fix: use dependency injection for image-downloader fs mocks

mock.module("fs/promises") on Linux Bun poisons the entire mock
tracking system — not just fs mocks, but ALL mock/spy assertions
(octokit mocks, spyOn(global, "fetch"), etc.) stop recording calls.

Replace with dependency injection: downloadCommentImages() now accepts
an optional fsDeps parameter. Tests pass mock mkdir/writeFile directly.
Production callers omit it and get real fs. No mock.module anywhere.

Fixes 18 test failures blocking CI on Forgejo Linux runners.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

prds/6-e2e-test-forgejo-triggers.md

@@ -31,6 +31,18 @@ Systematically exercise each untested scenario on the live Forgejo instance (git
   - Ensure the workflow has the correct permissions for each trigger type
   - **Validation**: Workflow YAML includes all required `on:` triggers
 
+- [x] **M1.5: Fix image-downloader test failures blocking CI**
+
+  - **Problem**: All 18 `downloadCommentImages` tests failed on Forgejo CI (Linux Bun 1.2.21) while passing locally on macOS. This blocked all PRs.
+  - **Root cause**: `mock.module("fs/promises")` on Linux Bun poisons the entire mock tracking system — not just fs mocks, but ALL mock/spy assertions in the file (octokit parameter mocks, `spyOn(global, "fetch")`, etc.) stopped recording calls. The only passing test used a **negative** assertion (`not.toHaveBeenCalledWith`) which trivially passes when tracking is broken.
+  - **History of failed attempts**:
+    1. Upstream uses `spyOn(fs, "mkdir")` — works on GitHub CI (Bun 1.2.12/ubuntu) but fails on Forgejo Linux (different `fs/promises` object references between test and source modules)
+    2. `96ec04e` tried `_fsForTest` mutable export — abandoned
+    3. `cff360d` tried dependency injection — replaced
+    4. `dab92ef` tried `mock.module("fs/promises")` — works on macOS, poisons mock tracking on Linux
+  - **Fix**: Dependency injection via optional `fsDeps` parameter on `downloadCommentImages()`. Tests pass mock `mkdir`/`writeFile` directly — no module-level mocking at all. Production callers omit the parameter and get real `fs`. Zero `mock.module` usage in the test file.
+  - **Validation**: 797/797 tests pass locally, typecheck clean. Awaiting CI confirmation.
+
 - [ ] **M2: Test PR review comment trigger**
 
   - Create a test PR on the repo
@@ -89,6 +101,7 @@ Systematically exercise each untested scenario on the live Forgejo instance (git
 | Forgejo webhook payloads differ from expected            | Log raw payloads, compare with GitHub format, adapt parsing |
 | Trigger type not supported by Forgejo Actions            | Document as known limitation, skip gracefully               |
 | Action creates unwanted branches/comments during testing | Use a dedicated test repo or clean up after each test       |
+| Bun mock.module breaks mock tracking on Linux            | Use dependency injection for fs mocks, never mock.module    |
 
 ## Success Criteria
 

src/github/utils/image-downloader.ts

@@ -2,6 +2,15 @@ import fs from "fs/promises";
 import path from "path";
 import type { Octokits } from "../api/client";
 
+/** Injectable fs operations for cross-platform test compatibility.
+ *  On Linux Bun, mock.module("fs/promises") poisons the entire mock tracking
+ *  system, and spyOn(fs, "mkdir") gives test/source different object refs.
+ *  Dependency injection sidesteps all of this. */
+export interface FsDeps {
+  mkdir: typeof fs.mkdir;
+  writeFile: typeof fs.writeFile;
+}
+
 // Build regexes at call time, reading GITHUB_SERVER_URL from process.env
 // directly. The config.ts export freezes the value at module load time,
 // which can be wrong when the Forgejo CI runner injects its own
@@ -63,11 +72,13 @@ export async function downloadCommentImages(
   owner: string,
   repo: string,
   comments: CommentWithImages[],
+  fsDeps?: FsDeps,
 ): Promise<Map<string, string>> {
+  const { mkdir, writeFile } = fsDeps ?? fs;
   const urlToPathMap = new Map<string, string>();
   const downloadsDir = "/tmp/github-images";
 
-  await fs.mkdir(downloadsDir, { recursive: true });
+  await mkdir(downloadsDir, { recursive: true });
 
   const { imageRegex, htmlImgRegex } = buildImageRegexes();
 
@@ -217,7 +228,7 @@ export async function downloadCommentImages(
           const arrayBuffer = await imageResponse.arrayBuffer();
           const buffer = Buffer.from(arrayBuffer);
 
