Allow compatible workflows to share an existing job cache

README.md

@@ -134,6 +134,24 @@ builds something different its own `key`:
           key: ${{ matrix.target }}
 ```
 
+### Share an existing job's cache
+
+Compatible jobs in different workflows can reuse an existing cache by naming
+its owning workflow and job. Use the same `key` and `working-directory`, and
+keep the target, toolchain and build flags compatible:
+
+```yaml
+      - uses: tensorlakeai/github-runners@v1
+        with:
+          languages: rust
+          cache-owner: build_platform_dataplane_on_merge.yaml/build_binary
+          key: dataplane-platform-dataplane-release-glibc-2.34-x86_64-gnu-and-musl
+```
+
+The owner job needs no configuration change. Existing entries are reused;
+branches and pull requests still save only to their own ref. Builds restore
+archives to local disk and do not share a writable build directory.
+
 ## What is cached
 
 | Language | Detected from | Cached |
@@ -195,6 +213,7 @@ of many small files.
 | `languages` | `auto` | `auto`, `none`, or a comma list of `rust`, `node`, `go`, `python`. |
 | `paths` | | Extra files or directories to cache, one per line, relative to `working-directory` or starting with `~/`. |
 | `key` | | Separates caches for jobs that build different things, such as matrix entries. |
+| `cache-owner` | | Reuse another job's cache in this repository, as `workflow.yml/job-id`; use its `key` and `working-directory`. |
 | `working-directory` | `.` | Where to search for lockfiles. |
 | `save` | `auto` | `auto` saves to the job's own branch or pull request when dependencies changed, except in merge queue runs; `true` also saves in merge queue runs; `false` never saves. |
 | `default-branch` | the repository's | The fallback cache for branches, and never expired. |

action.yml

@@ -27,6 +27,13 @@ inputs:
       build different things, for example the target triple.
     required: false
     default: ""
+  cache-owner:
+    description: >-
+      Share an existing job's cache in this repository. Use workflow.yml/job-id
+      and the same key and working-directory as that job. Branch isolation is
+      preserved. Leave empty to use this workflow and job.
+    required: false
+    default: ""
   working-directory:
     description: Directory to search for lockfiles.
     required: false

src/lib/config.js

@@ -48,6 +48,16 @@ function workflowIdentity() {
   return ref.replace(/@.*$/, '');
 }
 
+function cacheIdentity() {
+  const owner = core.getInput('cache-owner', '');
+  if (!owner) return [workflowIdentity(), process.env.GITHUB_JOB || ''];
+  const match = /^([A-Za-z0-9_.-]+\.ya?ml)\/([A-Za-z_][A-Za-z0-9_-]*)$/.exec(owner);
+  if (!match) throw new Error('Input cache-owner must be workflow.yml/job-id in this repository.');
+  const repository = process.env.GITHUB_REPOSITORY || '';
+  if (!repository) throw new Error('Input cache-owner requires GITHUB_REPOSITORY.');
+  return [`${repository}/.github/workflows/${match[1]}`, match[2]];
+}
+
 function readEvent() {
   try {
     return JSON.parse(fs.readFileSync(process.env.GITHUB_EVENT_PATH, 'utf8'));
@@ -83,10 +93,11 @@ function load() {
   const workingDirectory = path.resolve(process.env.GITHUB_WORKSPACE || process.cwd(), core.getInput('working-directory', '.'));
   const key = core.getInput('key', '');
   const languages = parseLanguages(core.getInput('languages', 'auto'));
+  const [workflow, job] = cacheIdentity();
   const scope = sha256(JSON.stringify([
     process.env.GITHUB_REPOSITORY || '',
-    workflowIdentity(),
-    process.env.GITHUB_JOB || '',
+    workflow,
+    job,
     path.relative(process.env.GITHUB_WORKSPACE || process.cwd(), workingDirectory),
     key,
   ])).slice(0, 20);

test/cache.test.js

@@ -167,6 +167,60 @@ test('looks up the own ref, then the base branch, then the default branch', () =
   assert.deepEqual(config.refs(''), ['refs/heads/feature']);
 });
 
+test('cache-owner reuses the existing scope and keeps repository and build keys isolated', () => {
+  job({ inputs: { key: 'release-linux' } });
+  const existing = config.load().scope;
+  const legacy = config.sha256(JSON.stringify(['acme/app', 'acme/app/.github/workflows/ci.yml', 'build', '', 'release-linux'])).slice(0, 20);
+  assert.equal(existing, legacy, 'default scopes remain compatible with existing entries');
+
+  job({ inputs: { 'cache-owner': 'ci.yml/build', key: 'release-linux' } });
+  process.env.GITHUB_WORKFLOW_REF = 'acme/app/.github/workflows/publish.yml@refs/heads/main';
+  process.env.GITHUB_JOB = 'helper';
+  assert.equal(config.load().scope, existing);
+
+  process.env.INPUT_KEY = 'release-musl';
+  assert.notEqual(config.load().scope, existing, 'different build keys stay isolated');
+  process.env.INPUT_KEY = 'release-linux';
+  process.env.GITHUB_REPOSITORY = 'acme/other';
+  assert.notEqual(config.load().scope, existing, 'cache-owner cannot cross repositories');
+});
+
+test('rejects malformed cache owners', () => {
+  for (const owner of ['../ci.yml/build', 'acme/app/ci.yml/build', 'ci.yml', 'ci.yml/../build', 'ci.yml/1build']) {
+    job({ inputs: { 'cache-owner': owner } });
+    assert.throws(() => config.load(), /cache-owner must be/);
+  }
+  job({ inputs: { 'cache-owner': 'ci.yml/build' } });
+  delete process.env.GITHUB_REPOSITORY;
+  assert.throws(() => config.load(), /requires GITHUB_REPOSITORY/);
+});
+
+test('another workflow restores the owner cache and saves only to its own branch', async () => {
+  const main = job({ inputs: { key: 'release-linux' } });
+  rustProject(main);
+  const dependency = path.join('target', 'debug', 'deps', 'libserde.rlib');
+  write(path.join(main.workspace, dependency), 'built by the owner workflow');
+  await main.restore();
+  await main.save();
+  const mainEntries = entries('rust');
+
+  // Fresh runners use the same absolute repository/home paths. Remove the
+  // build output and switch workflow identity without changing those paths.
+  fs.rmSync(path.join(main.workspace, 'target'), { recursive: true });
+  process.env.GITHUB_WORKFLOW_REF = 'acme/app/.github/workflows/publish.yml@refs/pull/9/merge';
+  process.env.GITHUB_JOB = 'helper';
+  process.env.GITHUB_EVENT_NAME = 'pull_request';
+  process.env.GITHUB_REF = 'refs/pull/9/merge';
+  process.env.GITHUB_BASE_REF = 'main';
+  process.env['INPUT_CACHE-OWNER'] = 'ci.yml/build';
+  assert.equal((await main.restore()).restored, 'rust');
+  assert.equal(fs.readFileSync(path.join(main.workspace, dependency), 'utf8'), 'built by the owner workflow');
+  write(path.join(main.workspace, 'Cargo.lock'), 'version = 4\n# helper dependency change\n');
+  await main.save();
+  assert.equal(entries('rust', 'refs/pull/9/merge').length, 1);
+  assert.deepEqual(entries('rust'), mainEntries, 'the shared owner cache keeps default-branch entries intact');
+});
+
 test('balances shards by size', () => {
   const files = [900, 500, 400, 300, 200, 100].map((size, index) => ({ path: `f${index}`, size }));
   const shards = archive.plan(files, 1000);