internal/pkg: stream decompress artifact

The tarArtifact predates FileArtifact pipelining. This migrates decompression and buffering into a standalone artifact implementation.

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

internal/pkg/pkg.go

@@ -508,6 +508,8 @@ const (
 	KindExecNet
 	// KindFile is the kind of [Artifact] returned by [NewFile].
 	KindFile
+	// KindDecompress is the kind of [Artifact] returned by [NewDecompress].
+	KindDecompress
 
 	// _kindEnd is the total number of kinds and does not denote a kind.
 	_kindEnd
@@ -795,6 +797,38 @@ func (c *Cache) getReader(r io.Reader) *bufio.Reader {
 // putReader adds br to brPool.
 func (c *Cache) putReader(br *bufio.Reader) { c.brPool.Put(br) }
 
+// bufioReadCloser is the concrete type of value returned by Cache.getReaderRC.
+type bufioReadCloser struct {
+	// Saved close error.
+	closeErr error
+	// Synchronises calls to Close.
+	closeOnce sync.Once
+
+	// For backing freelist.
+	c *Cache
+	// Underlying reader.
+	r io.ReadCloser
+	// Allocated from c.
+	*bufio.Reader
+}
+
+// Close closes the underlying reader, saves its return value, and returns the
+// [bufio.Reader] instance to the backing [Cache].
+func (brc *bufioReadCloser) Close() error {
+	brc.closeOnce.Do(func() {
+		br := brc.Reader
+		brc.Reader = nil
+		brc.c.putReader(br)
+		brc.closeErr = brc.r.Close()
+	})
+	return brc.closeErr
+}
+
+// getReaderRC is like getReader, but returns an [io.ReadCloser].
+func (c *Cache) getReaderRC(r io.ReadCloser) io.ReadCloser {
+	return &bufioReadCloser{c: c, r: r, Reader: c.getReader(r)}
+}
+
 // getWriter is like [bufio.NewWriter] but for bwPool.
 func (c *Cache) getWriter(w io.Writer) *bufio.Writer {
 	bw := c.bwPool.Get().(*bufio.Writer)