bug#72088: eglot: Segregates language servers in a way that hampers polyglot language servers

[Eli Zaretskii]

> Date: Fri, 12 Jul 2024 13:33:56 +0200
> From: Niels Thykier <niels@thykier.net>
> 
> Hi
> 
> Thanks for providing LSP support in emacs. :)
> 
> 
> For reference, I used emacs 29.4 from Debian when I tested. If this 
> feature has changed in emacs 30, then I would not have seen it yet (I am 
> not tracking emacs/eglot development a lot).
> 
> 
> I am working on a language server for Debian packaging (called 
> `debputy`). The Debian packaging files consists of several different 
> file formats with cross references between the formats. As a 
> consequence, I built my language server to be a "polyglot" (multi-language).
> 
> While testing with eglot, I noticed that my language server was not 
> informed of all changes. As far as I can see, eglot spawns a language 
> server per file format (major mode) and then only informs the language 
> server of events (didOpen/didChange) for that major mode. This happens 
> even through it is exactly the same command + args for both major modes.
> 
> The next result is that my language server when used via emacs + eglot, 
> requires the user to save a file before the language server can "see" 
> the effect when you are working in a different file.
> 
> I was a bit in doubt whether the spec accounted for polyglot language 
> servers, so I asked in the LSP github for clarification. They made the 
> argument that a language server should be able to see events for other 
> file formats and suggested that I contacted you (the eglot maintainers) 
> about this 
> (https://github.com/microsoft/language-server-protocol/issues/1964). 
> They acknowledged this was implicit and then listed the two key features 
> they saw as supporting this implicit requirement (both of them boils 
> down to DocumentSelector as I read it).
> 
> I tried a static registration with a DocumentSelector on `**/debian/*` 
> (I think it was hover docs, though it might have been completion), but 
> my debugging suggested that my instances did not get didOpen/didChange 
> events "across" the file formats even with this registration.
> 
> Can we work towards a solution where I can ask eglot to provide 
> didOpen/didChange events across formats? :)

João, any comments or suggestions?