Re: [PATCH v7 1/7] run-command: add duplicate_output_fn to run_processes_parallel_opts

> > +                     } else {
> > +                             if (opts->duplicate_output)
> > +                                     opts->duplicate_output(&pp->children[i].err,
> > +                                            strlen(pp->children[i].err.buf) - n,
>
> Looking at how this is used in patch 7 I think it would be better to
> pass a const char*, length pair rather than a struct strbuf*, offset pair.
> i.e.
>         opts->duplicate_output(pp->children[i].err.buf +
> pp->children[i].err.len - n, n, ...)
>
> That would make it clear that we do not expect duplicate_output() to
> alter the buffer and would avoid the duplicate_output() having to add
> the offset to the start of the buffer to find the new data.