rails-erb-loader: Webpack dev server with rails-erb-loader and spring hangs

This was originally posted at webpacker https://github.com/rails/webpacker/issues/785.

Using rails-erb-loader@^5.2.1 and webpacker gem 3.0.1, rails 5.1.4 and spring 2.0.2.

I’ve the problem that in my current project, webpack-dev-server hangs when first started:

→   ./bin/webpack-dev-server
 11% building modules 10/10 modules 0 active
Project is running at http://localhost:3035/
webpack output is served from /packs/
Content not from webpack is served from/Users/.../public/packs
404s will fallback to /index.html
 66% building modules 468/469 modules 1 active ...xternal_application_form/index.js.erb

It sometimes hangs at some other point, eg.:

 67% building modules 479/480 modules 1 active ..._custom_question_answers_form.vue.erb

so the hang is not consistent, but it always displays some .erb at the end of output.

If I cancel the dev server and restart it, it goes through without a problem:

→   ./bin/webpack-dev-server
 11% building modules 10/10 modules 0 active                                                                                                                                                                    Project is running at http://localhost:3035/
webpack output is served from /packs/
Content not from webpack is served from /Users/.../public/packs                                                                                                                           404s will fallback to /index.html
Hash: 6b8d3991aafe09cef922                                                                                                                                                                                      Version: webpack 3.5.6
Time: 8058ms
[...]

I noticed that it happens only when spring was not started before. If spring is started already (e.g. because of a rails console), the compilation goes through on first try without the need to cancel it first.

Additionally, when I disable spring by commenting out the spring loader from bin/rails, or by using DISABLE_SPRING=1 ./bin/webpack-dev-server it always goes through without a problem, too. So I suspect it to be the combination of spring and the rails erb loader that makes it hang somehow (or is anything else calling bin/rails during compiling?)

I tried to set a timeout for rails-erb-loader by editing node_modules/rails-erb-loader/index.js, suspecting a hang while compiling the erb assets, but this did not change anything (still hangs).

How can we debug this further to see where exactly it hangs and why?