pid1: add SurviveFinalKillSignal= to skip units on final sigterm/sigkill spree

Add a new boolean for units, SurviveFinalKillSignal=yes/no. Units that
set it will not have their process receive the final sigterm/sigkill in
the shutdown phase.

This is implemented by checking if a process is part of a cgroup marked
with a user.survive_final_kill_signal xattr (or a trusted xattr if we
can't set a user one, which were added only in kernel v5.7 and are not
supported in CentOS 8).

test/units/testsuite-82.sh

@@ -20,8 +20,8 @@ if [ -f /run/testsuite82.touch3 ]; then
     read -r x <&5
     test "$x" = "oinkoink"
 
-    # Check that no service is still around
-    test "$(systemctl show -P ActiveState testsuite-82-survive.service)" != "active"
+    # Check that the surviving service is still around
+    test "$(systemctl show -P ActiveState testsuite-82-survive.service)" = "active"
     test "$(systemctl show -P ActiveState testsuite-82-nosurvive.service)" != "active"
 
     # All succeeded, exit cleanly now
@@ -43,8 +43,8 @@ elif [ -f /run/testsuite82.touch2 ]; then
     systemd-notify --fd=3 --pid=parent 3<"$T"
     rm "$T"
 
-    # Check that no service is still around
-    test "$(systemctl show -P ActiveState testsuite-82-survive.service)" != "active"
+    # Check that the surviving service is still around
+    test "$(systemctl show -P ActiveState testsuite-82-survive.service)" = "active"
     test "$(systemctl show -P ActiveState testsuite-82-nosurvive.service)" != "active"
 
     # Test that we really are in the new overlayfs root fs
@@ -57,6 +57,9 @@ elif [ -f /run/testsuite82.touch2 ]; then
     mount --bind /original-root /run/nextroot
     mount
 
+    # Restart the unit that is not supposed to survive
+    systemd-run -p Type=exec --unit=testsuite-82-nosurvive.service sleep infinity
+
     # Now issue the soft reboot. We should be right back soon.
     touch /run/testsuite82.touch3
     systemctl --no-block soft-reboot
@@ -85,8 +88,8 @@ elif [ -f /run/testsuite82.touch ]; then
     systemd-notify --fd=3 --pid=parent 3<"$T"
     rm "$T"
 
-    # Check that no service survived, regardless of the configuration
-    test "$(systemctl show -P ActiveState testsuite-82-survive.service)" != "active"
+    # Check that the surviving service is still around
+    test "$(systemctl show -P ActiveState testsuite-82-survive.service)" = "active"
     test "$(systemctl show -P ActiveState testsuite-82-nosurvive.service)" != "active"
 
     # This time we test the /run/nextroot/ root switching logic. (We synthesize a new rootfs from the old via overlayfs)
@@ -107,6 +110,9 @@ elif [ -f /run/testsuite82.touch ]; then
     # Bind our current root into the target so that we later can return to it
     mount --bind / /run/nextroot/original-root
 
+    # Restart the unit that is not supposed to survive
+    systemd-run -p Type=exec --unit=testsuite-82-nosurvive.service sleep infinity
+
     # Now issue the soft reboot. We should be right back soon.
     touch /run/testsuite82.touch2
     systemctl --no-block soft-reboot
@@ -123,23 +129,17 @@ else
     systemd-notify --fd=3 --pid=parent 3<"$T"
     rm "$T"
 
-    # Create a script that can survive the soft reboot by ignoring SIGTERM (we
-    # do this instead of the argv[0][0] = '@' thing because that's so hard to
-    # do from a shell
-    T="/dev/shm/survive-$RANDOM.sh"
-    cat >$T <<EOF
-#!/bin/bash
-trap "" TERM
-systemd-notify --ready
-rm "$T"
-exec sleep infinity
-EOF
-    chmod +x "$T"
-    # This sets DefaultDependencies=no so that it remains running until the
-    # very end, and IgnoreOnIsolate=yes so that it isn't stopped via the
-    # "testsuite.target" isolation we do on next boot
-    systemd-run -p Type=notify -p DefaultDependencies=no -p IgnoreOnIsolate=yes --unit=testsuite-82-survive.service "$T"
-    systemd-run -p Type=exec -p DefaultDependencies=no -p IgnoreOnIsolate=yes --unit=testsuite-82-nosurvive.service sleep infinity
+    # Configure this transient unit to survive the soft reboot - it will not conflict with shutdown.target
+    # and it will be ignored on the isolate that happens in the next boot.
+    systemd-run -p Type=exec --unit=testsuite-82-survive.service \
+        --property SurviveFinalKillSignal=yes \
+        --property IgnoreOnIsolate=yes \
+        --property DefaultDependencies=no \
+        --property After=basic.target \
+        --property "Conflicts=reboot.target kexec.target poweroff.target halt.target emergency.target rescue.target" \
+        --property "Before=reboot.target kexec.target poweroff.target halt.target emergency.target rescue.target" \
+        sleep infinity
+    systemd-run -p Type=exec --unit=testsuite-82-nosurvive.service sleep infinity
 
     # Check that we can set up an inhibitor, and that busctl monitor sees the
     # PrepareForShutdownWithMetadata signal and that it says 'soft-reboot'.