Keep the hourly refresh on GitHub with a pacemaker chain

GitHub's cron scheduler creates this repository's scheduled runs four to
five hours late and drops the slots in between, so the hourly Hot CVEs
refresh cannot come from a cron. A workflow_dispatch call creates a run
within seconds, so a pacemaker workflow now sleeps until the next :05,
fires the refresh and its own successor, and keeps time by itself. Its
cron only restarts the chain if a link fails.

Hot CVEs loses its cron and the gate that was there to coexist with an
external trigger; nothing outside GitHub is involved any more.

.github/workflows/hot_cves.yml

@@ -1,15 +1,9 @@
 name: Hot CVEs List
 
-# The hourly trigger is a systemd timer on the VPS calling workflow_dispatch:
-# GitHub's own scheduler creates cron runs four to five hours late and drops
-# the slots in between. The cron below is the fallback and the watchdog. It
-# yields while the trigger is alive, takes over when it is not, and fails
-# loudly once the trigger has been dead for three hours so the failure email
-# says why.
+# Fired every hour at :05 by pacemaker.yml. No cron here: GitHub's scheduler
+# creates cron runs hours late, which is the whole reason the pacemaker exists.
 on:
   workflow_dispatch:
-  schedule:
-    - cron: '7 * * * *'
 
 permissions:
   contents: write
@@ -20,37 +14,7 @@ concurrency:
   cancel-in-progress: false
 
 jobs:
-  gate:
-    runs-on: ubuntu-latest
-    outputs:
-      run: ${{ steps.check.outputs.run }}
-      stale: ${{ steps.check.outputs.stale }}
-    steps:
-      - name: Yield to the hourly trigger while it is alive
-        id: check
-        env:
-          GH_TOKEN: ${{ github.token }}
-        run: |
-          if [ "$GITHUB_EVENT_NAME" != "schedule" ]; then
-            echo "run=true" >> "$GITHUB_OUTPUT"
-            exit 0
-          fi
-          last=$(gh api "repos/$GITHUB_REPOSITORY/commits?per_page=30" \
-            --jq 'map(select(.commit.message | startswith("Trending CVEs update")))[0].commit.committer.date')
-          age=$(( $(date -u +%s) - $(date -u -d "${last:-1970-01-01T00:00:00Z}" +%s) ))
-          echo "Last trending refresh: ${last:-none}, $((age / 60)) minutes ago"
-          if [ "$age" -lt 3300 ]; then
-            echo "Fresh enough, the hourly trigger is doing its job"
-            exit 0
-          fi
-          echo "run=true" >> "$GITHUB_OUTPUT"
-          if [ "$age" -gt 10800 ]; then
-            echo "stale=true" >> "$GITHUB_OUTPUT"
-          fi
-
   ScheduledRun:
-    needs: gate
-    if: needs.gate.outputs.run == 'true'
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
@@ -124,9 +88,3 @@ jobs:
         env:
           GH_TOKEN: ${{ github.token }}
         run: gh workflow run site.yml --ref main
-
-      - name: Fail loudly when the hourly trigger is dead
-        if: needs.gate.outputs.stale == 'true'
-        run: |
-          echo "::error::No trending refresh for over three hours: the external hourly trigger is not firing"
-          exit 1

.github/workflows/pacemaker.yml

@@ -0,0 +1,37 @@
+name: Pacemaker
+
+# GitHub's cron scheduler creates this repository's scheduled runs four to
+# five hours late and drops the slots in between, so the hourly refresh cannot
+# come from a cron. A workflow_dispatch call creates a run within seconds, so
+# each pacemaker run sleeps until the next :05, fires the Hot CVEs refresh and
+# its own successor, and the chain keeps time by itself. The cron below only
+# restarts the chain if a link ever fails; while the chain is alive the
+# concurrency group holds the extra run back and the next link cancels it.
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: '7 * * * *'
+
+permissions:
+  actions: write
+
+concurrency:
+  group: pacemaker
+  cancel-in-progress: false
+
+jobs:
+  tick:
+    runs-on: ubuntu-latest
+    timeout-minutes: 70
+    steps:
+      - name: Sleep until the next :05, then fire the hourly refresh
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          now=$(date -u +%s)
+          next=$(( now / 3600 * 3600 + 300 ))
+          [ "$next" -gt "$now" ] || next=$(( next + 3600 ))
+          echo "Sleeping $(( (next - now) / 60 )) minutes until $(date -u -d @"$next" +%H:%M) UTC"
+          sleep $(( next - now ))
+          gh workflow run hot_cves.yml --ref main
+          gh workflow run pacemaker.yml --ref main