contractcourt/chain_watcher test: do proper state rollback

The tests didn't really roll back the channel state, so we would only
rely on the state number to determine whether we had lost state. Now we
properly roll back the channel to a previous state, in preparation for
upcoming changes.

contractcourt/utils_test.go

@@ -1,10 +1,16 @@
 package contractcourt
 
 import (
+	"fmt"
+	"io"
+	"io/ioutil"
 	"os"
+	"path/filepath"
 	"runtime/pprof"
 	"testing"
 	"time"
+
+	"github.com/lightningnetwork/lnd/channeldb"
 )
 
 // timeout implements a test level timeout.
@@ -24,3 +30,69 @@ func timeout(t *testing.T) func() {
 		close(done)
 	}
 }
+
+func copyFile(dest, src string) error {
+	s, err := os.Open(src)
+	if err != nil {
+		return err
+	}
+	defer s.Close()
+
+	d, err := os.Create(dest)
+	if err != nil {
+		return err
+	}
+
+	if _, err := io.Copy(d, s); err != nil {
+		d.Close()
+		return err
+	}
+
+	return d.Close()
+}
+
+// copyChannelState copies the OpenChannel state by copying the database and
+// creating a new struct from it. The copied state and a cleanup function are
+// returned.
+func copyChannelState(state *channeldb.OpenChannel) (
+	*channeldb.OpenChannel, func(), error) {
+
+	// Make a copy of the DB.
+	dbFile := filepath.Join(state.Db.Path(), "channel.db")
+	tempDbPath, err := ioutil.TempDir("", "past-state")
+	if err != nil {
+		return nil, nil, err
+	}
+
+	cleanup := func() {
+		os.RemoveAll(tempDbPath)
+	}
+
+	tempDbFile := filepath.Join(tempDbPath, "channel.db")
+	err = copyFile(tempDbFile, dbFile)
+	if err != nil {
+		cleanup()
+		return nil, nil, err
+	}
+
+	newDb, err := channeldb.Open(tempDbPath)
+	if err != nil {
+		cleanup()
+		return nil, nil, err
+	}
+
+	chans, err := newDb.FetchAllChannels()
+	if err != nil {
+		cleanup()
+		return nil, nil, err
+	}
+
+	// We only support DBs with a single channel, for now.
+	if len(chans) != 1 {
+		cleanup()
+		return nil, nil, fmt.Errorf("found %d chans in the db",
+			len(chans))
+	}
+
+	return chans[0], cleanup, nil
+}