fix(inccommand): improve preview buffer number handling (#27087)

src/nvim/ex_getln.c

@@ -2252,6 +2252,7 @@ static buf_T *cmdpreview_open_buf(void)
 ///
 /// @return Pointer to command preview window if succeeded, NULL if failed.
 static win_T *cmdpreview_open_win(buf_T *cmdpreview_buf)
+  FUNC_ATTR_NONNULL_ALL
 {
   win_T *save_curwin = curwin;
 
@@ -2523,10 +2524,10 @@ static bool cmdpreview_may_show(CommandLineState *s)
   cmdpreview_prepare(&cpinfo);
 
   // Open preview buffer if inccommand=split.
-  if (!icm_split) {
-    cmdpreview_bufnr = 0;
-  } else if ((cmdpreview_buf = cmdpreview_open_buf()) == NULL) {
-    abort();
+  if (icm_split && (cmdpreview_buf = cmdpreview_open_buf()) == NULL) {
+    // Failed to create preview buffer, so disable preview.
+    set_string_option_direct("icm", -1, "nosplit", OPT_FREE, SID_NONE);
+    icm_split = false;
   }
   // Setup preview namespace if it's not already set.
   if (!cmdpreview_ns) {

test/functional/ui/inccommand_spec.lua

@@ -3250,3 +3250,54 @@ it("with 'inccommand' typing :filter doesn't segfault or leak memory #19057", fu
   feed('i')
   assert_alive()
 end)
+
+it("'inccommand' value can be changed multiple times #27086", function()
+  clear()
+  local screen = Screen.new(30, 20)
+  common_setup(screen, 'split', 'foo1\nfoo2\nfoo3')
+  for _ = 1, 3 do
+    feed(':%s/foo/bar')
+    screen:expect([[
+      {12:bar}1                          |
+      {12:bar}2                          |
+      {12:bar}3                          |
+      {15:~                             }|*7
+      {11:[No Name] [+]                 }|
+      |1| {12:bar}1                      |
+      |2| {12:bar}2                      |
+      |3| {12:bar}3                      |
+      {15:~                             }|*4
+      {10:[Preview]                     }|
+      :%s/foo/bar^                   |
+    ]])
+    feed('<Esc>')
+    command('set inccommand=nosplit')
+    feed(':%s/foo/bar')
+    screen:expect([[
+      {12:bar}1                          |
+      {12:bar}2                          |
+      {12:bar}3                          |
+      {15:~                             }|*16
+      :%s/foo/bar^                   |
+    ]])
+    feed('<Esc>')
+    command('set inccommand=split')
+  end
+end)
+
+it("'inccommand' disables preview if preview buffer can't be created #27086", function()
+  clear()
+  meths.buf_set_name(0, '[Preview]')
+  local screen = Screen.new(30, 20)
+  common_setup(screen, 'split', 'foo1\nfoo2\nfoo3')
+  eq('split', meths.get_option_value('inccommand', {}))
+  feed(':%s/foo/bar')
+  screen:expect([[
+    {12:bar}1                          |
+    {12:bar}2                          |
+    {12:bar}3                          |
+    {15:~                             }|*16
+    :%s/foo/bar^                   |
+  ]])
+  eq('nosplit', meths.get_option_value('inccommand', {}))
+end)

test/functional/ui/inccommand_user_spec.lua

@@ -1,6 +1,8 @@
 local helpers = require('test.functional.helpers')(after_each)
 local Screen = require('test.functional.ui.screen')
+local meths = helpers.meths
 local clear = helpers.clear
+local eq = helpers.eq
 local exec_lua = helpers.exec_lua
 local insert = helpers.insert
 local feed = helpers.feed
@@ -543,6 +545,22 @@ describe("'inccommand' for user commands", function()
       test_preview_break_undo()
     end)
   end)
+
+  it('disables preview if preview buffer cannot be created #27086', function()
+    command('set inccommand=split')
+    meths.buf_set_name(0, '[Preview]')
+    exec_lua([[
+      vim.api.nvim_create_user_command('Test', function() end, {
+        nargs = '*',
+        preview = function(_, _, _)
+          return 2
+        end
+      })
+    ]])
+    eq('split', meths.get_option_value('inccommand', {}))
+    feed(':Test')
+    eq('nosplit', meths.get_option_value('inccommand', {}))
+  end)
 end)
 
 describe("'inccommand' with multiple buffers", function()