[perf] Kill tasks directly rather than through the supervisor

When lots of things happen in lexical, it's necessary to kill some
outstanding tasks that won't finish. When lots of things need to be
killed (as in when a lot of files are changed), killing them can
become a bottleneck. This is because we were using terminate_child,
which makes a genserver call through the supervisor and waits for the
pid to exit before it returns. This, in turn would cause the
supervisor to back up and the task queue to die because of timeouts.

Replacing this with `Process.exit` removes the bottleneck and doesn't
seem to have any adverse effects.

apps/server/lib/lexical/server/task_queue.ex

@@ -71,7 +71,7 @@ defmodule Lexical.Server.TaskQueue do
     defp maybe_log_task(reason, request_id),
       do: Logger.warning("Request id #{request_id} failed with reason #{inspect(reason)}")
 
-    defp as_task(request_id, {m, f, a}) do
+    defp as_task(request_id, {m, f, a} = mfa) do
       handler = fn ->
         try do
           case apply(m, f, a) do
@@ -93,7 +93,7 @@ defmodule Lexical.Server.TaskQueue do
         end
       end
 
-      run_task(handler)
+      run_task(handler, mfa)
     end
 
     defp write_reply(response) do
@@ -119,21 +119,18 @@ defmodule Lexical.Server.TaskQueue do
     end
 
     defp write_error(id, message, code \\ :internal_error) do
-      error =
-        ResponseError.new(
-          code: code,
-          message: message
-        )
+      error = ResponseError.new(code: code, message: message)
 
       Transport.write(%{id: id, error: error})
     end
 
-    defp run_task(fun) when is_function(fun) do
-      Task.Supervisor.async_nolink(task_supervisor_name(), fun)
+    defp run_task(fun, mfa) when is_function(fun) do
+      task = Task.Supervisor.async_nolink(task_supervisor_name(), fun)
+      %Task{task | mfa: mfa}
     end
 
     defp cancel_task(%Task{} = task) do
-      Task.Supervisor.terminate_child(task_supervisor_name(), task.pid)
+      Process.exit(task.pid, :canceled)
     end
   end