split "validate old ghcs" into a separate workflow

.github/workflows/old-ghcs.skip.yml

@@ -0,0 +1,48 @@
+name: Validate old ghcs Skip
+
+# This Workflow is special and contains a workaround for a known limitation of GitHub CI.
+#
+# The problem: We don't want to run the "old ghcs" jobs on PRs which contain only changes
+# to the docs, since these jobs take a long time to complete without providing any benefit.
+# We therefore use path-filtering in the workflow triggers for the old ghcs jobs, namely
+# "paths-ignore: doc/**". But the "Validate old ghcs post job" is a required job, therefore
+# a PR cannot be merged unless the "Validate old ghcs post job" completes succesfully, which
+# it doesn't do if we filter it out.
+#
+# The solution: We use a second job with the same name which always returns the exit code 0.
+# The logic implemented for "required" workflows accepts if 1) at least one job with that name
+# runs through, AND 2) If multiple jobs of that name exist, then all jobs of that name have to
+# finish successfully.
+on:
+  push:
+    paths:
+      - 'doc/**'
+      - '**/README.md'
+      - 'CONTRIBUTING.md'
+      - "changelog.d/**"
+      # only top level for these, because various test packages have them too
+      - "*/ChangeLog.md"
+      - "*/changelog.md"
+      - "release-notes/**"
+    branches:
+      - master
+  pull_request:
+    paths:
+      - 'doc/**'
+      - '**/README.md'
+      - 'CONTRIBUTING.md'
+      - "changelog.d/**"
+      - "*/ChangeLog.md"
+      - "*/changelog.md"
+      - "release-notes/**"
+  release:
+    types:
+      - created
+
+jobs:
+  validate-old-ghcs-post-job:
+    if: always()
+    name: Validate old ghcs post job
+    runs-on: ubuntu-latest
+    steps:
+      - run: exit 0

.github/workflows/old-ghcs.yml

@@ -0,0 +1,126 @@
+name: Validate old ghcs
+
+# See: https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#concurrency.
+concurrency:
+  group: ${{ github.ref }}-${{ github.workflow }}
+  cancel-in-progress: true
+
+# Note: This workflow file contains the required job "Validate old ghcs post job". We are using path
+# filtering here to ignore PRs which only change documentation. This can cause a problem, see the
+# workflow file "old-ghcs.skip.yml" for a description of the problem and the solution provided in
+# that file.
+on:
+  push:
+    paths-ignore:
+      - "doc/**"
+      - "**/README.md"
+      - "CONTRIBUTING.md"
+      - "changelog.d/**"
+      # only top level for these, because various test packages have them too
+      - "*/ChangeLog.md"
+      - "*/changelog.md"
+      - "release-notes/**"
+    branches:
+      - master
+  pull_request:
+    paths-ignore:
+      - "doc/**"
+      - "**/README.md"
+      - "CONTRIBUTING.md"
+      - "changelog.d/**"
+      - "*/ChangeLog.md"
+      - "*/changelog.md"
+      - "release-notes/**"
+  release:
+    types:
+      - created
+  workflow_call:
+
+env:
+  # We choose a stable ghc version across all os's
+  # which will be used to do the next release
+  GHC_FOR_RELEASE: "9.4.8"
+  COMMON_FLAGS: "-j 2 -v"
+
+jobs:
+
+  validate-old-ghcs:
+    name: Validate old ghcs ${{ matrix.extra-ghc }}
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        extra-ghc:
+          ["8.4.4", "8.2.2", "8.0.2"]
+          ## GHC 7.10.3 does not install on ubuntu-22.04 with ghcup.
+          ## Older GHCs are not supported by ghcup in the first place.
+      fail-fast: false
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install prerequisites for old GHCs
+        run: |
+          sudo apt-get update
+          sudo apt-get install libncurses5 libtinfo5
+
+      - name: Install extra compiler
+        run: ghcup install ghc ${{ matrix.extra-ghc }}
+
+      - name: GHCup logs
+        # Does anyone know why the explicit "always()" here and below?
+        # By default steps always run.
+        if: always()
+        run: cat /usr/local/.ghcup/logs/*
+
+      - name: Install primary compiler
+        uses: haskell-actions/setup@v2
+        id: setup-haskell
+        with:
+          ghc-version: ${{ env.GHC_FOR_RELEASE }}
+          cabal-version: latest
+
+      - name: GHC versions
+        run: |
+          ghc --version
+          "ghc-${{ matrix.extra-ghc }}" --version
+
+      # This will get us an outdated cache, because this workflow runs before
+      # validate.yml can update it. As a result, we end up rebuilding the PR.
+      # The only way to fix this is to move it back into validate.yml, because
+      # we can't force this to run synchronously with steps in a different
+      # workflow.
+      - uses: actions/cache@v4
+        with:
+          path: |
+            ${{ steps.setup-haskell.outputs.cabal-store }}
+          key: ${{ runner.os }}-${{ env.GHC_FOR_RELEASE }}-${{ github.sha }}
+          restore-keys: ${{ runner.os }}-${{ env.GHC_FOR_RELEASE }}-
+
+      - name: Validate build
+        run: sh validate.sh ${{ env.COMMON_FLAGS }} -s build
+
+      - name: "Validate lib-suite-extras --extra-hc ghc-${{ matrix.extra-ghc }}"
+        env:
+          EXTRA_GHC: ghc-${{ matrix.extra-ghc }}
+        run: sh validate.sh ${{ env.COMMON_FLAGS }} --lib-only -s lib-suite-extras --extra-hc "${{ env.EXTRA_GHC }}"
+
+  # We use this job as a summary of the workflow
+  # It will fail if any of the previous jobs does
+  # This way we can use it exclusively in branch protection rules
+  # and abstract away the concrete jobs of the workflow, including their names.
+  # See also the "skip" workflows, which these must match.
+  validate-old-ghcs-post-job:
+    if: always()
+    name: Validate old ghcs post job
+    runs-on: ubuntu-latest
+    # IMPORTANT! Any job added to the workflow should be added here too
+    # (This one is true because of the abstraction described above, and the
+    # corresponding branch protection rules. ++bsa)
+    needs: [validate-old-ghcs]
+
+    steps:
+      - run: |
+          echo "jobs info: ${{ toJSON(needs) }}"
+      - if: contains(needs.*.result, 'failure') || contains(needs.*.result, 'cancelled')
+        run: exit 1

