WIP: service preventing systemd suspend

examples/signal-handling/sleeping.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+
+printf '%s%s%s\n' 'START. Can be terminated with "kill -TERM ' "$$" '"'
+while :; do
+  printf '%s\n' 'sleeping 10s'
+  sleep 10s
+  printf '%s\n' 'awoke from sleep'
+done
+printf '%s\n' 'END'
+
+# When executing `kill -TERM $PID` in another terminal,
+# this script terminates instantly.

examples/signal-handling/sleeping2.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+set -eu -o pipefail
+
+function handle_sigterm(){
+  printf '%s\n' 'Ignored SIGTERM' >&2
+}
+
+# Handle SIGTERM
+trap 'handle_sigterm' TERM
+
+printf '%s%s%s\n' 'START. Can be terminated with "kill -TERM ' "$$" '"'
+while :; do
+  printf '%s\n' 'sleeping 10s'
+  sleep 10s
+  printf '%s\n' 'awoke from sleep'
+done
+printf '%s\n' 'END'
+
+# When executing `kill -TERM $PID` in another terminal,
+# the command `sleep 10s` continues.
+# After it has finished, `Ignored SIGTERM` is printed and the endless loop continues.

examples/suspend-impossible.nix

@@ -0,0 +1,90 @@
+{ config, pkgs, lib, ... }:
+
+# Relevant references:
+# How to prevent systemd suspend from stopping any service with a dont-sleep.service. https://askubuntu.com/q/830442/1002706
+
+# systemctl show test-block-suspend.service -p TimeoutStopUSec
+#=> TimeoutStopUSec=infinity
+
+# systemctl cat test-block-suspend.service
+
+# systemctl list-dependencies --before test-block-suspend.service
+#=> test-block-suspend.service
+#=> ├─shutdown.target
+#=> ├─sleep.target
+#=> │ ├─systemd-hibernate.service
+#=> │ ├─systemd-hybrid-sleep.service
+#=> │ ├─systemd-suspend-then-hibernate.service
+#=> │ └─systemd-suspend.service
+#=> └─suspend.target
+#=>   └─post-resume.service
+
+{
+  # Alternative approach to adding `conflicts`, `before` and `ExecStop`.
+  #
+  # Immediately before entering system suspend, all executables in /usr/lib/systemd/system-sleep/ are run.
+  # Execution of the sleep action is not continued until all have finished.
+  # TODO: This happens after my screen got blank, wifi disabled, screen locker activated, etc.
+#  environment.etc."systemd/system-sleep/test-block-suspend".source =
+#    pkgs.writeShellScript "test-block-suspend" ''
+#      set -eu -o pipefail
+#      if [ "$1" == "pre" ]; then
+#        while :; do
+#          systemctl is-active --quiet test-block-suspend.service || exit 0
+#          ${pkgs.coreutils}/bin/echo 'Waiting until service has finished ...'
+#          ${pkgs.coreutils}/bin/sleep 1s
+#        done
+#      fi
+#    '';
+
+  systemd.services."test-block-suspend" = {
+    description = "Inhibit suspend";
+
+    # If suspend is initiated, this service is stopped.
+    conflicts = [  "sleep.target" ];
+    # Start-up of `sleep.target`
+    # is delayed until this service has finished starting up.
+    before = [ "sleep.target" ];
+
+    serviceConfig = {
+      # `simple`:
+      #   As soon as the main process (defined in the `ExecStart`) is started (forked),
+      #   start-up is considered as finished.
+      # `notify`:
+      #   The service sends a "READY=1" notification message via `sd_notify` when it has finished starting up.
+      #   Requires `NotifyAccess`.
+      #   See https://www.freedesktop.org/software/systemd/man/latest/systemd-notify.html
+      Type = "notify";
+      NotifyAccess = "main";
+
+      # To stop this service `ExecStop` is run instead of sending SIGTERM to the main process of the service.
+      # The command specified by `ExecStop` does never end,
+      # thus stopping this service takes forever.
+      # TODO: However, the sleep action continues while `ExecStop` is running. As a reslut, the system suspends.
+      ExecStop = "${pkgs.coreutils}/bin/sleep infinity";
+      TimeoutStopSec = "infinity";
+
+      # This service executes a shell script as main process.
+      ExecStart  = pkgs.writeShellScript "test-block-suspend-execstart" ''
+        set -eu -o pipefail
+
+        #handle_sigterm(){
+        #  #while :; do
+        #    printf '%s\n' 'Ignoring SIGTERM ...' >&2
+        #  #  ${pkgs.coreutils}/bin/sleep 5s
+        #  #done
+        #}
+        #trap 'handle_sigterm' TERM
+
+        #printf '%s\n' 'Start-up 30s ...'
+        #${pkgs.coreutils}/bin/sleep 30s
+        systemd-notify READY=1
+
+        while :; do
+          printf '%s\n' 'Still alive'
+          ${pkgs.coreutils}/bin/sleep 5s
+        done
+      '';
+    };
+  };
+}

examples/systemd-timer.nix

@@ -1,19 +1,30 @@
 { config, pkgs, ... }:
 
 # TODO Note: One can specify ExecStart and ExecStop. Maybe to pause some script during shutdown/suspend? This would be nice for backups. Just finish the current snapshot then pause.
-#
-# ExecStop=
-#   Commands to execute to stop the service started via ExecStart=.
-#   The command that asks the service to stop should wait for it to do so!
-#   After the commands configured in this option are run, it is implied that the service is stopped, and any processes remaining for it are terminated.
-#
-#   If this option is not specified, the process is terminated when service stop is requested.
 
