chainntnfs.TxConfStatus, error) { // 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, chainntnfs.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.GetBlock(blockHash) if err != nil { return nil, chainntnfs.TxNotFoundManually, fmt.Errorf("unable to get block with hash "+ "%v: %v", blockHash, err) } // For every transaction in the block, check which one matches // our request. If we find one that does, we can dispatch its // confirmation details. for txIndex, tx := range block.Transactions { if !confRequest.MatchesTx(tx) { continue } return &chainntnfs.TxConfirmation{ Tx: tx, BlockHash: blockHash, BlockHeight: height, TxIndex: uint32(txIndex), }, 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/output script has been spent by a transaction on-chain. When // intending to be notified of the spend of an output script, a nil outpoint // must be used. The heightHint should represent the earliest height in the // chain of the transaction that spent the outpoint/output script. // // Once a spend of has been detected, the details of the spending event will be // sent across the 'Spend' channel. func (b *BitcoindNotifier) RegisterSpendNtfn(outpoint *wire.OutPoint, pkScript []byte, heightHint uint32) (*chainntnfs.SpendEvent, error) { // 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. ntfn, err := b.txNotifier.RegisterSpend(outpoint, pkScript, heightHint) if err != nil { return nil, err } // We'll then request the backend to notify us when it has detected the // outpoint/output script as spent. // // TODO(wilmer): use LoadFilter API instead. if outpoint == nil || *outpoint == chainntnfs.ZeroOutPoint { _, addrs, _, err := txscript.ExtractPkScriptAddrs( pkScript, b.chainParams, ) if err != nil { return nil, fmt.Errorf("unable to parse script: %v", err) } if err := b.chainConn.NotifyReceived(addrs); err != nil { return nil, err } } else { ops := []*wire.OutPoint{outpoint} if err := b.chainConn.NotifySpent(ops); 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 ntfn.HistoricalDispatch == nil { return ntfn.Event, nil } // Otherwise, we'll need to dispatch a historical rescan to determine if // the outpoint was already spent at a previous height. // // We'll short-circuit the path when dispatching the spend of a script, // rather than an outpoint, as there aren't any additional checks we can // make for scripts. if ntfn.HistoricalDispatch.OutPoint == chainntnfs.ZeroOutPoint { select { case b.notificationRegistry <- ntfn.HistoricalDispatch: case <-b.quit: return nil, chainntnfs.ErrChainNotifierShuttingDown } return ntfn.Event, nil } // When dispatching spends of outpoints, there are a number of checks we // can make to start our rescan from a better height or completely avoid // it. // // We'll start by checking 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( ntfn.HistoricalDispatch.SpendRequest, nil, ) if err != nil { return nil, err } return ntfn.Event, nil } // Since the outpoint was spent, as it no longer exists within the UTXO // set, we'll determine when it happened by scanning the chain. // // 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) > ntfn.HistoricalDispatch.StartHeight { ntfn.HistoricalDispatch.StartHeight = uint32(blockHeight) } } // Now that we've determined the starting point of our rescan, we can // dispatch it and return. select { case b.notificationRegistry <- ntfn.HistoricalDispatch: case <-b.quit: return nil, chainntnfs.ErrChainNotifierShuttingDown } return ntfn.Event, nil } // historicalSpendDetails attempts to manually scan the chain within the given // height range for a transaction that spends the given outpoint/output script. // If one is found, the spend details are assembled and returned to the caller. // If the spend is not found, a nil spend detail will be returned. func (b *BitcoindNotifier) historicalSpendDetails( spendRequest chainntnfs.SpendRequest, startHeight, endHeight uint32) ( *chainntnfs.SpendDetail, error) { // 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 nil, chainntnfs.ErrChainNotifierShuttingDown default: } // First, we'll fetch the block for the current height. blockHash, err := b.chainConn.GetBlockHash(int64(height)) if err != nil { return nil, fmt.Errorf("unable to retrieve hash for "+ "block with height %d: %v", height, err) } block, err := b.chainConn.GetBlock(blockHash) if err != nil { return nil, fmt.Errorf("unable to retrieve block "+ "with hash %v: %v", blockHash, err) } // Then, we'll manually go over every input in every transaction // in it and determine whether it spends the request in // question. If we find one, we'll dispatch the spend details. for _, tx := range block.Transactions { matches, inputIdx, err := spendRequest.MatchesTx(tx) if err != nil { return nil, err } if !matches { continue } txHash := tx.TxHash() return &chainntnfs.SpendDetail{ SpentOutPoint: &tx.TxIn[inputIdx].PreviousOutPoint, SpenderTxHash: &txHash, SpendingTx: tx, SpenderInputIndex: inputIdx, SpendingHeight: int32(height), }, nil } } return nil, nil } // RegisterConfirmationsNtfn registers an intent to be notified once the target // txid/output script has reached numConfs confirmations on-chain. When // intending to be notified of the confirmation of an output script, a nil txid // must be used. The heightHint should represent the earliest height at which // the txid/output script could have been included in the chain. // // Progress on the number of confirmations left can be read from the 'Updates' // channel. Once it has reached all of its confirmations, a notification will be // sent across the 'Confirmed' channel. func (b *BitcoindNotifier) RegisterConfirmationsNtfn(txid *chainhash.Hash, pkScript []byte, numConfs, heightHint uint32) (*chainntnfs.ConfirmationEvent, error) { // 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. ntfn, err := b.txNotifier.RegisterConf( txid, pkScript, numConfs, heightHint, ) if err != nil { return nil, err } if ntfn.HistoricalDispatch == nil { return ntfn.Event, nil } select { case b.notificationRegistry <- ntfn.HistoricalDispatch: return ntfn.Event, nil case <-b.quit: return nil, chainntnfs.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. If // they do not provide one, then a notification will be dispatched immediately // for the current tip of the chain upon a successful registration. 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 } }