chainio: implement Blockbeat

In this commit, a minimal implementation of `Blockbeat` is added to
synchronize block heights, which will be used in `ChainArb`, `Sweeper`,
and `TxPublisher` so blocks are processed sequentially among them.

chainio/blockbeat.go

@@ -0,0 +1,54 @@
+package chainio
+
+import (
+	"fmt"
+
+	"github.com/btcsuite/btclog/v2"
+	"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 = clog.WithPrefix(logPrefix)
+
+	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
+}