internal/pkg: streaming archive reader/writer

This is much more robust and efficient than the simple buffering implementation for larger files. Allocations happen almost exclusively in WalkDir.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/report.go

@@ -26,7 +26,17 @@ func padSize[T int | int64](sz T) T {
 	return (wordSize - (sz)%wordSize) % wordSize
 }
 
-// WriteReport writes a report of all available [PArtifact] to w.
+// countWriter records total amount of data written.
+type countWriter uint64
+
+// Write records the size of p.
+func (w *countWriter) Write(p []byte) (n int, err error) {
+	n = len(p)
+	*w += countWriter(n)
+	return
+}
+
+// WriteReport writes a report of all available [Artifact] to w.
 func WriteReport(msg message.Msg, w io.Writer, c *pkg.Cache) error {
 	var (
 		zero [wordSize]byte
@@ -90,12 +100,12 @@ func WriteReport(msg message.Msg, w io.Writer, c *pkg.Cache) error {
 		}
 
 		// existence of status implies cured artifact
-		var n int
+		var n countWriter
 		if pathname, _, err := c.Cure(a); err != nil {
 			return err
-		} else if n, err = pkg.Flatten(
+		} else if err = pkg.Write(
 			os.DirFS(pathname.String()), ".",
-			io.Discard,
+			&n,
 		); err != nil {
 			return err
 		}