objectstore/packed: Add initial pack reading support

1 files changed, 41 insertions, 0 deletions

diff --git a/objectstore/packed/entry_inflate.go b/objectstore/packed/entry_inflate.go
new file mode 100644
index 00000000..5f1c476e
--- /dev/null
+++ b/objectstore/packed/entry_inflate.go
@@ -0,0 +1,41 @@
+package packed
+
+import (
+	"bytes"
+	"compress/zlib"
+	"fmt"
+	"io"
+)
+
+// zlibReaderAt opens a zlib reader starting at data offset within pack.
+func zlibReaderAt(pack *packFile, offset int) (io.ReadCloser, error) {
+	if offset < 0 || offset > len(pack.data) {
+		return nil, fmt.Errorf("objectstore/packed: pack %q zlib offset out of bounds", pack.name)
+	}
+	return zlib.NewReader(bytes.NewReader(pack.data[offset:]))
+}
+
+// inflateAt inflates one entry payload from data offset.
+//
+// When expectedSize is non-negative, the inflated length must match.
+func inflateAt(pack *packFile, offset int, expectedSize int64) ([]byte, error) {
+	reader, err := zlibReaderAt(pack, offset)
+	if err != nil {
+		return nil, err
+	}
+	defer func() { _ = reader.Close() }()
+
+	body, err := io.ReadAll(reader)
+	if err != nil {
+		return nil, err
+	}
+	if expectedSize >= 0 && int64(len(body)) != expectedSize {
+		return nil, fmt.Errorf(
+			"objectstore/packed: pack %q inflated size mismatch: got %d want %d",
+			pack.name,
+			len(body),
+			expectedSize,
+		)
+	}
+	return body, nil
+}