security: use openFileWithinRoot for A2UI file serving

[thebenignhacker]

Summary

• Replace hand-rolled path traversal defense in A2UI file-serving handler with the project's existing openFileWithinRoot primitive from fs-safe.ts
• Closes a TOCTOU vulnerability where lstat + realpath checks were performed separately from the subsequent fs.readFile
• Mirrors the established pattern already used in server.ts for canvas host file serving

Test plan

• Build passes
• Existing tests pass
• Manual: verify A2UI assets load correctly in the control UI
• Manual: verify path traversal attempts (e.g. ../../../etc/passwd) are rejected

[greptile-apps]

_{1 file reviewed, 1 comment}

_{Edit Code Review Agent Settings | Greptile}

[greptile-apps]

Additional Comments (1)

src/canvas-host/a2ui.ts
HEAD requests return body

handleA2uiHttpRequest explicitly allows HEAD, but it always reads the file and calls res.end(...) with a body (both for HTML and non-HTML). For HEAD, Node will typically drop the body bytes, but you still do the file I/O and may send an unexpected response. Consider short-circuiting on req.method === "HEAD" after setting headers (and still closing result.handle) so HEAD is actually header-only.

Prompt To Fix With AI

This is a comment left during a code review.
Path: src/canvas-host/a2ui.ts
Line: 179:223

Comment:
**HEAD requests return body**

`handleA2uiHttpRequest` explicitly allows `HEAD`, but it always reads the file and calls `res.end(...)` with a body (both for HTML and non-HTML). For `HEAD`, Node will typically drop the body bytes, but you still do the file I/O and may send an unexpected response. Consider short-circuiting on `req.method === "HEAD"` after setting headers (and still closing `result.handle`) so `HEAD` is actually header-only.

How can I resolve this? If you propose a fix, please make it concise.

[thebenignhacker]

Addressed the HEAD request feedback — pushed a fix that returns headers only (Content-Type, Cache-Control) without reading the file body for HEAD requests.

Ready for review.

[steipete]

Merged via squash.

• Prepared head SHA: 64547d6
• Merge commit: 7467fcc

Thanks @abdelsfane!