CI all test is the only required test

This means we can just use the `needs` of that test to determine which
CI actions are required for status checks to pass.

.ci/all_check.py

@@ -0,0 +1,32 @@
+#!/usr/bin/env python3
+"""
+Makes sure:
+ * All jobs are listed in the 'all' job
+ * Only existing tests are listed
+"""
+
+import sys
+import yaml
+
+CI_PATH = '.github/workflows/ci.yaml'
+ALL_TEST = 'all'
+
+
+def main():
+    ci_yaml_fp = open(CI_PATH, 'r')
+    ci_yaml_parsed = yaml.load(ci_yaml_fp, Loader=yaml.FullLoader)
+
+    all_jobs = set(ci_yaml_parsed['jobs'].keys()) - {ALL_TEST}
+    all_needs = set(ci_yaml_parsed['jobs'][ALL_TEST]['needs'])
+
+    if all_jobs - all_needs:
+        sys.exit(f'Not all jobs mentioned in {ALL_TEST}.needs: '
+                 f'{all_jobs - all_needs}')
+
+    if all_needs - all_jobs:
+        sys.exit(f'Non-existing jobs found in {ALL_TEST}.needs: '
+                 f'{all_needs - all_jobs}')
+
+
+if __name__ == '__main__':
+    main()

.github/workflows/ci.yaml

@@ -97,14 +97,41 @@ jobs:
         run: |
           cabal v2-run unittests
 
-  # Mandatory check on GitHub
+  # Mechanism copied from https://github.com/clash-lang/clash-compiler/
   all:
     name: All jobs finished
     needs:
       - cabal
       - stack
     runs-on: ubuntu-latest
     steps:
-      - name: Void
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: Check dependencies for failures
+        run: |
+          # Test all dependencies for success/failure
+          set -x
+          success="${{ contains(needs.*.result, 'success') }}"
+          fail="${{ contains(needs.*.result, 'failure') }}"
+          set +x
+
+          # Test whether success/fail variables contain sane values
+          if [[ "${success}" != "true" && "${success}" != "false" ]]; then exit 1; fi
+          if [[ "${fail}"    != "true" && "${fail}"    != "false" ]]; then exit 1; fi
+
+          # We want to fail if one or more dependencies fail. For safety, we introduce
+          # a second check: if no dependencies succeeded something weird is going on.
+          if [[ "${fail}" == "true" || "${success}" == "false" ]]; then
+            echo "One or more dependency failed, or no dependency succeeded."
+            exit 1
+          fi
+
+      - name: Install dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get -y install python3-yaml
+
+      - name: Check that the 'all' job depends on all other jobs
         run: |
-          echo "All good"
+          .ci/all_check.py