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

[Niels Thykier]

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? :)

Best regards,
Niels