Debugging a crashing language server - visual-studio-code

Question

I apologize if I'm a bit low on details here, but the main issue is actually trying to find the problem with my code. I'm updating an older extension of my own that was based on the Language Server example (https://code.visualstudio.com/docs/extensions/example-language-server). I've run into an issue where when I run the client part of my code using F5, and the debug window fires, I get:
The CSSLint Language Client server crashed 5 times in the last 3 minutes. The server will not be restarted.
Ok... so... here's the thing. The problems view in my extension client code shows nothing. DevTools for that Code window shows nothing.
The problems view for my server code shows nothing. DevTools, ditto.
For the Extension Developer Host instance, DevTools does show this:
messageService.ts:126 The CSSLint Language Client server crashed 5 times in the last 3 minutes. The server will not be restarted.e.doShow # messageService.ts:126
But I can't dig into the details to find a bug. So the question is - assuming that my server code is failing, where exactly would the errors be available?

Answer 1

Here is what I usually do to track server crashes down (I assume your server is written in JavaScript / TypeScript).
Use the following server options:
let serverModule = "path to your server"
let debugOptions = { execArgv: ["--nolazy", "--debug=6009"] };
let serverOptions = {
run: { module: serverModule, transport: TransportKind.ipc },
debug: { module: serverModule, transport: TransportKind.ipc, options: debugOptions}
};
Key here is to use the TransportKind.ipc. Errors that happen in the server and printed to stdio will then show in the output channel associated to your server (the name of the output channel is the name passed to the LanguageClient)
If you want to debug the server startup / initialize sequence you can change the debugOptions to:
let debugOptions = { execArgv: ["--nolazy", "--debug-brk=6009"] };
If the extension is started in debug mode (e.g. for example launched from VS Code using F5) then the LanguageClient automatically starts the server in debug mode. If the extension is started normally (for example as a real extension in VS Code) then the server is started normally as well.
To make this all work you need a latest version of the LSP node npm module both for server can client (e.g. 2.6.x)