fix: clean up stale asset symlinks to prevent hot reload crash loop

[masenf]

fix #6159

When a Python module directory using rx.asset(shared=True) is renamed, stale symlinks in assets/external/ cause Granian to enter an infinite reload loop. Clean up broken symlinks and empty directories in assets/external/ in App.__call__ before compilation, so the cleanup runs on every hot reload, not just initial startup.

[greptile-apps]

Greptile Summary

This PR fixes a hot-reload crash loop (issue #6159) where renaming a Python module that uses rx.asset(shared=True) leaves stale symlinks in assets/external/, causing Granian's file watcher to continuously detect changes and re-trigger reloads. The fix adds a remove_stale_external_asset_symlinks() utility that cleans up broken symlinks and their empty parent directories before each compilation cycle in App.__call__.

Key changes:

• reflex/assets.py: New remove_stale_external_asset_symlinks() function walks assets/external/, unlinks broken symlinks, then removes empty directories deepest-first using a reverse-sorted rglob.
• reflex/app.py: Calls the cleanup at the top of App.__call__ (before _compile), so it runs on every hot-reload, not just initial startup.
• tests/units/assets/test_assets.py: Two new tests cover the happy path (stale symlink removed, valid symlink preserved) and the no-op path (missing external_dir).

Issues found:

• test_remove_stale_external_asset_symlinks creates and destroys assets/external/ relative to the real Path.cwd() rather than a tmp_path. The shutil.rmtree in its finally block would silently wipe real app assets if the tests are run from inside a Reflex project directory. Using monkeypatch.chdir(tmp_path) (as done in the companion test) would make it fully hermetic.
• The broken-symlink removal loop mutates the directory tree while a live rglob generator is still open; collecting candidates into a list first is a minor robustness improvement.

Confidence Score: 4/5

• Safe to merge after addressing the test isolation issue; the production logic is sound.
• The core fix in assets.py and the call-site change in app.py are correct and well-scoped. The only meaningful concern is in the test — it uses the real CWD rather than a temporary directory, which could destroy real assets/external/ content in certain environments. This does not affect production behaviour but is worth fixing before merging to avoid CI environment pollution.
• tests/units/assets/test_assets.py — the main test function needs monkeypatch.chdir(tmp_path) to be environment-safe.

Important Files Changed

Filename	Overview
reflex/assets.py	Adds remove_stale_external_asset_symlinks() that walks assets/external/, unlinks broken symlinks, and removes now-empty directories; logic is correct (including the not dirpath.is_symlink() guard), but mutating the directory tree while holding a live rglob generator is a minor robustness concern.
reflex/app.py	Calls remove_stale_external_asset_symlinks() at the start of App.__call__ (before _compile), ensuring cleanup runs on every hot-reload cycle; the placement is correct and the lazy import avoids circular-import issues.
tests/units/assets/test_assets.py	Adds two new tests for the cleanup function; the test_remove_stale_external_asset_symlinks test is missing monkeypatch.chdir(tmp_path), meaning it operates on the real CWD and shutil.rmtree in its finally block can destroy real app assets; test_remove_stale_symlinks_no_external_dir is implemented correctly.

Sequence Diagram

sequenceDiagram
    participant Granian as Granian (file watcher)
    participant App as App.__call__
    participant Cleanup as remove_stale_external_asset_symlinks
    participant FS as assets/external/ (filesystem)
    participant Compile as App._compile

    Granian->>App: hot reload triggered
    App->>Cleanup: remove_stale_external_asset_symlinks()
    Cleanup->>FS: rglob("*") — find all paths
    FS-->>Cleanup: list of paths (symlinks + dirs)
    Cleanup->>FS: unlink broken symlinks (target missing)
    Cleanup->>FS: rglob("*") — find remaining dirs
    FS-->>Cleanup: remaining directories
    Cleanup->>FS: rmdir() empty directories (deepest first)
    Cleanup-->>App: done
    App->>Compile: _compile(prerender_routes=...)
    Compile->>FS: rx.asset(shared=True) recreates symlinks
    Note over FS: No stale symlinks → no spurious<br/>file-change events → no reload loop
    Compile-->>App: compilation complete
    App-->>Granian: ASGI app returned

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_{Last reviewed commit: ff962fc}

[codspeed-hq]

Merging this PR will not alter performance

✅ 8 untouched benchmarks

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_{Comparing masenf/clean-up-old-asset-symlinks (4710809) with main (1b39a5a)}

[masenf]

@greptile-apps please re-review