channeldb+lnwallet: add ShutdownInfo with read and write methods

ShutdownInfo contains any info about a previous Shutdown message that we
have sent. This commit adds this type along with read and write methods
for it in the channel db. The existence of the ShutdownInfo on disk
represents the fact that we have previously sent the Shutdown message
and hence that we should resend it on re-establish.

channeldb/channel_test.go

@@ -1158,6 +1158,70 @@ func TestFetchWaitingCloseChannels(t *testing.T) {
 	}
 }
 
+// TestShutdownInfo tests that a channel's shutdown info can correctly be
+// persisted and retrieved.
+func TestShutdownInfo(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name      string
+		localInit bool
+	}{
+		{
+			name:      "local node initiated",
+			localInit: true,
+		},
+		{
+			name:      "remote node initiated",
+			localInit: false,
+		},
+	}
+
+	for _, test := range tests {
+		test := test
+
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+
+			testShutdownInfo(t, test.localInit)
+		})
+	}
+}
+
+func testShutdownInfo(t *testing.T, locallyInitiated bool) {
+	fullDB, err := MakeTestDB(t)
+	require.NoError(t, err, "unable to make test database")
+
+	cdb := fullDB.ChannelStateDB()
+
+	// First a test channel.
+	channel := createTestChannel(t, cdb)
+
+	// We haven't persisted any shutdown info for this channel yet.
+	_, err = channel.ShutdownInfo()
+	require.Error(t, err, ErrNoShutdownInfo)
+
+	// Construct a new delivery script and create a new ShutdownInfo object.
+	script := []byte{1, 3, 4, 5}
+
+	// Create a ShutdownInfo struct.
+	shutdownInfo := NewShutdownInfo(script, locallyInitiated)
+
+	// Persist the shutdown info.
+	require.NoError(t, channel.MarkShutdownSent(shutdownInfo))
+
+	// We should now be able to retrieve the shutdown info.
+	info, err := channel.ShutdownInfo()
+	require.NoError(t, err)
+	require.True(t, info.IsSome())
+
+	// Assert that the decoded values of the shutdown info are correct.
+	info.WhenSome(func(info ShutdownInfo) {
+		require.EqualValues(t, script, info.DeliveryScript.Val)
+		require.Equal(t, locallyInitiated, info.LocalInitiator.Val)
+	})
+}
+
 // TestRefresh asserts that Refresh updates the in-memory state of another
 // OpenChannel to reflect a preceding call to MarkOpen on a different
 // OpenChannel.