Description of problem:

When a ssh client disconnect from server, the user session gets closed through executing *pam_systemd* hooks, which leads to getting a "Hello" then "ReleaseSession" dbus call.
In case the remote program exits while *pam_systemd* executes the dbus call, the following error message is logged:
~~~
pam_systemd(sshd:session): Failed to release session: Interrupted system call
~~~

Digging into this the whole day, I could find that the root cause is a bug in ppoll() call in dbus_poll() code:
~~~
2787 static int bus_poll(sd_bus *bus, bool need_more, uint64_t timeout_usec) {
 :
2839         r = ppoll(p, n, m == (uint64_t) -1 ? NULL : timespec_store(&ts, m), NULL);
2840         if (r < 0)
2841                 return -errno;
 :
~~~

Here above on line 2839 there is no check for special errnos EAGAIN and EINTR, which can be returned legitimately, per poll(2) manpage:
~~~
ERRORS

       EINTR  A signal occurred before any requested event; see signal(7).

NOTES

       On some other UNIX systems, poll() can fail with the error EAGAIN if the system fails to allocate kernel-in‐
       ternal  resources,  rather  than  ENOMEM as Linux does.  POSIX permits this behavior.  Portable programs may
       wish to check for EAGAIN and loop, just as with EINTR.
~~~ 

I don't know if not having the session be closed properly is an issue or not, but likely it's possible that any dbus client calling dbus_poll() can hit this.

Version-Release number of selected component (if applicable):

systemd-219

How reproducible:

Always

Steps to Reproduce:

I could not automatize the reproducer. The reproducer consists in disconnecting the client, and pausing dbus-daemon so that pam_systemd "hangs" on dbus Hello, while stopping the remote command.

1. Create a script looping forever until told to exit

    ~~~
    # cat /usr/local/bin/diewhentold 
    #!/bin/sh
    /bin/rm /run/dienow 2>/dev/null
    while [ ! -e /run/dienow ]; do
    	sleep 0.5	
    done
    ~~~

2. Start the script through ssh

    ~~~
    $ ssh localhost /usr/local/bin/diewhentold
    ~~~

3. In another terminal, stop dbus-daemon through using gdb and prepare the command to let the script stop (but don't execute it yet)

    ~~~
    $ gdb /usr/bin/dbus-daemon $(pgrep dbus-daemon)
    (gdb) !touch /run/dienow
    ~~~

4. Background the ssh command and kill it

    ~~~
    $ ^Z
    $ kill %1
    ~~~

5. In gdb terminal, tell the script to execute by really executing the "touch" command

Actual results:

Oct 25 16:18:53 vm-rhel7 sshd[8096]: pam_systemd(sshd:session): Failed to release session: Interrupted system call

Expected results:

Session closed properly