Beat [2/4]: implement blockbeat

chainio/README.md

@@ -0,0 +1,152 @@
+# Chainio 
+
+`chainio` is a package designed to provide blockchain data access to various
+subsystems within `lnd`. When a new block is received, it is encapsulated in a
+`Blockbeat` object and disseminated to all registered consumers. Consumers may
+receive these updates either concurrently or sequentially, based on their
+registration configuration, ensuring that each subsystem maintains a
+synchronized view of the current block state.
+
+The main components include:
+
+- `Blockbeat`: An interface that provides information about the block.
+
+- `Consumer`: An interface that specifies how subsystems handle the blockbeat.
+
+- `BlockbeatDispatcher`: The core service responsible for receiving each block
+  and distributing it to all consumers.
+
+Additionally, the `BeatConsumer` struct provides a partial implementation of
+the `Consumer` interface. This struct helps reduce code duplication, allowing
+subsystems to avoid re-implementing the `ProcessBlock` method and provides a
+commonly used `NotifyBlockProcessed` method.
+
+
+### Register a Consumer
+
+Consumers within the same queue are notified **sequentially**, while all queues
+are notified **concurrently**. A queue consists of a slice of consumers, which
+are notified in left-to-right order. Developers are responsible for determining
+dependencies in block consumption across subsystems: independent subsystems
+should be notified concurrently, whereas dependent subsystems should be
+notified sequentially.
+
+To notify the consumers concurrently, put them in different queues,
+```go
+// consumer1 and consumer2 will be notified concurrently.
+queue1 := []chainio.Consumer{consumer1}
+blockbeatDispatcher.RegisterQueue(consumer1)
+
+queue2 := []chainio.Consumer{consumer2}
+blockbeatDispatcher.RegisterQueue(consumer2)
+```
+
+To notify the consumers sequentially, put them in the same queue,
+```go
+// consumers will be notified sequentially via,
+// consumer1 -> consumer2 -> consumer3
+queue := []chainio.Consumer{
+   consumer1,
+   consumer2,
+   consumer3,
+}
+blockbeatDispatcher.RegisterQueue(queue)
+```
+
+### Implement the `Consumer` Interface
+
+Implementing the `Consumer` interface is straightforward. Below is an example
+of how
+[`sweep.TxPublisher`](https://github.com/lightningnetwork/lnd/blob/5cec466fad44c582a64cfaeb91f6d5fd302fcf85/sweep/fee_bumper.go#L310)
+implements this interface.
+
+To start, embed the partial implementation `chainio.BeatConsumer`, which
+already provides the `ProcessBlock` implementation and commonly used
+`NotifyBlockProcessed` method,  and exposes `BlockbeatChan` for the consumer to
+receive blockbeats.
+
+```go
+type TxPublisher struct {
+   started atomic.Bool
+   stopped atomic.Bool
+
+   chainio.BeatConsumer
+
+   ...
+```
+
+We should also remember to initialize this `BeatConsumer`,
+
+```go
+...
+// Mount the block consumer.
+tp.BeatConsumer = chainio.NewBeatConsumer(tp.quit, tp.Name())
+```
+
+Finally, in the main event loop, read from `BlockbeatChan`, process the
+received blockbeat, and, crucially, call `tp.NotifyBlockProcessed` to inform
+the blockbeat dispatcher that processing is complete.
+
+```go
+for {
+      select {
+      case beat := <-tp.BlockbeatChan:
+         // Consume this blockbeat, usually it means updating the subsystem
+         // using the new block data.
+
+         // Notify we've processed the block.
+         tp.NotifyBlockProcessed(beat, nil)
+
+      ...
+```
+
+### Existing Queues
+
+Currently, we have a single queue of consumers dedicated to handling force
+closures. This queue includes `ChainArbitrator`, `UtxoSweeper`, and
+`TxPublisher`, with `ChainArbitrator` managing two internal consumers:
+`chainWatcher` and `ChannelArbitrator`. The blockbeat flows sequentially
+through the chain as follows: `ChainArbitrator => chainWatcher =>
+ChannelArbitrator => UtxoSweeper => TxPublisher`. The following diagram
+illustrates the flow within the public subsystems.
+
+```mermaid
+sequenceDiagram
+		autonumber
+		participant bb as BlockBeat
+		participant cc as ChainArb
+		participant us as UtxoSweeper
+		participant tp as TxPublisher
+
+		note left of bb: 0. received block x,<br>dispatching...
+
+    note over bb,cc: 1. send block x to ChainArb,<br>wait for its done signal
+		bb->>cc: block x
+		rect rgba(165, 0, 85, 0.8)
+      critical signal processed
+        cc->>bb: processed block
+      option Process error or timeout
+        bb->>bb: error and exit
+      end
+    end
+
+    note over bb,us: 2. send block x to UtxoSweeper, wait for its done signal
+		bb->>us: block x
+		rect rgba(165, 0, 85, 0.8)
+      critical signal processed
+        us->>bb: processed block
+      option Process error or timeout
+        bb->>bb: error and exit
+      end
+    end
+
+    note over bb,tp: 3. send block x to TxPublisher, wait for its done signal
+		bb->>tp: block x
+		rect rgba(165, 0, 85, 0.8)
+      critical signal processed
+        tp->>bb: processed block
+      option Process error or timeout
+        bb->>bb: error and exit
+      end
+    end
+```

