Dev server should not start when plugin config server to be middlewareMode

[axe-me]

Describe the bug

When a plugin has the dev server config middlewareMode to be true, the CLI should not try to start the dev server. In my case, I would use my own server instead of the vite built-in one.

config: () => ({
  server: {
      middlewareMode: true,
  },
}),

Reproduction

https://github.com/axe-me/dev-server-shouldnt-start

System Info

• vite version: 2.x
• Operating System: macOS
• Node version: 14
• Package manager (npm/yarn/pnpm) and version: npm 6.x

Logs (Optional if provided reproduction)

vite

error when starting dev server:
Error: Cannot call server.listen in middleware mode.

Solution

dont run await server.listen() when resolved config has middlewareMode enabled

[axe-me]

@underfin Thank you for looking at this. I created a PR as well yesterday, for some reason it did not link to this issue. #2095
Anyway the way you doing it looks better. I gonna close mine.

[yyx990803]

I don't know if this should be a supported case, since this is allowing a plugin to replace the actual Vite http server and breaks the fix in #2101 breaks the API contract where listen() should aways return the server instance. Starting a server in configureServer also feels quite wrong.

I think a better, more explicit usage is to expose a CLI binary from your plugin which starts the server and uses Vite in middleware instead. So instead of vite, the user runs vite-express instead.

[axe-me]

A CLI bin makes sense, that actually was my first attempt. The reason I switch to the plugin approach is the ViteDevServer not expose the handleHotUpdate hook which is needed for me to know when to restart the node server. #2077 (comment)

I could create a PR to add some lifecycle hooks options to the dev server if you think it's OK.

Another idea is to add a handler to dev server and invoke it in hmr.send, so that the node server would know all the events the hmr sending out. I can create a PR for this as well, see which way you prefer.

@yyx990803

[axe-me]

I just realized I could just connect to the WebSocket server created by vite. None of those proposals above aren't needed anymore.