LSP-support with eglot in nested projects

After switching from lsp-mode to eglot recently, I ran into a weird issue today: When opening a work-project eglot+gopls basically bailed on me. All I got was this error message:

Your workspace root is /path/monorepo, but your module root is /path/monorepo/project.

This message was emitted by gopls which is reacting to an initialize call with a root path that doesn’t match the information it can find inside the local go.mod file.

OK, so where is that “root path” coming from? Internally, eglot uses the function project-current which is part of the project.el library. Sadly, this function, out of the box, seems to only support detecting project root folders by looking at various VCS metadata. In my case though, /path/monorepo is the one that contains the .git folder while /path/monorepo/project doesn’t.

As a quick fix I thought, I could just add a .projectile file (since I’m using projectile anyway) to the project directory and let (project-current) detect that. project.el offers an extension point for detecting project folders with the project-find-functions hook. Luckily, Louis Roché seems to have faced a similar issue and provided that little snippet in his emacs.d repository:

(defun my-projectile-project-find-function (dir)
  (let ((root (projectile-project-root dir)))
    (and root (cons 'transient root))))

(projectile-mode t)

(with-eval-after-load 'project
  (add-to-list 'project-find-functions 'my-projectile-project-find-function))

If I now run (project-current) in a file inside /path/monorepo/project, I get the correct root path and gopls is finally happy! Problem solved!