 # TODO: https://unix.stackexchange.com/questions/619987/stop-systemd-service-before-suspend-start-again-after-resume
 
+# Suspend, hibernate, shutdown
+#
+# Some of the upstream system units:
+#   https://github.com/NixOS/nixpkgs/blob/5c43dee215c279dceee7861eec85862ad85cc330/nixos/modules/system/boot/systemd.nix#L104
+#   - suspend.target
+#   - hibernate.target
+#   - sleep.target
+#   - hybrid-sleep.target
+#   https://github.com/NixOS/nixpkgs/blob/5c43dee215c279dceee7861eec85862ad85cc330/nixos/modules/system/boot/systemd.nix#L115
+#   - reboot.target
+#   - poweroff.target
+#
+# BTRFS scrub prevents suspend2ram and proper shutdown
+# https://github.com/NixOS/nixpkgs/blob/5c43dee215c279dceee7861eec85862ad85cc330/nixos/modules/tasks/filesystems/btrfs.nix#L131
+#   conflicts = [ "shutdown.target" "sleep.target" ];
+#   before = [ "shutdown.target" "sleep.target" ];
+
 # Benefits of Systemd Timers over cron: https://wiki.archlinux.org/title/Systemd/Timers#Benefits
-# Example: https://nixos.wiki/wiki/Nix_Cookbook#Creating_periodic_services
-# Example: https://nixos.wiki/wiki/Systemd/Timers
+# Timer example: https://nixos.wiki/wiki/Nix_Cookbook#Creating_periodic_services
+# Timer example: https://nixos.wiki/wiki/Systemd/Timers
+
 {
   systemd.timers."hello-world" = {
     # description = "My Timer";
@@ -46,12 +57,19 @@
   systemd.services."hello-world" = {
     # description = "My Oneshot Service";
 
-    # TODO: Prevents suspend2ram or proper shutdown?
-    # https://github.com/NixOS/nixpkgs/blob/e9b4b56e5a20ac322c0c01ccab7ec697ab076ea0/nixos/modules/tasks/filesystems/btrfs.nix#L128-L130
+    # Prevent suspend2ram and proper shutdown.
+    #
+    # TODO Additionally:
+    #   Option I:
+    #     Use ExecStart and ExecStop. ExecStop initiates an early stop of the service and waits until this has finished.
+    #     In `serviceConfig`, set `Type` to `simple` (and not `oneshot`, otherwise ExecStop is not used).
+    #   Option II:
+    #     Set `TimeoutSec` to `infinity`.
     #
     # If the specified units are started, then this unit is stopped and vice versa.
     conflicts = [ "shutdown.target" "sleep.target" ];
-    # If the specified units are started at the same time as this unit, delay them until this unit has started.
+    # If this unit and one from `before` are being started,
+    # the start-up of the latter is delayed until this service has finished starting up.
     before = [ "shutdown.target" "sleep.target" ];
 
     # https://man.archlinux.org/man/systemd.service.5.en#OPTIONS
@@ -60,6 +78,27 @@
     # Example: https://github.com/NixOS/nixpkgs/blob/e9b4b56e5a20ac322c0c01ccab7ec697ab076ea0/nixos/modules/tasks/filesystems/btrfs.nix#L132-L142
     serviceConfig = {
       Type = "oneshot";
+
+      # Takes a boolean value that specifies whether the service shall be considered active even when all its processes exited.
+      # Defaults to `false`.
+      #RemainAfterExit = true;
+
+      # A shorthand for configuring both TimeoutStartSec= and TimeoutStopSec= to the specified value.
+      # TimeoutStopSec:
+      #   If no ExecStop= commands are specified, the service gets the SIGTERM immediately.
+      #   It configures the time to wait for the service itself to stop.
+      #   If it doesn't terminate in the specified time, it will be forcibly terminated by SIGKILL (see KillMode).
+      # SIGTERM and SIGKILL:
+      #   The SIGTERM signal is a generic signal used to cause program termination.
+      #   Unlike SIGKILL, this signal can be blocked, handled, and ignored.
+      #   It is the normal way to politely ask a program to terminate.
+      # SIGINT:
+      #   The SIGINT (“program interrupt”) signal is sent when the user types the INTR character (normally C-c).
+      # Example:
+      #   grow-partition.nix conflicts with `shutdown.target` and has an infinite `TimeoutStopSec` to make sure that resizing a partition is not interrupted.
+      #   https://github.com/NixOS/nixpkgs/blob/008d84ab67c4f4ccbd7c0996005e46c8bd32d675/nixos/modules/system/boot/grow-partition.nix#L34
+      #TimeoutSec = "infinity";
+
       PrivateTmp = true;
 
       #User = "myuser";
@@ -69,7 +108,16 @@
       # Takes one of the strings realtime, best-effort or idle.
       IOSchedulingClass = "idle";
 
-      #ExecStart = "${pkgs.python3.withPackages my-python-packages}/bin/netcup-dns";
+      #ExecStart = "${pkgs.coreutils}/bin/echo 'Hello World'";
+      # Commands to execute to stop the service started via ExecStart=.
+      # The command that asks the service to stop should wait for it to do so!
+      # After the commands configured in this option are run, it is implied that the service is stopped, and any processes remaining for it are terminated.
+      #
+      # If this option is not specified, the process is terminated when service stop is requested.
+      #ExecStop  = pkgs.writeShellScript "cancel-hello-world" ''
+      #  set -eu -o pipefail
+      #  echo 'Cancelling execution of hello-world'
+      #'';
     };
 
     # Packages required for the script.
@@ -87,7 +135,7 @@
     # Shell commands executed as the service's main process.
     script = ''
       set -eu -o pipefail
-      ${pkgs.coreutils}/bin/echo "Hello World"
+      ${pkgs.coreutils}/bin/echo 'Hello World'
     '';
   };
 }