bug#8705: 23.3; Emacs occasionally crashes (segfault) just after starting it

[Glenn Morris <rgm@gnu.org>]

Stefan Monnier wrote:

>> Any news on this bug? Debian's GNU Emacs 24.3.1 is still affected.
>
> Same question here.
 

Comments from https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=699325#17:


> What emacs appears to be doing is:
> 
> * vfork() in thread A
>   - parent: thread A suspended
>   - parent: threads B, C, ... (one of which is the Gtk GUI) continue
>   - child: "shares all memory with its parent, including the stack"
>     per vfork(2)
> 
> * child copies environ and modifies the copy as needed
> 
> RACE:
> child + parent thread A:
>   * changes the global environ pointer, potentially making it point to
>     a new mmap() that only exists in the child process (or something?)
>   * child: calls execvp()
>   * parent: thread A resumes and puts the old environ back
> parent threads B, C...
>   * threads B, C, ... continue their work and might call getenv()
> 
> If the child wins the race, everything's OK; if the parent's threads B,
> C... "win" the race, everything explodes. It seems that Gtk, in the
> parent's GUI thread, is now more likely to "win" the race and crash,
> because new features like touchscreen support have the side-effect that
> it calls getenv() more often.
> 
> On the upstream emacs bug, Troels Nielsen wrote:
> > In the meantime, retaining support for vfork would be nice, because
> > on some platforms, like Cygwin, fork is still very slow
> 
> but on Linux (and hopefully also *BSD and Hurd), fork() is quite fast,
> and considerably less crashy. I would suggest changing the vfork() call
> to fork(), making sure the environ rewriting is only done in the child
> side of the fork(), and seeing whether that helps.
> 
> Alternatively, emacs could use execvpe() instead of execve() on
> platforms where it exists (including all GNU platforms as far as I
> know), so that it does not need to alter the value of environ at all on
> such platforms. I think that would fix this?