-          await fs.writeFile(localPath, buffer);
+          await writeFile(localPath, buffer);
           console.log(`✓ Saved: ${localPath}`);
 
           urlToPathMap.set(originalUrl, localPath);

test/image-downloader.test.ts

@@ -9,22 +9,10 @@ import {
   setSystemTime,
 } from "bun:test";
 
-// Mock fs/promises at the module loader level. Bun hoists mock.module calls
-// before imports, so the source module's `import fs from "fs/promises"` gets
-// these mocks. This avoids cross-module jest.fn() tracking bugs on Linux Bun.
-const mockMkdir = mock(() => Promise.resolve(undefined as any));
-const mockWriteFile = mock(() => Promise.resolve(undefined as any));
-
-mock.module("fs/promises", () => ({
-  default: {
-    mkdir: mockMkdir,
-    writeFile: mockWriteFile,
-  },
-}));
-
 import {
   downloadCommentImages,
   type CommentWithImages,
+  type FsDeps,
 } from "../src/github/utils/image-downloader";
 import type { Octokits } from "../src/github/api/client";
 
@@ -35,6 +23,14 @@ describe("downloadCommentImages", () => {
   let fetchSpy: any;
   let savedServerUrl: string | undefined;
 
+  // DI mocks for fs — avoids mock.module which poisons mock tracking on Linux Bun
+  const mockMkdir = mock(() => Promise.resolve(undefined as any));
+  const mockWriteFile = mock(() => Promise.resolve(undefined as any));
+  const fsDeps: FsDeps = {
+    mkdir: mockMkdir as any,
+    writeFile: mockWriteFile as any,
+  };
+
   beforeEach(() => {
     // Pin GITHUB_SERVER_URL so tests pass regardless of runner environment.
     // The image-downloader reads process.env directly at call time.
@@ -88,7 +84,7 @@ describe("downloadCommentImages", () => {
     const mockOctokit = createMockOctokit();
     const comments: CommentWithImages[] = [];
 
-    await downloadCommentImages(mockOctokit, "owner", "repo", comments);
+    await downloadCommentImages(mockOctokit, "owner", "repo", comments, fsDeps);
 
     expect(mockMkdir).toHaveBeenCalledWith("/tmp/github-images", {
       recursive: true,
@@ -110,6 +106,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      fsDeps,
     );
 
     expect(result.size).toBe(0);
@@ -155,6 +152,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      fsDeps,
     );
 
     expect(mockOctokit.rest.issues.getComment).toHaveBeenCalledWith({
@@ -217,6 +215,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      fsDeps,
     );
 
     expect(mockOctokit.rest.pulls.getReviewComment).toHaveBeenCalledWith({
@@ -266,6 +265,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      fsDeps,
     );
 
     expect(mockOctokit.rest.pulls.getReview).toHaveBeenCalledWith({
@@ -315,6 +315,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      fsDeps,
     );
 
     expect(mockOctokit.rest.issues.get).toHaveBeenCalledWith({
@@ -365,6 +366,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      fsDeps,
     );
 
     expect(mockOctokit.rest.pulls.get).toHaveBeenCalledWith({
@@ -418,6 +420,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      fsDeps,
     );
 
     expect(fetchSpy).toHaveBeenCalledTimes(2);
@@ -471,6 +474,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      fsDeps,
     );
 
     expect(fetchSpy).toHaveBeenCalledTimes(1); // Only downloaded once
@@ -506,6 +510,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      fsDeps,
     );
 
     expect(result.size).toBe(0);
@@ -548,6 +553,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      fsDeps,
     );
 
     expect(result.size).toBe(0);
@@ -579,6 +585,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      fsDeps,
     );
 
     expect(result.size).toBe(0);
@@ -652,6 +659,7 @@ describe("downloadCommentImages", () => {
         "owner",
         "repo",
         comments,
+        fsDeps,
       );
       expect(result.get(url)).toBe(
         `/tmp/github-images/image-${1704067200000 + callIndex}-0${ext}`,
@@ -698,6 +706,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      fsDeps,
     );
 
     expect(fetchSpy).toHaveBeenCalledTimes(1);
@@ -745,6 +754,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      fsDeps,
     );
 
     expect(mockOctokit.rest.issues.getComment).toHaveBeenCalledWith({
@@ -811,6 +821,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      fsDeps,
     );
 
     expect(fetchSpy).toHaveBeenCalledTimes(2);
@@ -863,6 +874,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      fsDeps,
     );
 
     expect(fetchSpy).toHaveBeenCalledTimes(2);
@@ -911,6 +923,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      fsDeps,
     );
 
     expect(fetchSpy).toHaveBeenCalledTimes(1); // Only downloaded once
@@ -957,6 +970,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      fsDeps,
     );
 
     expect(fetchSpy).toHaveBeenCalledTimes(1);