chainio/blockbeat.go

@@ -0,0 +1,55 @@
+package chainio
+
+import (
+	"fmt"
+
+	"github.com/btcsuite/btclog/v2"
+	"github.com/lightningnetwork/lnd/build"
+	"github.com/lightningnetwork/lnd/chainntnfs"
+)
+
+// Beat implements the Blockbeat interface. It contains the block epoch and a
+// customized logger.
+//
+// TODO(yy): extend this to check for confirmation status - which serves as the
+// single source of truth, to avoid the potential race between receiving blocks
+// and `GetTransactionDetails/RegisterSpendNtfn/RegisterConfirmationsNtfn`.
+type Beat struct {
+	// epoch is the current block epoch the blockbeat is aware of.
+	epoch chainntnfs.BlockEpoch
+
+	// log is the customized logger for the blockbeat which prints the
+	// block height.
+	log btclog.Logger
+}
+
+// Compile-time check to ensure Beat satisfies the Blockbeat interface.
+var _ Blockbeat = (*Beat)(nil)
+
+// NewBeat creates a new beat with the specified block epoch and a customized
+// logger.
+func NewBeat(epoch chainntnfs.BlockEpoch) *Beat {
+	b := &Beat{
+		epoch: epoch,
+	}
+
+	// Create a customized logger for the blockbeat.
+	logPrefix := fmt.Sprintf("Height[%6d]:", b.Height())
+	b.log = build.NewPrefixLog(logPrefix, clog)
+
+	return b
+}
+
+// Height returns the height of the block epoch.
+//
+// NOTE: Part of the Blockbeat interface.
+func (b *Beat) Height() int32 {
+	return b.epoch.Height
+}
+
+// logger returns the logger for the blockbeat.
+//
+// NOTE: Part of the private blockbeat interface.
+func (b *Beat) logger() btclog.Logger {
+	return b.log
+}

chainio/blockbeat_test.go

@@ -0,0 +1,28 @@
+package chainio
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/lightningnetwork/lnd/chainntnfs"
+	"github.com/stretchr/testify/require"
+)
+
+var errDummy = errors.New("dummy error")
+
+// TestNewBeat tests the NewBeat and Height functions.
+func TestNewBeat(t *testing.T) {
+	t.Parallel()
+
+	// Create a testing epoch.
+	epoch := chainntnfs.BlockEpoch{
+		Height: 1,
+	}
+
+	// Create the beat and check the internal state.
+	beat := NewBeat(epoch)
+	require.Equal(t, epoch, beat.epoch)
+
+	// Check the height function.
+	require.Equal(t, epoch.Height, beat.Height())
+}

