fix(ui): don't crash if maximum UI count reached (#37636)

src/nvim/api/ui.c

@@ -162,10 +162,13 @@ void nvim_ui_attach(uint64_t channel_id, Integer width, Integer height, Dict opt
                   "UI already attached to channel: %" PRId64, channel_id);
     return;
   }
+  if (!ui_can_attach_more()) {
+    api_set_error(err, kErrorTypeException, "Maximum UI count reached");
+    return;
+  }
 
   if (width <= 0 || height <= 0) {
-    api_set_error(err, kErrorTypeValidation,
+    api_set_error(err, kErrorTypeValidation, "Expected width > 0 and height > 0");
-                  "Expected width > 0 and height > 0");
     return;
   }
   RemoteUI *ui = xcalloc(1, sizeof(RemoteUI));

src/nvim/ui.c

@@ -371,9 +371,14 @@ void do_autocmd_uienter_all(void)
   }
 }
 
+bool ui_can_attach_more(void)
+{
+  return ui_count < MAX_UI_COUNT;
+}
+
 void ui_attach_impl(RemoteUI *ui, uint64_t chanid)
 {
-  if (ui_count == MAX_UI_COUNT) {
+  if (ui_count >= MAX_UI_COUNT) {
     abort();
   }
   if (!ui->ui_ext[kUIMultigrid] && !ui->ui_ext[kUIFloatDebug]
@@ -419,7 +424,7 @@ void ui_detach_impl(RemoteUI *ui, uint64_t chanid)
     }
   }
 
-  if (shift_index == MAX_UI_COUNT) {
+  if (shift_index >= MAX_UI_COUNT) {
     abort();
   }
 

test/functional/api/ui_spec.lua

@@ -71,6 +71,23 @@ describe('nvim_ui_attach()', function()
       pcall_err(request, 'nvim_ui_attach', 40, 10, { rgb = false })
     )
   end)
+
+  it('does not crash if maximum UI count is reached', function()
+    t.skip(t.is_os('win'), 'n.connect() hangs on Windows')
+    local server = api.nvim_get_vvar('servername')
+    local screens = {} --- @type test.functional.ui.screen[]
+    for i = 1, 16 do
+      screens[i] = Screen.new(nil, nil, nil, n.connect(server))
+    end
+    eq(
+      -- 0 is kErrorTypeException
+      { false, { 0, 'Maximum UI count reached' } },
+      { n.connect(server):request('nvim_ui_attach', 80, 24, {}) }
+    )
+    for i = 1, 16 do
+      screens[i]:detach()
+    end
+  end)
 end)
 
 describe('nvim_ui_send', function()
@@ -100,9 +117,9 @@ describe('nvim_ui_send', function()
     poke_eventloop()
 
     screen:expect([[
-        ^                                                  |
+      ^                                                  |
-        {1:~                                                 }|*8
+      {1:~                                                 }|*8
-                                                          |
+                                                        |
     ]])
 
     eq('Hello world', table.concat(read_data))
@@ -130,9 +147,9 @@ describe('nvim_ui_send', function()
     poke_eventloop()
 
     screen:expect([[
-        ^                                                  |
+      ^                                                  |
-        {1:~                                                 }|*8
+      {1:~                                                 }|*8
-                                                          |
+                                                        |
     ]])
 
     eq('', table.concat(read_data))

test/functional/ui/embed_spec.lua

@@ -299,7 +299,7 @@ end)
 
 describe('--embed --listen UI', function()
   it('waits for connection on listening address', function()
-    t.skip(t.is_os('win'))
+    t.skip(t.is_os('win'), 'n.connect() hangs on Windows')
     clear()
     local child_server = assert(n.new_pipename())
     fn.jobstart({