Not very fancy yet, but should do for now.

Contribute the example saver for displaying livestream feeds.

Internal reference: b/130370819.

Now the colors are only hardcoded in one place, making it easier to override them.

Not doing so but running xsecurelock via shell expansion means that the shell
will keep holding on to the sleep lock FD, preventing suspend.

(cleaner)

The easiest way to do that was to execl() a separate binary for the
placeholder (thereby closing all close-on-exec file descriptors, even
those we don't control, e.g. X11's), also making "ps" more descriptive
about what this process is for, and making sure any e.g. internal mutexes
by libraries we use are freed.

Fixes "systemd does not suspend laptop on closing the lid".

Otherwise the sigsuspend() call to wait for SIGCHLD does nothing and just hangs.

Also, refactor to prevent this issue from happening in the future.

$0 didn't contain the full path, so exec failed.

Fixes killing saver children twice twice.

- Always have a separate "dummy process" in each process group to prevent
  the PGID from getting deallocated when the leader dies. This makes
  the logic that kills the rest of a process group when the leader
  dies race-free.

- Do the killing the rest of the process group inside WaitPgrp rather than in
  its callers.

This signal is now sent from the main program to the saver to reset possible
idle timers in the savers, without intending to restart the saver.

This is part of the effort to make the auth child protocol more flexible and to
properly run auth on top of the saver as a window.

That way they will still get waitpid'd on and won't stay zombies.

Main changes:

- No longer support the case of sigprocmask failing. The new code will in that
  case have a race that can cause xsecurelock to hang; in the old code a similar
  (preexisting) race could cause other processes to get killed.
- Use only one sigprocmask call to retrieve and set the new mask.
- Do the sigprocmask calls outside the loop, not inside (fixes a race that
  appears to have caused hangs for me; repro: just hold escape).

Use criticial section in WaitPgrp.

Use explicit_bzero for password data.

This may fix some cases of old screen content appearing after resume from
suspend. Cannot reproduce these issues here though.

The C standard does not guarantee that memset won't be optimized away by
a compiler, which means that memset for password data is not safe.

Also clear password data in signal handler for SIGTERM, otherwise it
could happen that password data is left in memory that way.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>

A signal race exists in WaitPgrp that could result in a kill
initiated by xsecurelock against a process that does not belong to it.

This race can happen if waitpid has been called for a child that has
already been terminated and SIGTERM is received before the pid has been
reset to 0.

In that case SIGTERM will call kill for a pid that has been already
released to the operating system and could thus have been assigned to a
new process again.

To prevent this, the criticial section blocks SIGTERM and also SIGCHLD
if WaitPgrp is supposed to block until a child status changes. If the
function is supposed to block, sigsuspend() restores SIGCHLD and SIGTERM
again and waits until a signal is delivered. These signals are blocked
and unblocked for each iteration of the loop. This is required to
actually get interrupted/killed by a SIGTERM in non-blocking mode.

If no critical section could be installed due to failure in calling
sigprocmask the old behaviour is kept.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>

Having a conditional loop without breaks or returns is a better
foundation to implement a criticial section without duplicating
cleanups throughout the code.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>

Centralized pid handling into WaitPgrp.