Acquire rewatch build lock before initialization

[cknitt]

Motivation

build.lock is meant to prevent multiple rewatch build processes from mutating the same project build artifacts at the same time.

Before this change, build::build called initialize_build before acquiring build.lock. That was unsafe because initialization is not read-only: it reads the previous compile state and calls cleanup_previous_build, which can remove stale .ast, .iast, .cmi, .cmj, .cmt, .cmti, and generated JS artifacts under lib/bs.

If two rewatch build processes started at the same time in the same project, both could enter initialization and cleanup concurrently. Only later, when each reached incremental_build, would one process wait on build.lock. By then, both processes may already have inspected or mutated the previous build state, which can lead to racey cleanup and nondeterministic dirty-module state.

Summary

• Acquire build.lock before build::build runs initialization.
• Keep the lock held through initialization, compilation, post-build cleanup, and build.ninja writes.
• Split the incremental compile body into an unlocked helper so incremental_build keeps its existing public locking behavior while full builds can use a wider lock boundary.
• Add regression coverage proving build::build waits for an existing build.lock before initialization starts.

[cknitt]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🎉

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[pkg-pr-new]

Open in StackBlitz

rescript

npm i https://pkg.pr.new/rescript@8409

@rescript/darwin-arm64

npm i https://pkg.pr.new/@rescript/darwin-arm64@8409

@rescript/darwin-x64

npm i https://pkg.pr.new/@rescript/darwin-x64@8409

@rescript/linux-arm64

npm i https://pkg.pr.new/@rescript/linux-arm64@8409

@rescript/linux-x64

npm i https://pkg.pr.new/@rescript/linux-x64@8409

@rescript/runtime

npm i https://pkg.pr.new/@rescript/runtime@8409

@rescript/win32-x64

npm i https://pkg.pr.new/@rescript/win32-x64@8409

commit: e6d9fa2