Fix pool buffer bypass caused by *os.File WriterTo

On Go 1.26+, *os.File implements WriterTo. When ioCopier's reader is an
*os.File (the common case for commandStage), io.CopyBuffer detects
WriterTo and calls File.WriteTo instead of using the provided pool
buffer. File.WriteTo's sendfile path fails (the dest is not a network
connection), so it falls back to genericWriteTo → io.Copy, which
allocates a fresh 32KB buffer on every call — defeating the sync.Pool
entirely.

Fix: wrap the reader in readerOnly{} to strip all interfaces except
Read, forcing io.CopyBuffer to use the pool buffer.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: znull

pipe/iocopier.go

@@ -26,6 +26,10 @@ var copyBufPool = sync.Pool{
 	},
 }
 
+// readerOnly wraps an io.Reader, hiding any other interfaces (such as
+// WriterTo) so that io.CopyBuffer is forced to use the provided buffer.
+type readerOnly struct{ io.Reader }
+
 func newIOCopier(w io.WriteCloser) *ioCopier {
 	return &ioCopier{
 		w:    w,
@@ -41,7 +45,13 @@ func (s *ioCopier) Name() string {
 func (s *ioCopier) Start(_ context.Context, _ Env, r io.ReadCloser) (io.ReadCloser, error) {
 	go func() {
 		bp := copyBufPool.Get().(*[]byte)
-		_, err := io.CopyBuffer(s.w, r, *bp)
+		// Strip all interfaces except Read from r so that
+		// io.CopyBuffer always uses the provided pool buffer.
+		// Without this, *os.File's WriterTo (added in Go 1.26)
+		// causes CopyBuffer to call File.WriteTo, which falls
+		// back to io.Copy with a fresh allocation, bypassing
+		// the pool entirely.
+		_, err := io.CopyBuffer(s.w, readerOnly{r}, *bp)
 		copyBufPool.Put(bp)
 		// We don't consider `ErrClosed` an error (FIXME: is this
 		// correct?):