Re: SIGHUP on pty closure

[Christopher Faylor <cgf-use-the-mailinglist-please at cygwin dot com>]

On Mon, Jul 25, 2011 at 12:36:58PM +0200, Marco atzeri wrote:
>On 7/21/2011 11:43 PM, Marco atzeri wrote:
>> looking on the mc subshell issue, I found that mc
>> suppose that the subshell will receive a SIGHUP
>> when mc exit and close the master side of pty.
>>
>> Is such assumption wrong or it is a missing piece of
>> cygwin pty implementation ?
>>
>>
>>
>> ------------- extract from subshell.c --------------
>> /* Attach all our standard file descriptors to the pty */
>>
>> /* This is done just before the fork, because stderr must still */
>> /* be connected to the real tty during the above error messages; */
>> /* otherwise the user will never see them. */
>>
>> dup2 (subshell_pty_slave, STDIN_FILENO);
>> dup2 (subshell_pty_slave, STDOUT_FILENO);
>> dup2 (subshell_pty_slave, STDERR_FILENO);
>>
>> close (subshell_pipe[READ]);
>> close (subshell_pty_slave); /* These may be FD_CLOEXEC, but just in
>> case... */
>> /* Close master side of pty. This is important; apart from */
>> /* freeing up the descriptor for use in the subshell, it also */
>> /* means that when MC exits, the subshell will get a SIGHUP and */
>> /* exit too, because there will be no more descriptors pointing */
>> /* at the master side of the pty and so it will disappear. */
>> close (subshell_pty);
>>
>> /* Execute the subshell at last */
>>
>> switch (subshell_type)
>> {
>> case BASH:
>> execl (shell, "bash", "-rcfile", init_file, (char *) NULL);
>> break;
>> ----------------------------------------------------------
>
>It seems that mc is correct in the expectation.
>
>http://pubs.opengroup.org/onlinepubs/9699919799/functions/close.html
>
>"If fildes refers to the master side of a pseudo-terminal, and this is 
>the last close, a SIGHUP signal shall be sent to the controlling 
>process, if any, for which the slave side of the pseudo-terminal is the 
>controlling terminal. It is unspecified whether closing the master side 
>of the pseudo-terminal flushes all queued input and output."
>
>
>I don't find such implementation on cygwin
>
>fhandler_pty_master::close ()
>
>Am I looking in the wrong place ?

(checked into this a little more)

Sort of.  If the process is doing a read, it is supposed to detect that
the tty has been closed and a SIGHUP is supposed to be sent.  It is not
precisely the same thing as sending a SIGHUP when the master closes but
I'm surprised that, in principle, it doesn't amount to the same thing.

Just see any of the SIGHUPs in fhandler_tty.cc.  They are all supposed
to be dealing with this scenario.

So, unless bash is not waiting for input (which is unlikely) this should
work.

cgf

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