feat(lsp): run handler in coroutine to support async response (#21026)

To illustrate a use-case this also changes `window/showMessageRequest`
to use `vim.ui.select`

runtime/lua/vim/lsp/handlers.lua

@@ -81,17 +81,32 @@ M['window/workDoneProgress/create'] = function(_, result, ctx)
 end
 
 --see: https://microsoft.github.io/language-server-protocol/specifications/specification-current/#window_showMessageRequest
+---@param result lsp.ShowMessageRequestParams
 M['window/showMessageRequest'] = function(_, result)
-  local actions = result.actions
+  local actions = result.actions or {}
-  print(result.message)
+  local co, is_main = coroutine.running()
+  if co and not is_main then
+    local opts = {
+      prompt = result.message .. ': ',
+      format_item = function(action)
+        return (action.title:gsub('\r\n', '\\r\\n')):gsub('\n', '\\n')
+      end,
+    }
+    vim.ui.select(actions, opts, function(choice)
+      -- schedule to ensure resume doesn't happen _before_ yield with
+      -- default synchronous vim.ui.select
+      vim.schedule(function()
+        coroutine.resume(co, choice or vim.NIL)
+      end)
+    end)
+    return coroutine.yield()
+  else
     local option_strings = { result.message, '\nRequest Actions:' }
     for i, action in ipairs(actions) do
       local title = action.title:gsub('\r\n', '\\r\\n')
       title = title:gsub('\n', '\\n')
       table.insert(option_strings, string.format('%d. %s', i, title))
     end
-
-  -- window/showMessageRequest can return either MessageActionItem[] or null.
     local choice = vim.fn.inputlist(option_strings)
     if choice < 1 or choice > #actions then
       return vim.NIL
@@ -99,6 +114,7 @@ M['window/showMessageRequest'] = function(_, result)
       return actions[choice]
     end
   end
+end
 
 --see: https://microsoft.github.io/language-server-protocol/specifications/specification-current/#client_registerCapability
 M['client/registerCapability'] = function(_, _, ctx)

runtime/lua/vim/lsp/protocol.lua

@@ -20,6 +20,14 @@ function transform_schema_to_table()
 end
 --]=]
 
+---@class lsp.ShowMessageRequestParams
+---@field type lsp.MessageType
+---@field message string
+---@field actions nil|lsp.MessageActionItem[]
+
+---@class lsp.MessageActionItem
+---@field title string
+
 local constants = {
   DiagnosticSeverity = {
     -- Reports an error.
@@ -39,6 +47,7 @@ local constants = {
     Deprecated = 2,
   },
 
+  ---@enum lsp.MessageType
   MessageType = {
     -- An error message.
     Error = 1,

runtime/lua/vim/lsp/rpc.lua

@@ -391,6 +391,7 @@ function Client:handle_body(body)
     -- Schedule here so that the users functions don't trigger an error and
     -- we can still use the result.
     schedule(function()
+      coroutine.wrap(function()
         local status, result
         status, result, err = self:try_call(
           client_errors.SERVER_REQUEST_HANDLER_ERROR,
@@ -429,6 +430,7 @@ function Client:handle_body(body)
           result = nil
         end
         self:send_response(decoded.id, err, result)
+      end)()
     end)
     -- This works because we are expecting vim.NIL here
   elseif decoded.id and (decoded.result ~= vim.NIL or decoded.error ~= vim.NIL) then