feat(rar): add support for multi-volume rar archives

rar.go

@@ -20,6 +20,12 @@ func init() {
 	RegisterFormat(Rar{})
 }
 
+type rarReader interface {
+	Next() (*rardecode.FileHeader, error)
+	io.Reader
+	io.WriterTo
+}
+
 type Rar struct {
 	// If true, errors encountered during reading or writing
 	// a file within an archive will be logged and the
@@ -28,6 +34,23 @@ type Rar struct {
 
 	// Password to open archives.
 	Password string
+
+	// Name for a multi-volume archive. When Name is specified,
+	// the named file is extracted (rather than any io.Reader that
+	// may be passed to Extract). If the archive is a multi-volume
+	// archive, this name will also be used by the decoder to derive
+	// the filename of the next volume in the volume set.
+	Name string
+
+	// FS is an fs.FS exposing the files of the archive. Unless Name is
+	// also specified, this does nothing. When Name is also specified,
+	// FS defines the fs.FS that from which the archive will be opened,
+	// and in the case of a multi-volume archive, from where each subsequent
+	// volume of the volume set will be loaded.
+	//
+	// Typically this should be a DirFS pointing at the directory containing
+	// the volumes of the archive.
+	FS fs.FS
 }
 
 func (Rar) Extension() string { return ".rar" }
@@ -65,7 +88,26 @@ func (r Rar) Extract(ctx context.Context, sourceArchive io.Reader, handleFile Fi
 		options = append(options, rardecode.Password(r.Password))
 	}
 
-	rr, err := rardecode.NewReader(sourceArchive, options...)
+	if r.FS != nil {
+		options = append(options, rardecode.FileSystem(r.FS))
+	}
+
+	var (
+		rr  rarReader
+		err error
+	)
+
+	// If a name has been provided, then the sourceArchive stream is ignored
+	// and the archive is opened directly via the filesystem (or provided FS).
+	if r.Name != "" {
+		var or *rardecode.ReadCloser
+		if or, err = rardecode.OpenReader(r.Name, options...); err == nil {
+			rr = or
+			defer or.Close()
+		}
+	} else {
+		rr, err = rardecode.NewReader(sourceArchive, options...)
+	}
 	if err != nil {
 		return err
 	}

rar_test.go

@@ -0,0 +1,40 @@
+package archives
+
+import (
+	"context"
+	"crypto/sha1"
+	"encoding/hex"
+	"io"
+	"testing"
+)
+
+func TestRarExtractMultiVolume(t *testing.T) {
+	// Test files testdata/test.part*.rar were created by:
+	//   seq 0 2000 > test.txt
+	//   rar a -v1k test.rar test.txt
+	rar := Rar{
+		Name: "test.part01.rar",
+		FS:   DirFS("testdata"),
+	}
+
+	const expectedSHA1Sum = "4da7f88f69b44a3fdb705667019a65f4c6e058a3"
+	if err := rar.Extract(context.Background(), nil, func(_ context.Context, info FileInfo) error {
+		f, err := info.Open()
+		if err != nil {
+			return err
+		}
+		defer f.Close()
+
+		h := sha1.New()
+		if _, err = io.Copy(h, f); err != nil {
+			return err
+		}
+
+		if got := hex.EncodeToString(h.Sum(nil)); got != expectedSHA1Sum {
+			t.Errorf("expected %s, got %s", expectedSHA1Sum, got)
+		}
+		return nil
+	}); err != nil {
+		t.Error(err)
+	}
+}