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

@@ -81,22 +81,38 @@ 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
-  print(result.message)
-  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
+  local actions = result.actions or {}
+  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
-    return actions[choice]
+    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
+    local choice = vim.fn.inputlist(option_strings)
+    if choice < 1 or choice > #actions then
+      return vim.NIL
+    else
+      return actions[choice]
+    end
   end
 end
 

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

@@ -391,44 +391,46 @@ 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()
-      local status, result
-      status, result, err = self:try_call(
-        client_errors.SERVER_REQUEST_HANDLER_ERROR,
-        self.dispatchers.server_request,
-        decoded.method,
-        decoded.params
-      )
-      local _ = log.debug()
-        and log.debug(
-          'server_request: callback result',
-          { status = status, result = result, err = err }
+      coroutine.wrap(function()
+        local status, result
+        status, result, err = self:try_call(
+          client_errors.SERVER_REQUEST_HANDLER_ERROR,
+          self.dispatchers.server_request,
+          decoded.method,
+          decoded.params
         )
-      if status then
-        if result == nil and err == nil then
-          error(
-            string.format(
-              'method %q: either a result or an error must be sent to the server in response',
-              decoded.method
+        local _ = log.debug()
+          and log.debug(
+            'server_request: callback result',
+            { status = status, result = result, err = err }
+          )
+        if status then
+          if result == nil and err == nil then
+            error(
+              string.format(
+                'method %q: either a result or an error must be sent to the server in response',
+                decoded.method
+              )
             )
-          )
+          end
+          if err then
+            assert(
+              type(err) == 'table',
+              'err must be a table. Use rpc_response_error to help format errors.'
+            )
+            local code_name = assert(
+              protocol.ErrorCodes[err.code],
+              'Errors must use protocol.ErrorCodes. Use rpc_response_error to help format errors.'
+            )
+            err.message = err.message or code_name
+          end
+        else
+          -- On an exception, result will contain the error message.
+          err = rpc_response_error(protocol.ErrorCodes.InternalError, result)
+          result = nil
         end
-        if err then
-          assert(
-            type(err) == 'table',
-            'err must be a table. Use rpc_response_error to help format errors.'
-          )
-          local code_name = assert(
-            protocol.ErrorCodes[err.code],
-            'Errors must use protocol.ErrorCodes. Use rpc_response_error to help format errors.'
-          )
-          err.message = err.message or code_name
-        end
-      else
-        -- On an exception, result will contain the error message.
-        err = rpc_response_error(protocol.ErrorCodes.InternalError, result)
-        result = nil
-      end
-      self:send_response(decoded.id, err, result)
+        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