Bug#921835: xfce4-screensaver: wastes CPU while the display is off

[Adam Borowski <kilobyte@angband.pl>]

Package: xfce4-screensaver
Version: 0.1.3-2
Severity: normal

I left home for the weekend, just ssh-ed in and I see:
  PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
 2375 root      20   0  823412 223376 169752 R  96.3   2.7   1946:54 Xorg
24967 kilobyte  20   0   31540   5736   4496 R  34.9   0.1   1:26.92 zoom
24966 kilobyte  20   0   31144   5188   4320 S  28.2   0.1   1:11.75 zoom

The screen has been locked for nearly a day, thus it's obviously off (which
I can't verify).  For further information, I left this bug report
half-written for while; the screensaver has in the meantime rolled over to
something else that takes just 20% X + 2*7% for screensavers proper, then to
euler3d at 27% X + 2*13%.  That's better than nearly two full cores as for
zoom, but the question is: why?

Once the monitor gets suspended/slept/powered off, there's completely no
point in drawing anything.  On the other hand, a computer with two cores
worth of activity for a prolonged time wastes a significant amount of
electricity, which is bad both for the environment and for users' wallets.

Thus: could you please stop spawning new "draw something" processes once
the saver has gone past the first (visual) stage?


Meow!
-- System Information:
Debian Release: buster/sid
  APT prefers unstable-debug
  APT policy: (500, 'unstable-debug'), (500, 'unstable'), (500, 'testing'), (150, 'experimental')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 5.0.0-rc5-debug-00035-g12a002e2de92 (SMP w/6 CPU cores)
Locale: LANG=C.UTF-8, LC_CTYPE=C.UTF-8 (charmap=UTF-8), LANGUAGE=C.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: sysvinit (via /sbin/init)

Versions of packages xfce4-screensaver depends on:
ii  libc6               2.28-6
ii  libcairo2           1.16.0-2
ii  libdbus-1-3         1.12.12-1
ii  libdbus-glib-1-2    0.110-4
ii  libgarcon-1-0       0.6.2-1
ii  libgdk-pixbuf2.0-0  2.38.0+dfsg-7
ii  libgl1              1.1.0-1
ii  libglib2.0-0        2.58.3-1
ii  libgtk-3-0          3.24.5-1
ii  libpam0g            1.1.8-4
ii  libpango-1.0-0      1.42.4-6
ii  libsystemd0         240-5
ii  libx11-6            2:1.6.7-1
ii  libxext6            2:1.3.3-1+b2
ii  libxfce4ui-2-0      4.12.1-3
ii  libxfconf-0-2       4.12.1-1
ii  libxklavier16       5.4-4
ii  libxrandr2          2:1.5.1-1
ii  libxss1             1:1.2.3-1
ii  libxxf86vm1         1:1.1.4-1+b2

xfce4-screensaver recommends no packages.

xfce4-screensaver suggests no packages.

-- no debconf information