https://github.com/vercel/next.js/pull/83961 lets you run both `next dev` and `next build` at the same time, however it's still problematic to run two instances of `next dev` or two instances of `next build` at the same time with the same `distDir`.
This sort of thing can happen to anyone by accident, but we've seen reports of this sort of behavior with AI agents.
https://github.com/vercel/next.js/pull/84378 adds a lockfile around just the persistent cache database in Turbopack, which helps, but it's not the only reason this is problematic, so we also need a lock around all of Next.js itself.
I was not able to find a cross-platform lockfile implementation in node that I felt was of sufficient quality, so on POSIX platforms this uses the recently-stabilized Rust stdlib implementation, which appears to be derived from rustc's own lockfile implementation, which has seen widespread use/deployment.
On Windows, since we'd rather have an advisory lock than a mandatory one, this emulates that by instead opening a file with write permissions, and a sharing mode that prohibits shared write access. That means that other processes can safely read the (empty) lockfile without blowing up.
<img width="500" src="https://github.com/user-attachments/assets/1ee4a6c6-2280-4f64-8bf2-ec5369c26db1" />
<img width="500" src="https://github.com/user-attachments/assets/2b04b566-c0b4-42ce-af34-912f3f6afa72" />
Tested on Windows as well:
<img width="1224" height="690" alt="Screenshot 2025-10-07 at 5 44 33 PM" src="https://github.com/user-attachments/assets/e36085b6-9c32-40ac-aca6-4c83e2a53842" />
Jimmy Lai
