Diff against the merge base and gate on relevance in sim-tests.yml

This fixes the sim-tests workflow diffing against the PR base branch tip
instead of the merge base, and folds the path-based skip decision into
the job so a required check cannot hang on a skipped run.

- Added a step that resolves and verifies the merge base of the PR's
  base and head commits, failing loudly with an ::error:: annotation if
  either does not resolve.
- The base-tip bug made every file changed on master since the PR
  branched look PR-authored, and every file deleted on master look
  newly added, which is exactly the false-attribution problem this
  migration exists to remove.
- Removed the pull_request `paths:` filter and instead skip inside the
  gate step, printing a distinct ::notice:: for test/needs-o2-dev and
  for "no changed file under DATA/, MC/, test/, RelVal/".
- Moved the gate step before the CVMFS check, so a PR that is skipped
  does not fail on a broken runner.
- Pruned everything but logs from o2dpg_tests/ after the upload step.
- Added a comment noting the alienv -c re-parsing hazard.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: sawenzel

.github/workflows/sim-tests.yml

@@ -2,12 +2,7 @@
 name: Simulation tests
 
 'on':
-  pull_request:
-    paths:
-      - 'DATA/**'
-      - 'MC/**'
-      - 'test/**'
-      - 'RelVal/**'
+  pull_request: {}
   workflow_dispatch:
     inputs:
       tag:
@@ -31,40 +26,63 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v4
         with:
-          # The changed-file logic diffs against the PR base, so the full
+          # The changed-file logic diffs against the merge base, so the full
           # history is needed, not a shallow clone.
           fetch-depth: 0
 
-      - name: Check the CVMFS environment
+      - name: Resolve the diff base
+        id: base
+        if: github.event_name == 'pull_request'
+        env:
+          BASE_SHA: ${{ github.event.pull_request.base.sha }}
+          HEAD_SHA: ${{ github.event.pull_request.head.sha }}
         run: |
           set -eu
-          test -d /cvmfs/alice.cern.ch || {
-            echo "::error title=CVMFS unavailable::/cvmfs/alice.cern.ch is not mounted on this runner"
+          git rev-parse --verify "$BASE_SHA^{commit}" >/dev/null || {
+            echo "::error title=Cannot resolve diff base::pull request base commit $BASE_SHA does not resolve in this checkout"
             exit 1
           }
-          test -x /cvmfs/alice.cern.ch/bin/alienv || {
-            echo "::error title=CVMFS unavailable::/cvmfs/alice.cern.ch/bin/alienv is missing"
+          git rev-parse --verify "$HEAD_SHA^{commit}" >/dev/null || {
+            echo "::error title=Cannot resolve diff head::pull request head commit $HEAD_SHA does not resolve in this checkout"
             exit 1
           }
+          echo "sha=$(git merge-base "$BASE_SHA" "$HEAD_SHA")" >> "$GITHUB_OUTPUT"
 
-      - name: Skip when the pull request opted into the source build
-        id: sentinel
+      - name: Skip when not relevant or opted into the source build
+        id: gate
         if: github.event_name == 'pull_request'
         env:
-          BASE_SHA: ${{ github.event.pull_request.base.sha }}
+          BASE_SHA: ${{ steps.base.outputs.sha }}
           HEAD_SHA: ${{ github.event.pull_request.head.sha }}
         run: |
           set -eu
-          if git diff --name-only "$BASE_SHA" "$HEAD_SHA" | grep -qx 'test/needs-o2-dev' ; then
+          changed=$(git diff --diff-filter=AMR --name-only "$BASE_SHA" "$HEAD_SHA")
+          if grep -qx 'test/needs-o2-dev' <<< "$changed" ; then
             echo "::notice title=Skipped::this pull request touches test/needs-o2-dev, so it is tested by build/O2DPG/sim/o2dev against O2 dev instead"
             echo "skip=true" >> "$GITHUB_OUTPUT"
+          elif ! grep -qE '^(DATA/|MC/|test/|RelVal/)' <<< "$changed" ; then
+            echo "::notice title=Skipped::no changed file matches DATA/, MC/, test/ or RelVal/"
+            echo "skip=true" >> "$GITHUB_OUTPUT"
           else
             echo "skip=false" >> "$GITHUB_OUTPUT"
           fi
 
+      - name: Check the CVMFS environment
+        if: steps.gate.outputs.skip != 'true'
+        run: |
+          set -eu
+          test -d /cvmfs/alice.cern.ch || {
+            echo "::error title=CVMFS unavailable::/cvmfs/alice.cern.ch is not mounted on this runner"
+            exit 1
+          }
+          test -x /cvmfs/alice.cern.ch/bin/alienv || {
+            echo "::error title=CVMFS unavailable::/cvmfs/alice.cern.ch/bin/alienv is missing"
+            exit 1
+          }
+
       - name: Resolve the O2PDPSuite tag
         id: tag
-        if: steps.sentinel.outputs.skip != 'true'
+        if: steps.gate.outputs.skip != 'true'
         env:
           REQUESTED_TAG: ${{ inputs.tag }}
           PR_BODY: ${{ github.event.pull_request.body }}
@@ -88,14 +106,19 @@ jobs:
           echo "tag=$tag" >> "$GITHUB_OUTPUT"
 
       - name: Run the O2DPG tests
-        if: steps.sentinel.outputs.skip != 'true'
+        if: steps.gate.outputs.skip != 'true'
         env:
           O2PDPSUITE_TAG: ${{ steps.tag.outputs.tag }}
-          O2DPG_TEST_HASH_BASE: ${{ github.event.pull_request.base.sha }}
+          O2DPG_TEST_HASH_BASE: ${{ steps.base.outputs.sha }}
           O2DPG_TEST_HASH_HEAD: ${{ github.event.pull_request.head.sha }}
           JOBS: 8
         run: |
           set -eu
+          # Everything after "-c" is joined into one string and re-evaluated
+          # by the CVMFS alienv via "bash -c \"$*\"", so this quoting is only
+          # applied once here and then discarded. Safe for the runner's
+          # actual workspace path, but a space or "$" in that path away from
+          # breaking.
           /cvmfs/alice.cern.ch/bin/alienv setenv "O2PDPSuite/$O2PDPSUITE_TAG" -c \
             env O2DPG_ROOT="$PWD" O2DPG_MC_CONFIG_ROOT="$PWD" \
                 O2DPG_TEST_REPO_DIR="$PWD" \
@@ -116,3 +139,7 @@ jobs:
             o2dpg_tests/**/*mergerlog*
           if-no-files-found: ignore
           retention-days: 14
+
+      - name: Prune test artifacts
+        if: always()
+        run: find o2dpg_tests -type f ! -name '*.log' ! -name '*serverlog*' ! -name '*workerlog*' ! -name '*mergerlog*' -delete || true