Windows perf: WinPython locator scans C:\, D:\, E:\ drive roots on every refresh

[eleanorjboyd]

Summary

The WinPython locator scans the root of C:\, D:\, and E:\ (plus ~/Downloads, ~/Desktop, ~/Documents, Program Files, Program Files (x86), etc.) on every refresh RPC. There is no cache, no opt-in, and no equivalent locator on macOS/Linux. This is the most plausible single cause of the Windows-vs-macOS p90 gap (Windows p90 ~11.3s on the slow cohort vs ~0.8s on macOS).

Full investigation: docs/windows-perf-investigation.md (Issue 1).

Where

• crates/pet-winpython/src/lib.rs — get_winpython_search_paths (~L304–332) and find (~L246–302)

What it does today

for drive in ['C', 'D', 'E'] {
    let drive_path = PathBuf::from(format!("{}:\\", drive));
    paths.push(drive_path.clone());          // root of the drive
    paths.push(drive_path.join("WinPython"));
    paths.push(drive_path.join("Python"));
}

For each existing search path, find() calls fs::read_dir(...) then path.is_dir() on every entry, then a second fs::read_dir inside matching directories.

Why this hurts on Windows

• Drive roots commonly contain hundreds of unrelated folders. Every is_dir() is a stat syscall that triggers Defender real-time scanning.
• ~/Downloads is a notorious slow path: large and full of zip/installer artifacts that Defender re-scans aggressively.
• find() has no cache (unlike WindowsStore and WindowsRegistry), so this work runs on every refresh RPC.
• WinPython is a niche portable distribution — the cost-benefit of scanning every drive root on every machine is poor.
• There is no equivalent locator on macOS/Linux. This asymmetry alone could account for a significant chunk of the 5× p90 gap.

Proposed fix

1. Drop drive-root and Program Files entries from get_winpython_search_paths(). Keep only ~/WinPython and an opt-in env var (e.g. WINPYTHON_HOME).
2. Add a per-process cache to WinPython::find() mirroring WindowsStore / WindowsRegistry.
3. Follow-up: make WinPython lazy — only run find() when the user opts in or when a workspace path resolves into a WinPython tree.

Estimated change size: ~10 lines for (1), ~30 lines for (2).

Validation

The extension can confirm the impact today by attaching RefreshPerformance.locators["WinPython"] (already emitted via the RefreshPerformance telemetry notification) to its pet.refresh event and slicing the slow Windows cohort. A long tail concentrated on WinPython would confirm the diagnosis.