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

[Niels Thykier <niels@thykier.net>]

Eli Zaretskii:
>> From: João Távora <joaotavora@gmail.com>
>> Date: Sat, 13 Jul 2024 17:29:04 +0100
>> Cc: Niels Thykier <niels@thykier.net>, 72088@debbugs.gnu.org
>>
>> Major modes in eglot-server-programs don't work? What was attempted? I couldn't understand. Look at
>> existing examples, like C/C++ file types being handled by the same server, and others.
> 
> AFAIU, the OP tried to use the same LSP server for several languages,
> or something like that.

Correct, I am using the same LSP server for several languages (major-modes).

I observe that eglot spawns an instance *per* major mode even though I 
use exactly the same definition in eglot-server-programs. Additionally, 
each instance only receives "didOpen" and "didChange" events for the 
given mode they were spawned for.

 From my PoV, ideally, it would be possible to ask eglot to spawn only 
one instance across all major modes and it would receive "didOpen" and 
"didChange" events for all the modes that it was registered for. Like 
the singleton pattern from programming languages.


Alternatively, I think it would also be possible to have an instance per 
major mode as long as they receive the "didOpen" and "didChange" for all 
relevant modes (that is, eglot sends the same "didOpen"/"didChange" 
event to multiple servers). The downside of the later approach is that 
the same computations happen twice and is sent back to eglot that then 
has deal with the duplication. Notably push diagnostics would then 
happen once per server per major mode.


Note since I am the author of the language server, I am looking for a 
way for eglot manage the life-cycle of the language server. Asking my 
consumers to manually start and stop the language server would make it a 
second-class language server at best.

Best regards,
Niels