[GRASS-dev] Open/close db drivers order

[Glynn Clements]

Radim Blazek wrote:

> I found, that db drivers must be closed in reverse order to the order
> in which were opened, otherwise it hangs (Linux). I vaguely remember
> that this was a problem years ago. Is it still an issue, anybody knows
> more about the problem?

It sounds like an issue with inheriting file descriptors. I.e.:

1. Opening the first driver causes the main process to get some file
descriptors for the pipes connecting it to the driver

2. Opening the second driver will result in the driver inheriting
copies of those descriptors from the main process.

3. When the main process closes the write end of the pipe whose read
end is the first driver's stdin, the first driver won't get EOF on its
stdin because the second driver still has a copy of the descriptor for
the write end of the pipe and hasn't closed it (it doesn't even know
it has it).

If that's what's happening, setting the close-on-exec flag for each
descriptor should fix it. E.g.

        void close_on_exec(int fd)
        {
        #ifndef __MINGW32__
            int flags = fcntl(fd, F_GETFD);
            fcntl(fd, F_SETFD, flags | FD_CLOEXEC);
        #endif
        }

	close_on_exec(p1[READ]);
	close_on_exec(p1[WRITE]);
	close_on_exec(p2[READ]);
	close_on_exec(p2[WRITE]);

p1[WRITE] is the one which is most likely to matter; that's the one
which needs to be closed (in all processes) for the child to get EOF
on stdin.

p1[READ] and p2[WRITE] are closed in the parent after the child has
been spawned, so they won't be inherited by subsequent child
processes.

-- 
Glynn Clements <glynn at gclements.plus.com>