Use setup-bazel GHA for disk and repository caching (#111)

Author: tom-osika

.github/ci.bazelrc

@@ -13,60 +13,29 @@ on:
 jobs:
   bazel_test:
     runs-on: ubuntu-latest
-    env:
-      # This is where bazelisk caches its downloads of Bazel.
-      BAZELISK_HOME: /home/runner/.cache/bazel_ci/bazelisk
     steps:
       - uses: actions/checkout@v6
       - name: Install Ubuntu dependencies
         run: .github/ci_setup.bash
-      # Restore the most appropriate caches.
-      #
-      # In the below: `github.ref` will be either "refs/pull/<pr_number>/merge"
-      # or "refs/heads/master"; `github.run_number` is a monotonically increasing
-      # serial number; and `github.run_attempt` is a typically 1.
-      #
-      # The `key` declared here will never match (since it encodes the current
-      # run_number) but illustrates the name we'll use to save the caches when
-      # we're done.
-      #
-      # However, one of the `restore-keys` patterns *should* always match. The
-      # effects we should see from the `restore-keys` patterns are:
-      #
-      # * When building the `master` branch, we'll always pull the most recently
-      #   updated master caches. Both restore keys say
-      #   "bazel_ci-refs/heads/master-" and the prior archives are named, e.g.,
-      #   "bazel_ci-refs/heads/master-####-#".  GitHub will use the *most
-      #   recently saved* archive name that matches the restore key prefix
-      #   (no matter the lexicographic ordering of the ####-# part).
-      #
-      # * Ditto for the first build of a PR; the first restore key will not
-      #   match anything, so it falls back to the master caches.
-      #
-      # * For subsequent builds of a PR, the first restore key, e.g.,
-      #   "bazel_ci-refs/pull/###/merge-" should match the most recently saved
-      #   PR key "bazel_ci-refs/pull/###/merge-####-#".
-      - uses: actions/cache/restore@v5
+      # Use setup-bazel for caching.
+      - uses: bazel-contrib/setup-bazel@0.15.0
         with:
-          path: ~/.cache/bazel_ci
-          key: bazel_ci-${{ github.ref }}-${{ github.run_number }}-${{ github.run_attempt }}
-          restore-keys: |
-            bazel_ci-${{ github.ref }}-
-            bazel_ci-refs/heads/master-
-      - name: Report cache sizes
-        run: |
-          du -ms ~/.cache/bazel_ci/* || true
+          # Use the cached Bazelisk version, or download if not found.
+          bazelisk-cache: true
+          # Enable the disk cache and share it across all workflows
+          # (i.e., across master and pull requests).
+          # We want all builds to start from a warm cache, and we're not too
+          # concerend about pull request builds "polluting" master builds.
+          disk-cache: true
+          # Share repository cache globally as well. If repository-cache is set to true,
+          # it uses the hash of a few bazel files as the key for the cache. You can set
+          # your own list of files to use to calculate the key by passing them below.
+          # Passing an empty string tells setup-bazel to use the same key
+          # every time, regardless of any changes to any files.
+          repository-cache: ""
+          # This dumps configuration details to the log.
+          bazelrc: common --announce_rc=yes
       # Actual testing.
       - name: Bazel Test
         run: |
-          ln -s .github/ci.bazelrc user.bazelrc
           bazelisk test //...
-      # Save the updated cache snapshots, so we never do the same work twice.
-      # By default, actions/cache only saves after a successful workflow, but
-      # our caches are bags of files where we only ever add new files (not change
-      # an existing files), so it's always safe to snapshot.
-      - uses: actions/cache/save@v5
-        if: always()
-        with:
-          path: ~/.cache/bazel_ci
-          key: bazel_ci-${{ github.ref }}-${{ github.run_number }}-${{ github.run_attempt }}