Supports server tools

[okuramasafumi]

Recently, RuboCop, a linter for Ruby, started to offer a server mode, where RuboCop accepts requests at some port.
I tried to use it with null-ls but I just got some error message, but I wasn't surprised because it works quite differently from the original non-server behavior.

I'm not familiar with other tools but they might support server feature in some forms in the future. So, I think it's better that null-ls supports tools that work as servers. Is it possible?

[jose-elias-alvarez]

So it seems like based on the documentation, rubocop --server should be a drop-in replacement for rubocop, since it'll start the server if it's not already running. In fact, these changes make it kind of work:

diff --git a/lua/null-ls/builtins/diagnostics/rubocop.lua b/lua/null-ls/builtins/diagnostics/rubocop.lua
index c92f907..4420477 100644
--- a/lua/null-ls/builtins/diagnostics/rubocop.lua
+++ b/lua/null-ls/builtins/diagnostics/rubocop.lua
@@ -60,9 +60,9 @@ return h.make_builtin({
     filetypes = { "ruby" },
     generator_opts = {
         command = "rubocop",
-        args = { "-f", "json", "--force-exclusion", "--stdin", "$FILENAME" },
+        args = { "--server", "-f", "json", "--force-exclusion", "--stdin", "$FILENAME" },
         to_stdin = true,
-        format = "json",
+        format = "json_raw",
         check_exit_code = function(code)
             return code <= 1
         end,

At a minimum, it looks like the first run will always fail because the following line is added to the output, which makes JSON parsing fail:

RuboCop server starting on 127.0.0.1:54781.

There's some other weird behavior (error messages that I had to suppress with json_raw, diagnostics not updating until the first change to the file) but I think it should theoretically work with our current architecture if someone is willing to put in the time to test it and put together a PR.

[okuramasafumi]

Maybe RuboCop should provide --quiet option to suppress some extra lines.
There also might be some bugs in RuboCop server mode. I'll have a look.

[jose-elias-alvarez]

Yep, the ideal would be if -f json forces only valid JSON output. Let me know if there's anything we can do on our end!