This isn't something user facing, rather a matter of making it easy to use merlin correctly in ocamllsp. You might be aware that ocamllsp runs the merlin pipeline in a dedicated thread to make sure the server stays responsive while merlin is busy. That works well until preprocessing is involved. chdir is not threadsafe and therefore merlin's use of it for running pp/ppx breaks ocamllsp in some subtle (and painfully hard to debug) ways. Thus my humble request from the merlin team is to accept a patch that removes the use of chdir.

I haven't yet written the patch, but I'm thinking that it can be done by using the spawn library. This library is successfully used by dune and ocamllsp itself so it is well tested. It's also light on the dependencies.

The only problem I could imagine is that merlin has some code for launching processes on Windows. So perhaps we can keep the current code for windows and add a cwd argument? Or improve spawn in a way that makes this custom code unnecessary? cc @MisterDA