Re: Closing fds twice when using remote helpers

> On Thu, May 16, 2019 at 07:08:34AM +0900, Mike Hommey wrote:
> 
> > > >> - Except, well, fds being what they are, we in fact just closed a fd
> > > >>   from a packed_git->pack_fd. So, when use_pack is later called, and
> > > >>   tries to mmap data from that pack, it fails because the file
> > > >>   descriptor was closed.
> > > 
> > > Either dup() the file descriptor, or mmap() before you call the
> > > consuming start_command().
> 
> If I understand your case correctly, the mmap() is not relevant.  The
> issue is that we close a random file descriptor accidentally, and it
> just happens to be a pack descriptor later on. Right?
> 
> > You seem to imply this is my code doing something. It's not. The process
> > that is doing all the things I noted above is an unmodified `git fetch`,
> > when using a remote-helper transport. The use_pack happens after the
> > transport is disposed because that's at the end of git fetch, when it
> > updates refs.
> 
> Yes, it's a bug in git.
> 
> > Anyway, it would seem the fix is to dup(out) when passing it as in to
> > start_command?
> 
> Generally, yes. It looks like maybe this spot?
> 
> diff --git a/transport-helper.c b/transport-helper.c
> index fcd2a58d0e..45cdf891ec 100644
> --- a/transport-helper.c
> +++ b/transport-helper.c
> @@ -433,7 +433,7 @@ static int get_importer(struct transport *transport, struct child_process *fasti
>  	struct helper_data *data = transport->data;
>  	int cat_blob_fd, code;
>  	child_process_init(fastimport);
> -	fastimport->in = helper->out;
> +	fastimport->in = xdup(helper->out);
>  	argv_array_push(&fastimport->args, "fast-import");
>  	argv_array_push(&fastimport->args, debug ? "--stats" : "--quiet");
>  
> 
> One thing I'd wonder, though: what is the contract between the helper
> and fast-import here? In the current code, when the helper closes its
> stdout, fast-import will see EOF. But not if we are holding on to
> another copy of the descriptor.