Refactor CLI to use spawn for better signal handling in watch mode

[nojaf]

Replaced execFileSync with spawn to prevent blocking the event loop, allowing for proper signal forwarding and cleanup on termination. This change enhances the user experience during watch mode by ensuring that the Rust watcher can exit cleanly and that the parent process does not terminate prematurely. Added signal handling for SIGINT, SIGTERM, SIGHUP, and SIGQUIT to manage child process termination effectively.

This is to prevent the terminal being blocked after sending ctrl + c during watch mode.

Problem goes away after this PR:

[pkg-pr-new]

Open in StackBlitz

rescript

npm i https://pkg.pr.new/rescript-lang/rescript@7844

@rescript/darwin-arm64

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/darwin-arm64@7844

@rescript/darwin-x64

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/darwin-x64@7844

@rescript/linux-arm64

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/linux-arm64@7844

@rescript/linux-x64

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/linux-x64@7844

@rescript/runtime

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/runtime@7844

@rescript/win32-x64

npm i https://pkg.pr.new/rescript-lang/rescript/@rescript/win32-x64@7844

commit: ba50294

[nojaf]

Worked on Windows as well.

[zth]

👍 from me, great stuff! But would be good if @cknitt had a look.

[cknitt]

Thanks! I wonder if this logic shouldn't be extracted to some helper function and used in other places, too, like bsc.js or in the rescript legacy code.

Actually we already have lib_dev/process.js with similar utility functions, so it could go there.

[nojaf]

Hmm, I'm not sure really, I’d prefer keeping this in the CLI runtime rather than lib_dev/process.js (which is dev/test-only and has different behavior).

bsc.js runs a single-shot compiler invocation without a watch loop. There’s no long-lived child to gracefully tear down on Ctrl+C, so execFileSync is appropriate and simpler. The problem we fixed only shows up in rescript.js because rewatch runs indefinitely and needs orderly signal forwarding/cleanup.

The user problem is with rescript.js, fixing it there seems fine to me.