chainio/consumer.go

@@ -0,0 +1,113 @@
+package chainio
+
+// BeatConsumer defines a supplementary component that should be used by
+// subsystems which implement the `Consumer` interface. It partially implements
+// the `Consumer` interface by providing the method `ProcessBlock` such that
+// subsystems don't need to re-implement it.
+//
+// While inheritance is not commonly used in Go, subsystems embedding this
+// struct cannot pass the interface check for `Consumer` because the `Name`
+// method is not implemented, which gives us a "mortise and tenon" structure.
+// In addition to reducing code duplication, this design allows `ProcessBlock`
+// to work on the concrete type `Beat` to access its internal states.
+type BeatConsumer struct {
+	// BlockbeatChan is a channel to receive blocks from Blockbeat. The
+	// received block contains the best known height and the txns confirmed
+	// in this block.
+	BlockbeatChan chan Blockbeat
+
+	// name is the name of the consumer which embeds the BlockConsumer.
+	name string
+
+	// quit is a channel that closes when the BlockConsumer is shutting
+	// down.
+	//
+	// NOTE: this quit channel should be mounted to the same quit channel
+	// used by the subsystem.
+	quit chan struct{}
+
+	// errChan is a buffered chan that receives an error returned from
+	// processing this block.
+	errChan chan error
+}
+
+// NewBeatConsumer creates a new BlockConsumer.
+func NewBeatConsumer(quit chan struct{}, name string) BeatConsumer {
+	// Refuse to start `lnd` if the quit channel is not initialized. We
+	// treat this case as if we are facing a nil pointer dereference, as
+	// there's no point to return an error here, which will cause the node
+	// to fail to be started anyway.
+	if quit == nil {
+		panic("quit channel is nil")
+	}
+
+	b := BeatConsumer{
+		BlockbeatChan: make(chan Blockbeat),
+		name:          name,
+		errChan:       make(chan error, 1),
+		quit:          quit,
+	}
+
+	return b
+}
+
+// ProcessBlock takes a blockbeat and sends it to the consumer's blockbeat
+// channel. It will send it to the subsystem's BlockbeatChan, and block until
+// the processed result is received from the subsystem. The subsystem must call
+// `NotifyBlockProcessed` after it has finished processing the block.
+//
+// NOTE: part of the `chainio.Consumer` interface.
+func (b *BeatConsumer) ProcessBlock(beat Blockbeat) error {
+	// Update the current height.
+	beat.logger().Tracef("set current height for [%s]", b.name)
+
+	select {
+	// Send the beat to the blockbeat channel. It's expected that the
+	// consumer will read from this channel and process the block. Once
+	// processed, it should return the error or nil to the beat.Err chan.
+	case b.BlockbeatChan <- beat:
+		beat.logger().Tracef("Sent blockbeat to [%s]", b.name)
+
+	case <-b.quit:
+		beat.logger().Debugf("[%s] received shutdown before sending "+
+			"beat", b.name)
+
+		return nil
+	}
+
+	// Check the consumer's err chan. We expect the consumer to call
+	// `beat.NotifyBlockProcessed` to send the error back here.
+	select {
+	case err := <-b.errChan:
+		beat.logger().Debugf("[%s] processed beat: err=%v", b.name, err)
+
+		return err
+
+	case <-b.quit:
+		beat.logger().Debugf("[%s] received shutdown", b.name)
+	}
+
+	return nil
+}
+
+// NotifyBlockProcessed signals that the block has been processed. It takes the
+// blockbeat being processed and an error resulted from processing it. This
+// error is then sent back to the consumer's err chan to unblock
+// `ProcessBlock`.
+//
+// NOTE: This method must be called by the subsystem after it has finished
+// processing the block.
+func (b *BeatConsumer) NotifyBlockProcessed(beat Blockbeat, err error) {
+	// Update the current height.
+	beat.logger().Debugf("[%s]: notifying beat processed", b.name)
+
+	select {
+	case b.errChan <- err:
+		beat.logger().Debugf("[%s]: notified beat processed, err=%v",
+			b.name, err)
+
+	case <-b.quit:
+		beat.logger().Debugf("[%s] received shutdown before notifying "+
+			"beat processed", b.name)
+	}
+}