bug#47596: File descriptor error when exiting emacs on android 11

[Eli Zaretskii <eliz@gnu.org>]

> From: Henrik Grimler <henrik@grimler.se>
> Cc: 47596@debbugs.gnu.org
> Date: Mon, 05 Apr 2021 10:14:10 +0200
> 
> Compiling this, on any optimisation level, is enough to trigger the
> error:
> 
> ```
> #include <stdio.h>
> int main()
> {
>   fdopen (2, "w");
>   fclose (stderr);
> }
> ```

Any idea why?  It makes no sense to me, and the above is an entirely
valid program, AFAICT.

> In emacs fdopen is run in init_standard_fds, where we have
> 
> ```
> [...]
> 
>   force_open (STDERR_FILENO, O_RDONLY);
> 
>   /* Set buferr if possible on platforms defining _PC_PIPE_BUF, as
>      they support the notion of atomic writes to pipes.  */
>   #ifdef _PC_PIPE_BUF
>     buferr = fdopen (STDERR_FILENO, "w");
>     if (buferr)
>       setvbuf (buferr, NULL, _IOLBF, 0);
>   #endif
> }
> ```

This just creates a copy of stderr that has special buffering.  Again,
entirely valid for a C program to do that.

> so I suppose there is either some very fundamental issue with 
> `fdopen (STDERR_FILENO, "w")` here, or the file descriptor sanitizer on
> android is broken.

Likely the latter, so I'm reluctant to make any changes in Emacs until
the issue you opened against Android gets some response that makes
sense.

> If I avoid that part of init_standard_fds, i.e. change the ifdef to
> something like `#if defined(_PC_PIPE_BUF) && !defined(__ANDROID__)`, I
> get an emacs that seem to fully work on android.

Feel free to make such changes in your copy, but I'd prefer not to
change Emacs until the root cause behind these weird problems becomes
clear.