Use workflow_run.head_sha to find corresponding tag

Author: sarahchen6

.github/workflows/pin-system-tests.yaml

@@ -7,8 +7,10 @@ on:
         description: 'The minor release branch name (e.g. release/v1.54.x)'
         required: true
         type: string
-  # run workflow when any branch or tag is created
-  create:
+  workflow_run:
+    workflows: [Create Release Branch]
+    types:
+      - completed
 
 jobs:
   # TODO: Remove this job after confirming the workflow succeeds for release branches
@@ -22,7 +24,7 @@ jobs:
 
   pin-system-tests:
     name: "Pin system tests"
-    if: github.event_name != 'create' || (contains(github.ref, 'tags/v') && endsWith(github.ref, '.0'))
+    if: github.event_name == 'workflow_dispatch' || github.event.workflow_run.conclusion == 'success'
     runs-on: ubuntu-latest
     permissions:
       contents: write
@@ -34,20 +36,34 @@ jobs:
           scope: DataDog/dd-trace-java
           policy: self.pin-system-tests.create-pr
 
+      - name: Checkout repository for tag lookup
+        if: github.event_name == 'workflow_run'
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # 6.0.2
+        with:
+          fetch-depth: 0 # fetch entire history, including tags
+
+      - name: Get release branch name from tag
+        if: github.event_name == 'workflow_run'
+        id: get-release-branch
+        run: |
+          HEAD_SHA="${{ github.event.workflow_run.head_sha }}"
+          TAG=$(git tag --points-at "$HEAD_SHA" | grep -E '^v[0-9]+\.[0-9]+\.0$' | head -n 1)
+          if [[ -z "$TAG" ]]; then
+            echo "ERROR: Could not find tag that matches pattern 'vX.Y.0' and points at commit $HEAD_SHA"
+            exit 1
+          fi
+          echo "branch=release/${TAG%.0}.x" >> $GITHUB_OUTPUT
+
       - name: Define base release branch
         id: define-base-branch
         run: |
           if [[ -n "${{ github.event.inputs.release-branch-name }}" ]]; then
             BASE_BRANCH=${{ github.event.inputs.release-branch-name }}
+          elif [[ "${{ github.event_name }}" == "workflow_run" ]]; then
+            BASE_BRANCH="${{ steps.get-release-branch.outputs.branch }}"
           else
-            # Convert tag (e.g. v1.54.0) to release branch name (e.g. release/v1.54.x)
-            TAG=${GITHUB_REF#refs/tags/}
-            if [[ "$TAG" =~ ^v[0-9]+\.[0-9]+\.0$ ]]; then
-              BASE_BRANCH="release/${TAG%.0}.x"
-            else
-              echo "ERROR: Tag $TAG is not in the expected format: vX.Y.0"
-              exit 1
-            fi
+            echo "ERROR: Unable to determine base branch."
+            exit 1
           fi
           echo "base_branch=${BASE_BRANCH}" >> $GITHUB_OUTPUT