build: fix ci cacheing to use recently built caches

.github/workflows/ci.yaml

@@ -20,16 +20,21 @@ jobs:
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
       - uses: actions/checkout@v2
-      - name: Mount bazel action cache
+      # Cache build and external artifacts so that the next ci build is incremental.
+      # Because caches cannot be updated, create a new cache based on the contents of
+      # hashFiles (modify this to capture sources in your repo that would cause bazel
+      # to rebuild artifacts, but it doesn't have to be perfect). On a cache miss, the
+      # restore-keys prefix will load the most recent bazel cache for the current branch
+      # or its parent. After the build, the new cache will be stored which will be used
+      # on the next build, and so forth.
+      - name: Mount bazel caches
         uses: actions/cache@v2
         with:
-          path: "~/.cache/bazel"
-          key: bazel
-      - name: Mount bazel repo cache
-        uses: actions/cache@v2
-        with:
-          path: "~/.cache/bazel-repo"
-          key: bazel-repo
+          path: |
+            "~/.cache/bazel"
+            "~/.cache/bazel-repo"
+          key: bazel-cache-${{ hashFiles('**/BUILD.bazel', '**/*.bzl', 'WORKSPACE') }}
+          restore-keys: bazel-cache-
       - name: bazel test //...
         env:
           # Bazelisk will download bazel to here, ensure it is cached between runs.

.github/workflows/release.yml

@@ -14,6 +14,14 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v2
+      - name: Mount bazel caches
+        uses: actions/cache@v2
+        with:
+          path: |
+            "~/.cache/bazel"
+            "~/.cache/bazel-repo"
+          key: bazel-cache-${{ hashFiles('**/BUILD.bazel', '**/*.bzl', 'WORKSPACE') }}
+          restore-keys: bazel-cache-
       - name: bazel test //...
         env:
           # Bazelisk will download bazel to here

README.md

@@ -23,6 +23,7 @@ Ready to get started? Copy this repo, then
 1. rename directory "mylang" similarly
 1. run `pre-commit install` to get lints (see CONTRIBUTING.md)
 1. if you don't need to fetch platform-dependent tools, then remove anything toolchain-related.
+1. update the `actions/cache@v2` bazel cache key in [.github/workflows/ci.yaml](.github/workflows/ci.yaml) and [.github/workflows/release.yml](.github/workflows/release.yml) to be a hash of your source files.
 1. delete this section of the README (everything up to the SNIP).
 
 ---- SNIP ----