Unify PR build into a single workflow with a dispatcher

- Introduce reusable-pr-build.yml that owns all build orchestration
  (common, test-latest-deps, test-native, muzzle, markdown checks, and
  the required-status-check aggregator).
- Convert build-pull-request.yml into a thin dispatcher: it listens to
  pull_request events (including 'labeled' for 'test native', 'test
  openj9', 'test windows') and calls reusable-pr-build.yml with the
  appropriate skip flags.
- Delete rebuild-pull-request-on-label.yml; the dispatcher absorbs its
  role via the 'labeled' trigger and the shared concurrency group.

Branch protection must be updated to require
'build / required-status-check' instead of 'required-status-check'.

Author: trask

.github/workflows/build-pull-request.yml

@@ -6,66 +6,32 @@ on:
       - opened
       - synchronize
       - reopened
-  # called from rebuild-pull-request-on-label.yml
-  workflow_call:
+      - labeled
 
 concurrency:
-  # fixed group name (not github.workflow) so that runs triggered by pull_request
-  # and runs triggered via workflow_call from rebuild-pull-request-on-label.yml
-  # share the group, otherwise the PR build would run twice in parallel whenever
-  # a "test *" label is added
   group: build-pull-request-${{ github.event.pull_request.number }}
   cancel-in-progress: true
 
 permissions:
   contents: read
 
 jobs:
-  common:
-    uses: ./.github/workflows/build-common.yml
+  build:
+    # On `labeled` events, only proceed when the added label affects what we
+    # build. All other event types (opened, synchronize, reopened) always
+    # proceed.
+    if: |
+      github.event.action != 'labeled' ||
+      github.event.label.name == 'test native' ||
+      github.event.label.name == 'test openj9' ||
+      github.event.label.name == 'test windows'
+    uses: ./.github/workflows/reusable-pr-build.yml
     with:
-      # it's rare for only the openj9 tests, openj9 smoke variants, or the windows smoke tests to break
+      # it's rare for only the openj9 tests, openj9 smoke variants, the windows
+      # smoke tests, or the native tests to break, so they are gated by labels.
+      # `test native` is applied automatically by .github/labeler.yml when the
+      # PR diff touches native-relevant paths; `test openj9` and `test windows`
+      # are applied manually.
+      skip-native-tests: ${{ !contains(github.event.pull_request.labels.*.name, 'test native') }}
       skip-openj9-tests: ${{ !contains(github.event.pull_request.labels.*.name, 'test openj9') }}
       skip-windows-smoke-tests: ${{ !contains(github.event.pull_request.labels.*.name, 'test windows') }}
-      cache-read-only: true
-
-  test-latest-deps:
-    uses: ./.github/workflows/reusable-test-latest-deps.yml
-    with:
-      cache-read-only: true
-
-  test-native:
-    uses: ./.github/workflows/reusable-native-tests.yml
-    with:
-      skip-native-tests: ${{ !contains(github.event.pull_request.labels.*.name, 'test native') }}
-
-  muzzle:
-    uses: ./.github/workflows/reusable-muzzle.yml
-    with:
-      cache-read-only: true
-
-  # this is not a required check to avoid blocking pull requests if external links break
-  markdown-check:
-    # release branches are excluded because the README.md javaagent download link has to be updated
-    # on release branches before the release download has been published
-    if: "!startsWith(github.ref_name, 'release/') && !startsWith(github.base_ref, 'release/')"
-    uses: ./.github/workflows/reusable-link-check.yml
-
-  markdown-lint-check:
-    uses: ./.github/workflows/reusable-markdown-lint-check.yml
-
-  required-status-check:
-    needs:
-      - common
-      - test-latest-deps
-      - muzzle
-      - markdown-lint-check
-    runs-on: ubuntu-latest
-    if: always()
-    steps:
-      - if: |
-          needs.common.result != 'success' ||
-          needs.test-latest-deps.result != 'success' ||
-          needs.muzzle.result != 'success' ||
-          needs.markdown-lint-check.result != 'success'
-        run: exit 1 # fail

.github/workflows/rebuild-pull-request-on-label.yml

@@ -0,0 +1,68 @@
+name: Reusable PR build
+
+# Always invoked from build-pull-request.yml (the dispatcher).
+# All gating decisions are made by the dispatcher and passed in as inputs.
+on:
+  workflow_call:
+    inputs:
+      skip-native-tests:
+        type: boolean
+        default: false
+      skip-openj9-tests:
+        type: boolean
+        default: false
+      skip-windows-smoke-tests:
+        type: boolean
+        default: false
+
+permissions:
+  contents: read
+
+jobs:
+  common:
+    uses: ./.github/workflows/build-common.yml
+    with:
+      skip-openj9-tests: ${{ inputs.skip-openj9-tests }}
+      skip-windows-smoke-tests: ${{ inputs.skip-windows-smoke-tests }}
+      cache-read-only: true
+
+  test-latest-deps:
+    uses: ./.github/workflows/reusable-test-latest-deps.yml
+    with:
+      cache-read-only: true
+
+  test-native:
+    uses: ./.github/workflows/reusable-native-tests.yml
+    with:
+      skip-native-tests: ${{ inputs.skip-native-tests }}
+
+  muzzle:
+    uses: ./.github/workflows/reusable-muzzle.yml
+    with:
+      cache-read-only: true
+
+  # this is not a required check to avoid blocking pull requests if external links break
+  markdown-check:
+    # release branches are excluded because the README.md javaagent download link has to be updated
+    # on release branches before the release download has been published
+    if: "!startsWith(github.ref_name, 'release/') && !startsWith(github.base_ref, 'release/')"
+    uses: ./.github/workflows/reusable-link-check.yml
+
+  markdown-lint-check:
+    uses: ./.github/workflows/reusable-markdown-lint-check.yml
+
+  required-status-check:
+    needs:
+      - common
+      - test-latest-deps
+      - muzzle
+      - markdown-lint-check
+    runs-on: ubuntu-latest
+    if: ${{ always() }}
+    steps:
+      - if: |
+          needs.common.result != 'success' ||
+          needs.test-latest-deps.result != 'success' ||
+          needs.muzzle.result != 'success' ||
+          needs.markdown-lint-check.result != 'success'
+        run: exit 1 # fail