fix(core): Isolate instance settings writes (no-changelog) (#39118)

Co-authored-by: cubic-dev-ai[bot] <191113872+cubic-dev-ai[bot]@users.noreply.github.com>

.agents/skills/conventions/SKILL.md

@@ -45,6 +45,9 @@ throw new UnexpectedError('message', { extra: { context } });
 **Testing:**
 - Vitest (unit), Playwright (E2E)
 - Mock external dependencies
+- Keep filesystem tests in a test-owned temporary directory
+- Set `N8N_USER_FOLDER` before importing settings code
+- Trace branches activated by mocks and isolate every reachable mutation
 - Work from package directory: `pushd packages/cli && pnpm test`
 
 **Database:**

.agents/skills/reproduce-bug/SKILL.md

@@ -73,6 +73,12 @@ Write a regression test that:
 - Asserts the CORRECT behavior (test will fail on current code)
 - Also includes a "happy path" test to prove the setup works
 
+Complete the test safety gate in `/AGENTS.md` before you run the test:
+- Trace side effects from imports, constructors, hooks, and mocked branches
+- Keep filesystem access in a test-owned temporary directory
+- Set `N8N_USER_FOLDER` before importing modules that resolve it
+- When a mock changes a state check, inspect and isolate the branch that it activates
+
 ## Step 8: Run and Score
 
 Run the test from the package directory (e.g., `cd packages/nodes-base && pnpm test <file>`).

AGENTS.md

@@ -342,6 +342,17 @@ Testing rules:
 - Run tests and `pnpm typecheck` from the owning package.
 - Confirm unit test cases with the user before you write them.
 - Mock external dependencies. Use `nock` for HTTP services.
+- Trace side effects from imports, constructors, hooks, and mocked branches before
+  you run a new or changed test.
+- Do not let tests read from or write to the developer's home directory,
+  `~/.n8n`, or other user-owned locations.
+- Use a test-owned temporary directory for filesystem tests. Set
++  `N8N_USER_FOLDER` before you import modules that resolve it. n8n writes to `${N8N_USER_FOLDER}/.n8n`, so expect the `.n8n` subfolder there.
+- When a mock changes a state check such as `existsSync()`, inspect the branch
+  that it activates. Mock every reachable filesystem mutation unless filesystem
+  behavior is under test.
+- Run tests that can initialize n8n settings with an isolated
+  `N8N_USER_FOLDER` first. Clean up only paths that the test created.
 - Reuse immutable hoisted `mock<T>(...)` fixtures. Do not replace typed entity mocks with `as unknown as T`.
 - Use `createVitestConfigWithDecorators` for Vitest packages that use `@n8n/di` decorators.
 - Check for unused computed properties after you change a Pinia store.

packages/core/src/execution-engine/__tests__/workflow-execute-process-process-run-execution-data.test.ts

@@ -23,6 +23,7 @@ vi.mock('node:fs', async (importActual) => ({
 	...(await importActual()),
 	existsSync: vi.fn().mockReturnValue(false),
 	renameSync: vi.fn(),
+	writeFileSync: vi.fn(),
 }));
 
 import { DirectedGraph } from '../partial-execution-utils';

packages/core/src/execution-engine/__tests__/workflow-execute.test.ts

@@ -64,6 +64,7 @@ vi.mock('node:fs', async (importActual) => ({
 	...(await importActual()),
 	existsSync: vi.fn().mockReturnValue(false),
 	renameSync: vi.fn(),
+	writeFileSync: vi.fn(),
 }));
 
 const nodeTypes = Helpers.NodeTypes();

packages/core/test/setup-mocks.ts

@@ -1 +1,23 @@
+import { mkdtempSync, rmSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+
 import 'reflect-metadata';
+
+const originalUserFolder = process.env.N8N_USER_FOLDER;
+const testUserFolder = mkdtempSync(join(tmpdir(), 'n8n-core-test-'));
+
+process.env.N8N_USER_FOLDER = testUserFolder;
+
+afterAll(() => {
+	if (originalUserFolder === undefined) {
+		delete process.env.N8N_USER_FOLDER;
+	} else {
+		process.env.N8N_USER_FOLDER = originalUserFolder;
+	}
+
+	rmSync(testUserFolder, { recursive: true, force: true });
+});
+
+// Best-effort cleanup for runs interrupted before afterAll executes (SIGINT/worker kill).
+process.on('exit', () => rmSync(testUserFolder, { recursive: true, force: true }));