fix: skip saving missing cache directories (#294)

* fix: skip saving missing cache directories

* ci: run cache regression tests on Node.js 24

test/cache-save.test.mjs

@@ -0,0 +1,47 @@
+import assert from 'node:assert/strict'
+import { mkdtemp, rm, access } from 'node:fs/promises'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { after, beforeEach, mock, test } from 'node:test'
+
+const directory = await mkdtemp(join(tmpdir(), 'pnpm-cache-save-'))
+after(() => rm(directory, { recursive: true, force: true }))
+let state
+let checkAccess
+const saveCache = mock.fn(async () => 1)
+mock.module('@actions/cache', { namedExports: { saveCache } })
+mock.module('@actions/core', {
+  namedExports: { getState: name => state[name], info: () => {} },
+})
+mock.module('fs/promises', { namedExports: { access: path => checkAccess(path) } })
+const { runSaveCache } = await import('../src/cache-save/run.ts')
+
+beforeEach(() => {
+  state = { cache_restored_key: 'old', cache_primary_key: 'new', cache_path: directory }
+  checkAccess = access
+  saveCache.mock.resetCalls()
+})
+
+test('skips saving a missing directory', async () => {
+  state.cache_path = join(directory, 'missing')
+  await runSaveCache()
+  assert.equal(saveCache.mock.callCount(), 0)
+})
+
+test('saves an existing directory with the primary key', async () => {
+  await runSaveCache()
+  assert.deepEqual(saveCache.mock.calls[0].arguments, [[directory], 'new'])
+})
+
+test('skips saving a primary-key cache hit', async () => {
+  state.cache_restored_key = 'new'
+  await runSaveCache()
+  assert.equal(saveCache.mock.callCount(), 0)
+})
+
+test('propagates unexpected filesystem errors', async () => {
+  const error = Object.assign(new Error('access denied'), { code: 'EACCES' })
+  checkAccess = async () => { throw error }
+  await assert.rejects(runSaveCache(), error)
+  assert.equal(saveCache.mock.callCount(), 0)
+})