fix(lsp): detect if Client:request resolved synchronously #33624

Problem:
In cases when the (in-process) LSP server responds to the request
immediately and calls `notify_reply_callback` the request will still be
marked as pending, because the code assumes that the response will occur
asynchronously. Then the request will be pending forever, because it was
already set as "completed" before we even set it as "pending".

A workaround is to wrap `notify_replay_callback` in `vim.shedule` ([like
so](https://github.com/neovim/neovim/pull/24338#issuecomment-2809568617)]
but that seems counterintuitive.

Solution:
Handle this case in Client:request().

(cherry picked from commit 8315697449)

runtime/lua/vim/lsp/client.lua

@@ -678,6 +678,12 @@ function Client:request(method, params, handler, bufnr)
   bufnr = vim._resolve_bufnr(bufnr)
   local version = lsp.util.buf_versions[bufnr]
   log.debug(self._log_prefix, 'client.request', self.id, method, params, handler, bufnr)
+
+  -- Detect if request resolved synchronously (only possible with in-process servers).
+  local already_responded = false
+  local request_registered = false
+
+  -- NOTE: rpc.request might call an in-process (Lua) server, thus may be synchronous.
   local success, request_id = self.rpc.request(method, params, function(err, result)
     handler(err, result, {
       method = method,
@@ -688,11 +694,15 @@ function Client:request(method, params, handler, bufnr)
     })
   end, function(request_id)
     -- Called when the server sends a response to the request (including cancelled acknowledgment).
-    self:_process_request(request_id, 'complete')
+    if request_registered then
+      self:_process_request(request_id, 'complete')
+    end
+    already_responded = true
   end)
 
-  if success and request_id then
+  if success and request_id and not already_responded then
     self:_process_request(request_id, 'pending', bufnr, method)
+    request_registered = true
   end
 
   return success, request_id

test/functional/plugin/lsp_spec.lua

@@ -1252,6 +1252,67 @@ describe('LSP', function()
       }
     end)
 
+    it('request should not be pending for sync responses (in-process LS)', function()
+      clear()
+
+      --- @type boolean
+      local pending_request = exec_lua(function()
+        local function server(dispatchers)
+          local closing = false
+          local srv = {}
+          local request_id = 0
+
+          function srv.request(method, _params, callback, notify_reply_callback)
+            if method == 'textDocument/formatting' then
+              callback(nil, {})
+            elseif method == 'initialize' then
+              callback(nil, {
+                capabilities = {
+                  textDocument = {
+                    formatting = true,
+                  },
+                },
+              })
+            elseif method == 'shutdown' then
+              callback(nil, nil)
+            end
+            request_id = request_id + 1
+            if notify_reply_callback then
+              notify_reply_callback(request_id)
+            end
+            return true, request_id
+          end
+
+          function srv.notify(method)
+            if method == 'exit' then
+              dispatchers.on_exit(0, 15)
+            end
+          end
+          function srv.is_closing()
+            return closing
+          end
+          function srv.terminate()
+            closing = true
+          end
+
+          return srv
+        end
+
+        local client_id = assert(vim.lsp.start({ cmd = server }))
+        local client = assert(vim.lsp.get_client_by_id(client_id))
+
+        local ok, request_id = client:request('textDocument/formatting', {})
+        assert(ok)
+
+        local has_pending = client.requests[request_id] ~= nil
+        vim.lsp.stop_client(client_id)
+
+        return has_pending
+      end)
+
+      eq(false, pending_request, 'expected no pending requests')
+    end)
+
     it('should trigger LspRequest autocmd when requests table changes', function()
       local expected_handlers = {
         { NIL, {}, { method = 'finish', client_id = 1 } },