Re: [PATCH v3 2/3] sha1_file: open window into packfiles with O_CLOEXEC

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Oct 28, 2016, 11:11 UTC
Message-ID: <alpine.DEB.2.20.1610281306320.3264@virtualbox>
In-Reply-To: <xmqq60od42s0.fsf@gitster.mtv.corp.google.com>

Hi,

On Thu, 27 Oct 2016, Junio C Hamano wrote:

Show 27 quoted lines

> Junio C Hamano <gitster@pobox.com> writes:
> 
> > Linus Torvalds <torvalds@linux-foundation.org> writes:
> >
> >> On Thu, Oct 27, 2016 at 4:36 PM, Junio C Hamano <gitster@pobox.com> wrote:
> >>>
> >>> Would the best endgame shape for this function be to open with
> >>> O_NOATIME (and retry without), and then add CLOEXEC with fcntl(2)
> >>> but ignoring an error from it, I guess?  That would be the closest
> >>> to what we historically had, I would think.
> >>
> >> I think that's the best model.
> >
> > OK, so perhaps like this.
> 
> Hmph.  This may not fly well in practice, though.  
> 
> To Unix folks, CLOEXEC is not a huge correctness issue.  A child
> process may hold onto an open file descriptor a bit longer than the
> lifetime of the parent but as long as the child eventually exits,
> nothing is affected.  Over there, things are different.  The parent
> cannot even rename(2) or unlink(2) a file it created and closed
> while the child is still holding the file descriptor open and the
> lack of CLOEXEC will make the parent fail.  I do not know how well
> fcntl(2) emulation works on Windows, but I would not be surprised
> if J6t or Dscho comes back and says that FD_CLOEXEC given to F_SETFD
> would not work while O_CLOEXEC given to open(2) does.

You guys. I mean: You guys! You sure make my life hard. A brief look at mingw.h could have answered your implicit question:

	static inline int fcntl(int fd, int cmd, ...)
	{
		if (cmd == F_GETFD || cmd == F_SETFD)
			return 0;
		errno = EINVAL;
		return -1;
	}

So while you discuss in your Linux Ivory Tower how to optimize Git for Linux, and Linux only, I'll have to drop everything else and spend the rest of my Friday trying to find a way to adjust a file handle *immediately after opening it with undesired flags* (when it could have been opened with the desired flags, as suggested, to begin with).

Ciao, Johannes