Re: Closing fds twice when using remote helpers

> Am 15.05.19 um 13:43 schrieb Ævar Arnfjörð Bjarmason:
> > 
> > On Wed, May 15 2019, Mike Hommey wrote:
> > 
> >> Hi,
> >>
> >> I started getting a weird error message during some test case involving
> >> git-cinnabar, which is a remote-helper to access mercurial
> >> repositories.
> >>
> >> The error says:
> >> fatal: mmap failed: Bad file descriptor
> >>
> >> ... which was not making much sense. Some debugging later, and it turns
> >> out this is what happens:
> >>
> >> - start_command is called for fast-import
> 
> I guess, you request fast_import->out = -1.
> 
> >> - start_command is called again for git-remote-hg, passing the
> >>   fast_import->out as cmd->in.
> 
> OK.
> 
> >> - in start_command, we end up on the line of code that does
> >>   close(cmd->in), so fast_import->out/cmd->in is now closed
> 
> Yes. That's how the interface is specified.
> 
> >> - much later, in disconnect_helper, we call close(data->helper->out),
> >>   where data->helper is the cmd for fast-import, and that fd was already
> >> closed above.
> 
> That must is wrong. Passing a fd to start_command() relinquishes
> responsibility.
> 
> >> - 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().