fix(write-through): guard case-insensitive filesystem collisions before atomic write (#2831)

On macOS/Windows (case-folding filesystems), the write-through rename
silently clobbered a differently-cased file already occupying the target
path (uncontrolled repo files like README.md vs slug readme, or unicode
normalization variants between slugs). Refuse with
skipped: 'case_insensitive_collision' when the path exists on disk but no
exactly-named directory entry does; exact-case updates fall through and
case-sensitive filesystems are unaffected.

Fixes #2831

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/core/write-through.ts

@@ -21,8 +21,8 @@
  * only does "row exists + repo is a real dir → render + atomic write".
  */
 
-import { existsSync, statSync, mkdirSync, writeFileSync, renameSync, unlinkSync } from 'fs';
-import { dirname, join } from 'path';
+import { existsSync, statSync, mkdirSync, writeFileSync, renameSync, unlinkSync, readdirSync } from 'fs';
+import { basename, dirname, join } from 'path';
 import { randomBytes } from 'crypto';
 import type { BrainEngine } from './engine.ts';
 import { serializePageToMarkdown, resolvePageFilePath } from './markdown.ts';
@@ -56,8 +56,12 @@ export interface WriteThroughResult {
    *     DB write failed or targeted a different source).
    *   - path_escapes_source_root: the computed file path resolves outside the
    *     source's working tree (hostile slug row / symlinked subtree) — refused.
+   *   - case_insensitive_collision: on a case-insensitive filesystem
+   *     (macOS/Windows default), the target directory already holds a
+   *     differently-cased entry that the FS folds onto this page's file, so
+   *     writing would silently clobber the OTHER slug's file (#2831) — refused.
    */
-  skipped?: 'no_repo_configured' | 'repo_not_found' | 'source_repo_belongs_to_other_source' | 'page_not_found_after_write' | 'path_escapes_source_root';
+  skipped?: 'no_repo_configured' | 'repo_not_found' | 'source_repo_belongs_to_other_source' | 'page_not_found_after_write' | 'path_escapes_source_root' | 'case_insensitive_collision';
   /** Set when the render/write/rename itself threw (EACCES, ENOTDIR, disk full). */
   error?: string;
 }
@@ -146,6 +150,30 @@ export async function writePageThrough(
       frontmatterOverrides: opts.frontmatterOverrides,
     });
 
+    // #2831: two distinct DB slugs differing only by case (FOO vs foo) resolve
+    // to the SAME file on a case-insensitive filesystem — the second write
+    // would silently clobber the first slug's artifact. Refuse when the target
+    // dir holds a differently-cased entry that the FS folds onto our path:
+    // exact-case entry present → normal update, falls through; on a
+    // case-sensitive FS the variant path doesn't exist, so the guard is a
+    // no-op there.
+    const dir = dirname(filePath);
+    if (existsSync(dir)) {
+      const base = basename(filePath);
+      const entries = readdirSync(dir);
+      if (!entries.includes(base) && existsSync(filePath)) {
+        // The path exists on disk but no exactly-named entry does → the FS
+        // folded the name (case, or unicode normalization on APFS) onto a
+        // different slug's file.
+        const clash =
+          entries.find((e) => e.toLowerCase() === base.toLowerCase()) ?? '(normalization variant)';
+        opts.logger?.warn(
+          `[write-through] case-insensitive collision for ${slug}: '${clash}' already occupies ${filePath} — file not written (DB row is intact)`,
+        );
+        return { written: false, skipped: 'case_insensitive_collision' };
+      }
+    }
+
     mkdirSync(dirname(filePath), { recursive: true });
 
     // Atomic write: unique temp sibling + rename. Unique name (pid + random)

test/write-through.test.ts

@@ -172,6 +172,47 @@ describe('writePageThrough', () => {
     expect(walkFiles(globalDir).some((f) => f.endsWith('.md'))).toBe(false);
   });
 
+  test('[REGRESSION #2831] differently-cased entry occupying the target → skipped case_insensitive_collision, existing file untouched', async () => {
+    await engine.setConfig('sync.repo_path', brainDir);
+    const slug = 'wiki/ideas/note';
+    await seedPage(slug);
+
+    // A differently-cased file already occupies the target path's fold slot
+    // (e.g. an uncontrolled repo file, or another slug's normalization
+    // variant). On macOS/Windows the FS resolves `note.md` to it.
+    const dir = path.join(brainDir, 'wiki', 'ideas');
+    fs.mkdirSync(dir, { recursive: true });
+    fs.writeFileSync(path.join(dir, 'NOTE.md'), 'precious existing content');
+
+    // Detect whether THIS filesystem folds case (macOS/Windows: yes; Linux
+    // CI: no — there the two names are distinct files and no guard fires).
+    const caseInsensitiveFs = fs.existsSync(path.join(dir, 'note.md'));
+
+    const res = await writePageThrough(engine, slug, { sourceId: 'default' });
+
+    if (caseInsensitiveFs) {
+      expect(res).toEqual({ written: false, skipped: 'case_insensitive_collision' });
+      // The pre-existing file was NOT clobbered — the whole point of #2831.
+      expect(fs.readFileSync(path.join(dir, 'NOTE.md'), 'utf8')).toBe('precious existing content');
+    } else {
+      expect(res.written).toBe(true);
+      expect(fs.readFileSync(path.join(dir, 'NOTE.md'), 'utf8')).toBe('precious existing content');
+      expect(fs.existsSync(path.join(dir, 'note.md'))).toBe(true);
+    }
+  });
+
+  test('[#2831] exact-case rewrite of the same slug still updates (guard falls through)', async () => {
+    await engine.setConfig('sync.repo_path', brainDir);
+    const slug = 'wiki/ideas/rewrite-me';
+    await seedPage(slug);
+
+    const first = await writePageThrough(engine, slug, { sourceId: 'default' });
+    expect(first.written).toBe(true);
+    const second = await writePageThrough(engine, slug, { sourceId: 'default' });
+    expect(second.written).toBe(true);
+    expect(second.path).toBe(first.path);
+  });
+
   test('[REGRESSION] mkdir ENOTDIR (parent is a file) → error, no partial .md, no .tmp', async () => {
     await engine.setConfig('sync.repo_path', brainDir);
     // Block the `wiki/` directory by putting a FILE named "wiki" under the repo,