stdin-from-command: implemented suggestions in #4254

The code has been refactored so that the archiver is back to the original code, and the stderr is handled using a go routine to avoid deadlock.

internal/fs/fs_reader_command.go

@@ -0,0 +1,23 @@
+package fs
+
+import (
+	"io"
+	"os/exec"
+)
+
+// ReadCloserCommand wraps an exec.Cmd and its standard output to provide an
+// io.ReadCloser that waits for the command to terminate on Close(), reporting
+// any error in the command.Wait() function back to the Close() caller.
+type ReadCloserCommand struct {
+	Cmd    *exec.Cmd
+	Stdout io.ReadCloser
+}
+
+func (fp *ReadCloserCommand) Read(p []byte) (n int, err error) {
+	return fp.Stdout.Read(p)
+}
+
+func (fp *ReadCloserCommand) Close() error {
+	// No need to close fp.Stdout as Wait() closes all pipes.
+	return fp.Cmd.Wait()
+}