CI: Test on several Bazel and Ubuntu versions.

Also makes some improvements to the CI script taken from https://github.com/google/netkat.
They will allow us to make better use of the cache while avoiding hitting GitHub's cache limits.

Signed-off-by: Steffen Smolka <[email protected]>

.github/workflows/ci-build-proto.yml

@@ -14,30 +14,92 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-22.04
+
+    strategy:
+      matrix:
+        # We only test on the oldest version we want to support and latest.
+        # We trust that things also work for versions in the middle.
+        os: [ubuntu-22.04, ubuntu-latest]
+        # See Bazelisk README for legal values.
+        bazel_version: [7.x, latest]
+      # Don't abort other runs when one of them fails, to ease debugging.
+      fail-fast: false
+
+    runs-on: ${{ matrix.os }}
+
     env:
-      BAZEL: bazelisk-linux-amd64
+      # This tells Bazelisk (installed as `bazel`) to use specified version.
+      # https://github.com/bazelbuild/bazelisk?tab=readme-ov-file#how-does-bazelisk-know-which-bazel-version-to-run
+      USE_BAZEL_VERSION: ${{ matrix.bazel_version }}
+      CACHE_KEY: ${{ matrix.os }}_bazel-${{ matrix.bazel_version }}
+
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
       with:
         submodules: recursive
 
     - name: Mount bazel cache
-      uses: actions/cache@v2
+      uses: actions/cache/restore@v4
       with:
         # See https://docs.bazel.build/versions/master/output_directories.html
         path: "~/.cache/bazel"
         # Create a new cache entry whenever Bazel files change.
         # See https://docs.github.com/en/actions/guides/caching-dependencies-to-speed-up-workflows
-        key: bazel-${{ runner.os }}-build-${{ hashFiles('**/*.bzl', '**/*.bazel') }}
+        key: ${{ env.CACHE_KEY }}-${{ hashFiles('**/*.bazel*', '**/*.bzl') }}
         restore-keys: |
-          bazel-${{ runner.os }}-build-
+          ${{ env.CACHE_KEY }}
+
+    - name: Save start time
+      uses: josStorer/get-current-time@v2
+      id: start-time
+      with:
+        # Unix timestamp -- seconds since 1970.
+        format: X
 
     - name: Build proto/
       run: cd proto && bazel build //... && bazel test //...
 
+    - name: Build bazel/example/using-bzlmod/
+      run: cd bazel/example/using-bzlmod && bazel build //...
+
     - name: Build bazel/example/using-workspace/
       run: cd bazel/example/using-workspace && bazel build //...
+      # This is a legacy example that doesn't work beyond Bazel 7.x.
+      if: ${{ env.USE_BAZEL_VERSION == '7.x' }}
 
-    - name: Build bazel/example/using-bzlmod/
-      run: cd bazel/example/using-bzlmod && bazel build //...
+    - name: Save end time
+      # Always save the end time so we can calculate the build duration.
+      if: always()
+      uses: josStorer/get-current-time@v2
+      id: end-time
+      with:
+        # Unix timestamp -- seconds since 1970.
+        format: X
+
+    - name: Calculate build duration
+      # Always calculate the build duration so we can update the cache if needed.
+      if: always()
+      run: |
+        START=${{ steps.start-time.outputs.formattedTime }}
+        END=${{ steps.end-time.outputs.formattedTime }}
+        DURATION=$(( $END - $START ))
+        echo "duration=$DURATION" | tee "$GITHUB_ENV"
+
+    - name: Compress cache
+      # Always compress the cache so we can update the cache if needed.
+      if: always()
+      run: rm -rf $(bazel info repository_cache)
+
+    - name: Save bazel cache
+      uses: actions/cache/save@v4
+      # Only create a new cache entry if we're on the main branch or the build takes >3mins.
+      #
+      # NOTE: Even though `always()` evaluates to true, and `true && x == x`,
+      # the `always() &&` prefix is not redundant! The call to `always()` has a
+      # side effect, which is to override the default behavior of automagically
+      # canceling this step if a previous step failed.
+      # (Don't blame me, blame GitHub Actions!)
+      if: always() && (github.ref_name == 'main' || env.duration > 180)
+      with:
+        path: "~/.cache/bazel"
+        key: ${{ env.CACHE_KEY }}-${{ hashFiles('**/*.bazel*', '**/*.bzl') }}-${{ github.run_id }}