chanbackup: add functionality to allow external callers to backup channels

In this commit, we introduce a series of interfaces and methods that
will allow external callers to backup either all channels, or a specific
channel identified by its channel point. In order to abstract away the
details w.r.t _how_ we obtain the set of open channels, or their storage
mechanisms, we introduce a new LiveChannelSource interfaces. This
interfaces allows us to fetch all channels, a channel by its channel
point, and also all the known addresses for a node as we'll need this in
order to connect out to the node in the case of a recovery attempt.

chanbackup/backup.go

@@ -0,0 +1,99 @@
+package chanbackup
+
+import (
+	"fmt"
+	"net"
+
+	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/wire"
+	"github.com/lightningnetwork/lnd/channeldb"
+)
+
+// LiveChannelSource is an interface that allows us to query for the set of
+// live channels. A live channel is one that is open, and has not had a
+// commitment transaction broadcast.
+type LiveChannelSource interface {
+	// FetchAllChannels returns all known live channels.
+	FetchAllChannels() ([]*channeldb.OpenChannel, error)
+
+	// FetchChannel attempts to locate a live channel identified by the
+	// passed chanPoint.
+	FetchChannel(chanPoint wire.OutPoint) (*channeldb.OpenChannel, error)
+
+	// AddrsForNode returns all known addresses for the target node public
+	// key.
+	AddrsForNode(nodePub *btcec.PublicKey) ([]net.Addr, error)
+}
+
+// assembleChanBackup attempts to assemble a static channel backup for the
+// passed open channel. The backup includes all information required to restore
+// the channel, as well as addressing information so we can find the peer and
+// reconnect to them to initiate the protocol.
+func assembleChanBackup(chanSource LiveChannelSource,
+	openChan *channeldb.OpenChannel) (*Single, error) {
+
+	log.Debugf("Crafting backup for ChannelPoint(%v)",
+		openChan.FundingOutpoint)
+
+	// First, we'll query the channel source to obtain all the addresses
+	// that are are associated with the peer for this channel.
+	nodeAddrs, err := chanSource.AddrsForNode(openChan.IdentityPub)
+	if err != nil {
+		return nil, err
+	}
+
+	single := NewSingle(openChan, nodeAddrs)
+
+	return &single, nil
+}
+
+// FetchBackupForChan attempts to create a plaintext static channel backup for
+// the target channel identified by its channel point. If we're unable to find
+// the target channel, then an error will be returned.
+func FetchBackupForChan(chanPoint wire.OutPoint,
+	chanSource LiveChannelSource) (*Single, error) {
+
+	// First, we'll query the channel source to see if the channel is known
+	// and open within the database.
+	targetChan, err := chanSource.FetchChannel(chanPoint)
+	if err != nil {
+		// If we can't find the channel, then we return with an error,
+		// as we have nothing to  backup.
+		return nil, fmt.Errorf("unable to find target channel")
+	}
+
+	// Once we have the target channel, we can assemble the backup using
+	// the source to obtain any extra information that we may need.
+	staticChanBackup, err := assembleChanBackup(chanSource, targetChan)
+	if err != nil {
+		return nil, fmt.Errorf("unable to create chan backup: %v", err)
+	}
+
+	return staticChanBackup, nil
+}
+
+// FetchStaticChanBackups will return a plaintext static channel back up for
+// all known active/open channels within the passed channel source.
+func FetchStaticChanBackups(chanSource LiveChannelSource) ([]Single, error) {
+	// First, we'll query the backup source for information concerning all
+	// currently open and available channels.
+	openChans, err := chanSource.FetchAllChannels()
+	if err != nil {
+		return nil, err
+	}
+
+	// Now that we have all the channels, we'll use the chanSource to
+	// obtain any auxiliary information we need to craft a backup for each
+	// channel.
+	staticChanBackups := make([]Single, len(openChans))
+	for i, openChan := range openChans {
+		chanBackup, err := assembleChanBackup(chanSource, openChan)
+		if err != nil {
+			return nil, err
+		}
+
+		staticChanBackups[i] = *chanBackup
+	}
+
+	return staticChanBackups, nil
+}