allow saving cache to be optional (#131)

README.md

@@ -83,6 +83,30 @@ Default `""`.
   ```
 </details>
 
+### `cache-save`
+
+Whether to save caches at the end of the workflow.
+
+Set to `false` for pull requests to allow cache restoration without saving,
+which prevents PRs from polluting the cache while still benefiting from it.
+
+Default `true`.
+
+<details>
+  <summary>Examples</summary>
+
+  #### Disable cache saving on pull requests
+
+  ```yaml
+  - uses: bazel-contrib/setup-bazel@0.16.0
+    with:
+      bazelisk-cache: true
+      disk-cache: ${{ github.workflow }}
+      repository-cache: true
+      cache-save: ${{ github.event_name != 'pull_request' }}
+  ```
+</details>
+
 ### `disk-cache`
 
 Enable [`disk_cache`][2] and store it on GitHub based on contents of `BUILD` files.

action.yml

@@ -13,6 +13,10 @@ inputs:
     description: Extra contents to write to user .bazelrc
     required: false
     default: ""
+  cache-save:
+    description: Whether to save caches. Set to false for pull requests to allow restores but prevent saves.
+    required: false
+    default: "true"
   cache-version:
     description: Version of all caches
     required: false

config.js

@@ -5,6 +5,7 @@ const core = require('@actions/core')
 const github = require('@actions/github')
 
 const bazeliskVersion = core.getInput('bazelisk-version')
+const cacheSave = core.getBooleanInput('cache-save')
 const cacheVersion = core.getInput('cache-version')
 const moduleRoot = core.getInput('module-root')
 
@@ -137,6 +138,7 @@ core.exportVariable('BAZELISK_GITHUB_TOKEN', token)
 
 module.exports = {
   baseCacheKey,
+  cacheSave,
   bazeliskCache: {
     enabled: core.getBooleanInput('bazelisk-cache'),
     files: [`${moduleRoot}/.bazelversion`],

post.js

@@ -13,6 +13,11 @@ async function run() {
 }
 
 async function saveCaches() {
+  if (!config.cacheSave) {
+    core.info('Cache saving is disabled (cache-save: false)')
+    return
+  }
+
   await saveCache(config.bazeliskCache)
   await saveCache(config.diskCache)
   await saveCache(config.repositoryCache)