Env variable to control which server to run

To use, either patch Kakoune using
mawww/kakoune#5063 and set preferred servers
for each buffer like

	set-option -add buffer env KAK_LSP_LANGUAGE_SERVER=pylsp_main # or pylsp_test

Alternatively, use (untested)

	unset-option buffer lsp_cmd
	set-option buffer lsp_cmd "KAK_LSP_LANGUAGE_SERVER=pylsp_main %opt{lsp_cmd}"

kak-lsp.toml

@@ -271,12 +271,22 @@ roots = ["spago.dhall", "spago.yaml", "package.json", ".git", ".hg"]
 command = "purescript-language-server"
 args = ["--stdio"]
 
-[language_server.pylsp]
+[language_server.pylsp_main]
 filetypes = ["python"]
 roots = ["requirements.txt", "setup.py", ".git", ".hg"]
 command = "pylsp"
 settings_section = "_"
-[language_server.pylsp.settings._]
+[language_server.pylsp_main.settings._]
+# See https://github.com/python-lsp/python-lsp-server#configuration
+# pylsp.configurationSources = ["flake8"]
+pylsp.plugins.jedi_completion.include_params = true
+
+[language_server.pylsp_test]
+filetypes = ["python"]
+roots = ["requirements.txt", "setup.py", ".git", ".hg"]
+command = "pylsp" # TODO different path
+settings_section = "_"
+[language_server.pylsp_test.settings._]
 # See https://github.com/python-lsp/python-lsp-server#configuration
 # pylsp.configurationSources = ["flake8"]
 pylsp.plugins.jedi_completion.include_params = true

src/session.rs

@@ -13,6 +13,7 @@ use lsp_types::request::Request;
 use lsp_types::*;
 use regex::Regex;
 use std::collections::HashMap;
+use std::env;
 use std::path::PathBuf;
 use std::time::Duration;
 
@@ -107,9 +108,22 @@ pub fn start(
                     continue 'event_loop;
                 }
 
+
+            let maybe_server_filter = env::var("KAK_LSP_LANGUAGE_SERVER").ok();
+
                 let (language_id, servers) = cfg.unwrap();
                 let routes: Vec<_> = servers
                     .iter()
+                    .filter(
+                        |&server_name| {
+                            let Some(allowed) = maybe_server_filter.as_ref() else {return true; };
+                            if server_name != allowed {
+                                debug!("Not starting server {server_name} due to KAK_LSP_LANGUAGE_SERVER={allowed}");
+                                return false;
+                            }
+                            true
+                        }
+                    )
                     .map(|server_name| {
                         let language  = &languages[server_name];
                         let root = find_project_root(language_id, &language.roots, &request.meta.buffile);