tasks: Introduce elastic cloud runner mode

As the EC2 bare metal instances are expensive, we don't want them to run
permanently, but only in times of high demand. They should then
terminate themselves when the queue is running out of work.

Define "run out of work" as "the number of job-runner entries in the
AMQP queue drops below 10". At that level, our permanent PSI runners can
keep up. This is are more robust global criterion than checking if
`run-queue` encountered an empty queue, as that is more prone to
terminating only *some* of the instances while some others keep picking
up brand new queue entries.

Introduce an "idle poweroff" mode in which the `cockpit-tasks` main loop
exits with code 100 instead of slumbering when work is running low.
Configure the slice to automatically power off the machine once all
cockpit-tasks instances exited cleanly (we don't want this on failures,
so that we can ssh in and examine them). Use the `poweroff-immediate`
heavy hammer there, to avoid potential hangs on shutdown -- there is
nothing to rescue from the instance anyway.

Plumb that through the AWS Ansible role and document it.

ansible/aws/README.md

@@ -57,6 +57,8 @@ Create and configure the instance:
 
 If you run more than one at a time, set a custom host name with `-e hostname=cockpit-aws-tasks-2` or similar, so that GitHub test statuses remain useful to identify where a test runs.
 
+There is also an "elastic" mode where the tasks bots keep running until the AMQP queue runs low. Use that for situations where AWS instances act as extra high-demand capacity instead of being the primary runners. Enable that mode with `-e idle_poweroff=1`.
+
 Webhook setup
 -------------
 AWS runs our primary webhook. Deploy or update it with:

ansible/roles/tasks-systemd/tasks/main.yml

@@ -146,4 +146,5 @@
     export INSTANCES={{ instances | default(1) }}
     export TEST_NOTIFICATION_MX={{ notification_mx | default('') }}
     export TEST_NOTIFICATION_TO={{ notification_to | default('') }}
+    export IDLE_POWEROFF={{ idle_poweroff | default('') }}
     /run/install-service

tasks/container/cockpit-tasks

@@ -20,7 +20,17 @@ function update_bots() {
 }
 
 # wait between 1 and 10 minutes, with an override to speed up tests
+# in IDLE_POWEROFF mode, also check queue size
 function slumber() {
+    if [ -n "${IDLE_POWEROFF:-}" ] && [ -e ./inspect-queue ]; then
+        # only consider job-runner entries, not statistics or webhook
+        NUM_JOBS=$(./inspect-queue | grep --count '"job":')
+        if [ "$NUM_JOBS" -lt 10 ]; then
+            echo "Job queue running low, exiting"
+            exit 100
+        fi
+    fi
+
     if [ -n "${SLUMBER:-}" ]; then
         sleep "$SLUMBER"
     else

tasks/install-service

@@ -39,6 +39,9 @@ After=podman.socket
 [Service]
 Slice=cockpittasks.slice
 Restart=always
+# cockpit-tasks exits with 100 in IDLE_POWEROFF mode when queue is running low
+SuccessExitStatus=100
+RestartPreventExitStatus=100
 RestartSec=60
 # give image pull enough time
 TimeoutStartSec=10min
@@ -63,13 +66,24 @@ ExecStart=/usr/bin/podman run --name=cockpit-tasks-%i --hostname=${CONTAINER_HOS
     --env=GIT_AUTHOR_EMAIL=cockpituous@cockpit-project.org \
     --env=TEST_NOTIFICATION_MX=${TEST_NOTIFICATION_MX} \
     --env=TEST_NOTIFICATION_TO=${TEST_NOTIFICATION_TO} \
+    --env=IDLE_POWEROFF=${IDLE_POWEROFF:-} \
     ghcr.io/cockpit-project/tasks cockpit-tasks --verbose
 ExecStop=/usr/bin/podman rm -f cockpit-tasks-%i
 
 [Install]
 WantedBy=multi-user.target
 EOF
 
+# mode for elastic cloud runners
+if [ -n "${IDLE_POWEROFF:-}" ]; then
+    mkdir -p /etc/systemd/system/cockpittasks.slice.d
+    cat <<EOF > /etc/systemd/system/cockpittasks.slice.d/poweroff.conf
+    [Unit]
+    StopWhenUnneeded=yes
+    SuccessAction=poweroff-immediate
+EOF
+fi
+
 systemctl daemon-reload
 
 for i in `seq $INSTANCES`; do systemctl enable --now cockpit-tasks@$i; done