From a693ccda4b2065e9e949a1d8c12107639672aa9b Mon Sep 17 00:00:00 2001
From: Daz DeBoer <daz@gradle.com>
Date: Tue, 17 Aug 2021 16:15:28 -0600
Subject: [PATCH] Allow use of caches 'read-only'

To avoid evicting useful entries, some pipeline may benefit from using existing
cache entries without writing any changes back to the cache.

Fixes #62
---
 .github/workflows/prod.yml |  7 +++++++
 README.md                  | 12 ++++++++++++
 action.yml                 |  4 ++++
 src/post.ts                |  8 ++++++++
 4 files changed, 31 insertions(+)

diff --git a/.github/workflows/prod.yml b/.github/workflows/prod.yml
index 65163d0..5cd9f46 100644
--- a/.github/workflows/prod.yml
+++ b/.github/workflows/prod.yml
@@ -62,6 +62,13 @@ jobs:
         build-root-directory: __tests__/samples/basic
         arguments: test --no-daemon
         dependencies-cache-enabled: true
+    - name: Test dependencies-cache-enabled
+      uses: ./
+      with:
+        build-root-directory: __tests__/samples/basic
+        arguments: test --no-daemon
+        dependencies-cache-enabled: true
+        cache-read-only: true
 
   configuration-cache:
     strategy:
diff --git a/README.md b/README.md
index 2a4b1f2..a79fb7a 100644
--- a/README.md
+++ b/README.md
@@ -187,6 +187,18 @@ dependencies-cache-key: gradle/dependency-locks/**
 dependencies-cache-exact: true
 ```
 
+### Using the caches read-only
+
+Cache storage space is limited for GitHub actions, and writing new cache entries can trigger the deletion of exising entries.
+In some circumstances, it makes sense for a Gradle invocation to use any existing cache entries but not to write and changes back.
+For example, you may want to write cache entries for builds on your `main` branch, but not for any PR build invocations.
+
+Use the following configuration to avoid writing cache entries for the action invocation:
+
+```yaml
+cache-read-only: true
+```
+
 ## Build scans
 
 If your build publishes a [build scan](https://gradle.com/build-scans/) the `gradle-build-action` action will emit the link to the published build scan as an output named `build-scan-url`.
diff --git a/action.yml b/action.yml
index a91228e..b6b641a 100644
--- a/action.yml
+++ b/action.yml
@@ -51,6 +51,10 @@ inputs:
     description: Whether to restore only if exact match, default to 'false'
     required: false
     default: false
+  cache-read-only:
+    description: When 'true', existing entries will be read from the cache but no entries will be written
+    required: false
+    default: false
 
 outputs:
   build-scan-url:
diff --git a/src/post.ts b/src/post.ts
index 6119183..d119ceb 100644
--- a/src/post.ts
+++ b/src/post.ts
@@ -1,12 +1,20 @@
+import * as core from '@actions/core'
+
 import * as cacheWrapper from './cache-wrapper'
 import * as cacheDependencies from './cache-dependencies'
 import * as cacheConfiguration from './cache-configuration'
 
 // Invoked by GitHub Actions
 export async function run(): Promise<void> {
+    if (isCacheReadOnly()) return
+
     await cacheWrapper.cacheWrapperDist()
     await cacheDependencies.cacheDependencies()
     await cacheConfiguration.cacheConfiguration()
 }
 
+function isCacheReadOnly(): boolean {
+    return core.getBooleanInput('cache-read-only')
+}
+
 run()