Auto merge of #29197 - delan:ci-decision-job, r=jdm

Cancel main build if successful build already exists for commit

Homu pushes both the old and new commits for each merge, causing the main build to run twice for each commit.

This patch adds a “decision job” to the main build, which cancels the run if a successful main build already exists for the commit. The jobs that build Servo on each of the five platforms in turn block until that new job finishes.

---

- [ ] ~~`./mach build -d` does not report any errors~~
- [ ] ~~`./mach test-tidy` does not report any errors~~
- [x] These changes fix #28326

<!-- Either: -->
- [ ] There are tests for these changes OR
- [x] These changes do not require tests because they affect the CI configuration

.github/workflows/main.yml

@@ -13,9 +13,31 @@ env:
   SHELL: /bin/bash
 
 jobs:
+  decision:
+    name: Decision job
+    runs-on: ubuntu-20.04
+    steps:
+      - name: Previous successful build check
+        uses: actions/github-script@v6
+        with:
+          script: |
+            if ((await github.rest.actions.listWorkflowRuns({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              workflow_id: "main.yml",
+              head_sha: context.sha,
+              status: "success",
+            })).data.workflow_runs.length > 0)
+              await github.rest.actions.cancelWorkflowRun({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                run_id: context.runId,
+              });
+
   build-win:
     name: Build (Windows)
     runs-on: windows-2019
+    needs: ["decision"]
     steps:
       - name: Branch check
         if: ${{ !contains('refs/heads/auto refs/heads/try refs/heads/try-windows', github.ref) }}
@@ -43,6 +65,7 @@ jobs:
   build-uwp-x64:
     name: Build (Windows UWP x64)
     runs-on: windows-2019
+    needs: ["decision"]
     steps:
       - name: Branch check
         if: ${{ !contains('refs/heads/auto refs/heads/try refs/heads/try-windows', github.ref) }}
@@ -71,6 +94,7 @@ jobs:
   build-uwp-arm64:
     name: Build (Windows UWP arm64)
     runs-on: windows-2019
+    needs: ["decision"]
     steps:
       - name: Branch check
         if: ${{ !contains('refs/heads/auto refs/heads/try refs/heads/try-windows', github.ref) }}
@@ -97,6 +121,7 @@ jobs:
   build-mac:
     name: Build (macOS)
     runs-on: macos-12
+    needs: ["decision"]
     steps:
       - name: Branch check
         if: ${{ !contains('refs/heads/auto refs/heads/try refs/heads/try-mac', github.ref) }}
@@ -201,6 +226,7 @@ jobs:
   build-linux:
     name: Build (Linux)
     runs-on: ubuntu-20.04
+    needs: ["decision"]
     steps:
       - name: Branch check
         if: ${{ !contains('refs/heads/auto refs/heads/try refs/heads/try-linux refs/heads/try-wpt', github.ref) }}