Hi
I think you should probably loop for EINTR from read() but otherwise
this looks fine to me.
On Thu, Sep 18, 2014 at 07:51:37PM -0500, J Raynor wrote:
> Tmux is hanging on aix, and the problem appears to be a race condition
> in the implementation of forkpty.
>
> Aix has the behavior that, if data has been written to the slave side
> of a pty but has not been read yet by the master side, then the last
> close of the slave pty will hang until the data is read, or the master
> is closed.
>
> I'm able to trigger the hang by calling splitw with a bad shell
> command. What's happening is that after the call to fork in forkpty,
> the child process continues and the spawned shell writes a "command
> not found" message and then exits. It is able to do this before the
> parent process calls close(slave) at the end of its part of forkpty.
> By the time the parent calls close(slave), some data has been written
> to the slave, and the parent has the last open reference to the slave
> pty. So, the parent hangs in the close.
>
> Some type of coordination has to be done so that the parent (the tmux
> server process) can't end up with the last open reference to the
> slave. I've attached a patch that provides a possible solution.
> diff --git a/compat/forkpty-aix.c b/compat/forkpty-aix.c
> index db9c2e7..06f4706 100644
> --- a/compat/forkpty-aix.c
> +++ b/compat/forkpty-aix.c
> @@ -29,10 +29,13 @@
> pid_t
> forkpty(int *master, unused char *name, struct termios *tio, struct winsize
> *ws)
> {
> - int slave, fd;
> - char *path;
> + int slave, fd, pipe_fd[2];
> + char *path, dummy_buf;
> pid_t pid;
>
> + if (pipe(pipe_fd) == -1)
> + return (-1);
> +
> if ((*master = open("/dev/ptc", O_RDWR|O_NOCTTY)) == -1)
> return (-1);
>
> @@ -46,6 +49,10 @@ forkpty(int *master, unused char *name, struct termios
> *tio, struct winsize *ws)
> goto out;
> case 0:
> close(*master);
> + close(pipe_fd[1]);
> +
> + read(pipe_fd[0], &dummy_buf, 1);
> + close(pipe_fd[0]);
>
> fd = open(_PATH_TTY, O_RDWR|O_NOCTTY);
> if (fd >= 0) {
> @@ -80,13 +87,18 @@ forkpty(int *master, unused char *name, struct termios
> *tio, struct winsize *ws)
> dup2(slave, 2);
> if (slave > 2)
> close(slave);
> +
> return (0);
> }
>
> close(slave);
> + close(pipe_fd[0]);
> + close(pipe_fd[1]);
> return (pid);
>
> out:
> + close(pipe_fd[0]);
> + close(pipe_fd[1]);
> if (*master != -1)
> close(*master);
> if (slave != -1)
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