Simplify cache cleanup actions (#365)

In our old approach, a workflow file contains a job that uploads the PR
number as an artifact. While the PR is still open, the workflow_run
triggered by it will download the artifact and use the information to clean
up all except the last used cache associated with that original
workflow. When a PR is merged or closed, there will be a post-pr workflow
that uploads the PR number as an artifact and triggers a workflow_run that
clean up all caches associated with the PR. The reason we did it this way
was in the cache cleanup workflows, we did not find an easy way to get the
number of the PR triggering them. This is not convenient because we have to
add jobs uploading artifacts to workflow files.

After some experiments, we have found a reliable way to find the PR number
without using artifacts. The workflow_run's payload always contains the head
SHA of the commit that triggers it, whether the PR comes from a fork or
not. We can then use `gh pr list` to search for that head and obtain the PR
number.

.github/workflows/ci.yml

@@ -47,18 +47,3 @@ jobs:
       contents: read
       security-events: write
     uses: ./.github/workflows/codeql.yml
-
-  save_pr_number:
-    if: github.event_name != 'push'
-    runs-on: ubuntu-latest
-    steps:
-      - name: Save PR number
-        env:
-          PR_NUMBER: ${{ github.event.number }}
-        run: |
-          echo $PR_NUMBER > pr_number.txt
-      - uses: actions/upload-artifact@v4
-        with:
-          name: pr_number
-          path: pr_number.txt
-          retention-days: 1

.github/workflows/cleanup-cache-postpr.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   CleanUpCcacheCachePostPR:
-    name: Clean Up Ccahe Cache Post PR
+    name: Clean Up Ccache Cache Post PR
     runs-on: ubuntu-latest
     permissions:
       actions: write
@@ -17,14 +17,17 @@ jobs:
       GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
     steps:
       - uses: actions/checkout@v4
-      - name: Clean up ccahe
+      - name: Clean up ccache
         run: |
           gh extension install actions/gh-actions-cache
 
           REPO=${{ github.repository }}
 
-          gh run download ${{ github.event.workflow_run.id }} -n pr_number
-          pr_number=`cat pr_number.txt`
+          # For debugging cat ${GITHUB_EVENT_PATH} to see the payload.
+
+          pr_head_sha=${{ github.event.workflow_run.head_sha }}
+          pr_number=$(gh pr list --state all --search $pr_head_sha --json number --jq '.[0].number')
+          echo "Post-PR cache cleanup for PR ${pr_number}"
           BRANCH=refs/pull/${pr_number}/merge
 
           # Setting this to not fail the workflow while deleting cache keys.

.github/workflows/cleanup-cache.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   CleanUpCcacheCache:
-    name: Clean Up Ccahe Cache for ${{ github.event.workflow_run.name }}
+    name: Clean Up Ccache Cache for ${{ github.event.workflow_run.name }}
     runs-on: ubuntu-latest
     permissions:
       actions: write
@@ -17,7 +17,7 @@ jobs:
       GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
     steps:
       - uses: actions/checkout@v4
-      - name: Clean up ccahe
+      - name: Clean up ccache
         run: |
           gh extension install actions/gh-actions-cache
 
@@ -29,9 +29,12 @@ jobs:
           # Triggering workflow run name (e.g., LinuxClang)
           WORKFLOW_NAME="${{ github.event.workflow_run.name }}"
 
+          # For debugging, cat ${GITHUB_EVENT_PATH} to see the payload.
+
           if [[ $EVENT == "pull_request" ]]; then
-            gh run download ${{ github.event.workflow_run.id }} -n pr_number
-            pr_number=`cat pr_number.txt`
+            pr_head_sha=${{ github.event.workflow_run.head_sha }}
+            pr_number=$(gh pr list --search $pr_head_sha --json number --jq '.[0].number')
+            echo "Clean up cache for PR ${pr_number}"
             BRANCH=refs/pull/${pr_number}/merge
           else
             BRANCH=refs/heads/${{ github.event.workflow_run.head_branch }}
@@ -54,6 +57,7 @@ jobs:
           IFS=$'\n'
           for j in $cached_jobs
           do
+            # Delete all entries except the last used one
             old_keys=$(gh actions-cache list -L 100 -R $REPO -B $BRANCH --key "${j}-git-" --sort last-used | cut -f 1 | tail -n +2)
             for k in $old_keys
             do

.github/workflows/post-pr.yml

@@ -4,17 +4,13 @@ on:
     types:
       - closed
 
+# This workflow does not have the permission to clean up cache for PRs
+# originated from a fork. The purpose here is to trigger a workflow_run
+# cleanup-cache-postpr.yml that has the right permission.
+
 jobs:
-  cleanup:
+  noop:
     runs-on: ubuntu-latest
     steps:
-      - name: Save PR number
-        env:
-          PR_NUMBER: ${{ github.event.number }}
-        run: |
-          echo $PR_NUMBER > pr_number.txt
-      - uses: actions/upload-artifact@v4
-        with:
-          name: pr_number
-          path: pr_number.txt
-          retention-days: 1
+      - name: No OP
+        run: echo "This workflow is going to trigger CleanUpCachePostPR."