.github/workflows/validate.yml

@@ -237,65 +237,6 @@ jobs:
         if: matrix.ghc == env.GHC_FOR_SOLVER_BENCHMARKS
         run: sh validate.sh $FLAGS -s solver-benchmarks-run
 
-  validate-old-ghcs:
-    name: Validate old ghcs ${{ matrix.extra-ghc }}
-    runs-on: ubuntu-latest
-    needs: validate
-
-    strategy:
-      matrix:
-        extra-ghc:
-          ["8.4.4", "8.2.2", "8.0.2"]
-          ## GHC 7.10.3 does not install on ubuntu-22.04 with ghcup.
-          ## Older GHCs are not supported by ghcup in the first place.
-      fail-fast: false
-
-    steps:
-      - uses: actions/checkout@v4
-
-      - name: Install prerequisites for old GHCs
-        run: |
-          sudo apt-get update
-          sudo apt-get install libncurses5 libtinfo5
-
-      - name: Install extra compiler
-        run: ghcup install ghc ${{ matrix.extra-ghc }}
-
-      - name: GHCup logs
-        if: always()
-        run: cat /usr/local/.ghcup/logs/*
-
-      - name: Install primary compiler
-        uses: haskell-actions/setup@v2
-        id: setup-haskell
-        with:
-          ghc-version: ${{ env.GHC_FOR_RELEASE }}
-          cabal-version: latest
-
-      - name: GHC versions
-        run: |
-          ghc --version
-          "ghc-${{ matrix.extra-ghc }}" --version
-
-      # As we are reusing the cached build dir from the previous step
-      # the generated artifacts are available here,
-      # including the cabal executable and the test suite
-      - uses: actions/cache@v4
-        with:
-          path: |
-            ${{ steps.setup-haskell.outputs.cabal-store }}
-            dist-*
-          key: ${{ runner.os }}-${{ env.GHC_FOR_RELEASE }}-${{ github.sha }}
-          restore-keys: ${{ runner.os }}-${{ env.GHC_FOR_RELEASE }}-
-
-      - name: Validate build
-        run: sh validate.sh ${{ env.COMMON_FLAGS }} -s build
-
-      - name: "Validate lib-suite-extras --extra-hc ghc-${{ matrix.extra-ghc }}"
-        env:
-          EXTRA_GHC: ghc-${{ matrix.extra-ghc }}
-        run: sh validate.sh ${{ env.COMMON_FLAGS }} --lib-only -s lib-suite-extras --extra-hc "${{ env.EXTRA_GHC }}"
-
   build-alpine:
     name: Build statically linked using alpine
     runs-on: ubuntu-latest
@@ -412,7 +353,11 @@ jobs:
     if: github.ref == 'refs/heads/master'
 
     # IMPORTANT! Any job added to the workflow should be added here too
-    needs: [validate, validate-old-ghcs, build-alpine, dogfooding]
+    # (This doesn't appear to be true, or we'd have merges with failed
+    # bootstrap tests. The pre-merge CI check is sufficient, but we do
+    # need validate and build-alpine to save the corresponding artifacts.
+    # ++bsa)
+    needs: [validate, build-alpine]
 
     steps:
       - uses: actions/download-artifact@v4
@@ -453,7 +398,9 @@ jobs:
     name: Validate post job
     runs-on: ubuntu-latest
     # IMPORTANT! Any job added to the workflow should be added here too
-    needs: [validate, validate-old-ghcs, build-alpine, dogfooding]
+    # (This one is true because of the abstraction described above, and the
+    # corresponding branch protection rules. ++bsa)
+    needs: [validate, build-alpine, dogfooding]
 
     steps:
       - run: |