channeldb: boiler plate code for migration32

In preparation for the commit which will add the main logic for
migration 32 (which will migrate the MC store to use a more minimal
encoding), this commit just adds some of the code that the migration
will need to the package.

channeldb/migration32/codec.go

@@ -0,0 +1,146 @@
+package migration32
+
+import (
+	"encoding/binary"
+	"fmt"
+	"io"
+
+	"github.com/btcsuite/btcd/wire"
+	lnwire "github.com/lightningnetwork/lnd/channeldb/migration/lnwire21"
+)
+
+var (
+	// Big endian is the preferred byte order, due to cursor scans over
+	// integer keys iterating in order.
+	byteOrder = binary.BigEndian
+)
+
+// ReadElement is a one-stop utility function to deserialize any datastructure
+// encoded using the serialization format of the database.
+func ReadElement(r io.Reader, element interface{}) error {
+	switch e := element.(type) {
+	case *uint32:
+		if err := binary.Read(r, byteOrder, e); err != nil {
+			return err
+		}
+
+	case *lnwire.MilliSatoshi:
+		var a uint64
+		if err := binary.Read(r, byteOrder, &a); err != nil {
+			return err
+		}
+
+		*e = lnwire.MilliSatoshi(a)
+
+	case *[]byte:
+		bytes, err := wire.ReadVarBytes(r, 0, 66000, "[]byte")
+		if err != nil {
+			return err
+		}
+
+		*e = bytes
+
+	case *int64, *uint64:
+		if err := binary.Read(r, byteOrder, e); err != nil {
+			return err
+		}
+
+	case *bool:
+		if err := binary.Read(r, byteOrder, e); err != nil {
+			return err
+		}
+
+	case *int32:
+		if err := binary.Read(r, byteOrder, e); err != nil {
+			return err
+		}
+
+	default:
+		return UnknownElementType{"ReadElement", e}
+	}
+
+	return nil
+}
+
+// ReadElements deserializes a variable number of elements into the passed
+// io.Reader, with each element being deserialized according to the ReadElement
+// function.
+func ReadElements(r io.Reader, elements ...interface{}) error {
+	for _, element := range elements {
+		err := ReadElement(r, element)
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// UnknownElementType is an error returned when the codec is unable to encode or
+// decode a particular type.
+type UnknownElementType struct {
+	method  string
+	element interface{}
+}
+
+// Error returns the name of the method that encountered the error, as well as
+// the type that was unsupported.
+func (e UnknownElementType) Error() string {
+	return fmt.Sprintf("Unknown type in %s: %T", e.method, e.element)
+}
+
+// WriteElement is a one-stop shop to write the big endian representation of
+// any element which is to be serialized for storage on disk. The passed
+// io.Writer should be backed by an appropriately sized byte slice, or be able
+// to dynamically expand to accommodate additional data.
+func WriteElement(w io.Writer, element interface{}) error {
+	switch e := element.(type) {
+	case int64, uint64:
+		if err := binary.Write(w, byteOrder, e); err != nil {
+			return err
+		}
+
+	case uint32:
+		if err := binary.Write(w, byteOrder, e); err != nil {
+			return err
+		}
+
+	case int32:
+		if err := binary.Write(w, byteOrder, e); err != nil {
+			return err
+		}
+
+	case bool:
+		if err := binary.Write(w, byteOrder, e); err != nil {
+			return err
+		}
+
+	case lnwire.MilliSatoshi:
+		if err := binary.Write(w, byteOrder, uint64(e)); err != nil {
+			return err
+		}
+
+	case []byte:
+		if err := wire.WriteVarBytes(w, 0, e); err != nil {
+			return err
+		}
+
+	default:
+		return UnknownElementType{"WriteElement", e}
+	}
+
+	return nil
+}
+
+// WriteElements is writes each element in the elements slice to the passed
+// io.Writer using WriteElement.
+func WriteElements(w io.Writer, elements ...interface{}) error {
+	for _, element := range elements {
+		err := WriteElement(w, element)
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
+}