Re: Emacs can't start-process more than 30~40 processes (Was: Re: Emacs w3m `w3m-toggle-inline-images' cause segfault)

[Haojun Bao <baohaojun at gmail dot com>]

Haojun Bao <baohaojun@gmail.com> writes:

> I have reduced the test case in this mail 
>   http://cygwin.com/ml/cygwin/2009-07/msg00111.html
> to a simpler one:
>
>     $/bin/emacs --batch -q  --execute '(let ((num 0))
>       (while (< num 30)
>         (setq num (+ num 1))
>         (message "num is %d" num)
>         (start-process "hello" nil "/usr/bin/echo")))'
>     
> Emacs will coredump at the 30th process it tries to start on my XP.
>
> Now I think this should seem familiar to the experts. I tried to gdb it,
> the backtrace shows segfault is happening at the same place 
> (#0 0x610949d8 in fhandler_pipe::create () from /usr/bin/cygwin1.dll)
>
> You might need to tweak the (< num 30) to (< num 40) or bigger, also, to
> use gdb on it, you need write the lisp into a file and use `-l' to load
> the file:
>
> cat > ~/2.el <<End
> (let ((num 0))
>   (while (< num 40)
>     (setq num (+ num 1))
>     (message "num is %d" num)
>     (start-process "hello" nil "/usr/bin/echo")))
> End
>
> gdb --args ./emacs --batch -q  -l ~/2.el

I have debugged it again, and I think I have more clue. I have read the
how-cygheap-works.txt, and this might be a known problem.

It's because the cygheap space has been used up. With Procexp, I can see
cygwin1.dll is based 0x61000000, with size 0x300000 (3M). When segfault
is about to happen, cygheap_max is 0x6164e924, and the _csbrk is called
with a increase of 
    (gdb) p sbs
    $16 = 65544
This will increase cyghead to 0x6165e92c, but from Proxexp I can see
cygncurses-9.dll is based at 0x61650000.

Besides, this code snippet from pipe.cc fhandler_pipe::create didn't
check for NULL pointer, and directly caused the segfault.

      fhs[0] = (fhandler_pipe *) build_fh_dev (*piper_dev);
      fhs[1] = (fhandler_pipe *) build_fh_dev (*pipew_dev);

//bhj: we should check NULL here.

      mode |= mode & O_TEXT ?: O_BINARY;
      fhs[0]->init (r, FILE_CREATE_PIPE_INSTANCE | GENERIC_READ, mode);
      fhs[1]->init (w, FILE_CREATE_PIPE_INSTANCE | GENERIC_WRITE, mode);

My question is, is there anyway out of this? Can I just rebase
cygwin1.dll to the end of all other DLLs? 

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