fix: validate format in index file

pkg/appendable/index_file.go

@@ -54,6 +54,21 @@ func NewIndexFile(f io.ReadWriteSeeker, dataHandler DataHandler) (*IndexFile, er
 		} else if i > 1 {
 			panic("expected to only reset the first page once")
 		} else {
+			// validate the metadata
+			buf, err := tree.Metadata()
+			if err != nil {
+				return nil, fmt.Errorf("failed to read metadata: %w", err)
+			}
+			metadata := &FileMeta{}
+			if err := metadata.UnmarshalBinary(buf); err != nil {
+				return nil, fmt.Errorf("failed to unmarshal metadata: %w", err)
+			}
+			if metadata.Version != CurrentVersion {
+				return nil, fmt.Errorf("unsupported version: %d", metadata.Version)
+			}
+			if metadata.Format != dataHandler.Format() {
+				return nil, fmt.Errorf("unsupported format: %x", metadata.Format)
+			}
 			return &IndexFile{tree: tree, dataHandler: dataHandler}, nil
 		}
 	}

pkg/appendable/index_file_test.go

@@ -0,0 +1,36 @@
+package appendable
+
+import (
+	"testing"
+
+	"github.com/kevmo314/appendable/pkg/buftest"
+)
+
+type FormatHandler struct{ ReturnsFormat Format }
+
+func (f FormatHandler) Format() Format {
+	return f.ReturnsFormat
+}
+
+func (f FormatHandler) Synchronize(f1 *IndexFile, df []byte) error {
+	return nil
+}
+
+func (f FormatHandler) Parse(data []byte) []byte {
+	return nil
+}
+
+func TestIndexFile(t *testing.T) {
+	t.Run("validate metadata throws error if format doesn't match on second read", func(t *testing.T) {
+		f := buftest.NewSeekableBuffer()
+
+		if _, err := NewIndexFile(f, &FormatHandler{ReturnsFormat: Format(1)}); err != nil {
+			t.Fatal(err)
+		}
+
+		// try creating a new index file with a different format
+		if _, err := NewIndexFile(f, &FormatHandler{ReturnsFormat: Format(2)}); err == nil {
+			t.Fatal("expected error")
+		}
+	})
+}