vim-patch:9.1.1610: completion: hang or E684 when 'tagfunc' calls complete() (#35243)

Problem:  completion: hang (after 9.1.1471) or E684 (after 9.1.1410)
          when 'tagfunc' calls complete().
Solution: Check if complete() has been called immediately after getting
          matches instead of in the next loop iteration (zeertzjq).

related: vim/vim#1668
related: neovim/neovim#34416
related: neovim/neovim#35163
closes: vim/vim#17929

https://github.com/vim/vim/commit/982cda69764ac0575125bc8786177e48f5fd9420

src/nvim/insexpand.c

@@ -4608,12 +4608,6 @@ static int ins_compl_get_exp(pos_T *ini)
       }
     }
 
-    // If complete() was called then compl_pattern has been reset.
-    // The following won't work then, bail out.
-    if (compl_pattern.data == NULL) {
-      break;
-    }
-
     if (compl_autocomplete && type == CTRL_X_FUNCTION) {
       // LSP servers may sporadically take >1s to respond (e.g., while
       // loading modules), but other sources might already have matches.
@@ -4627,6 +4621,12 @@ static int ins_compl_get_exp(pos_T *ini)
     // get the next set of completion matches
     found_new_match = get_next_completion_match(type, &st, &start_pos);
 
+    // If complete() was called then compl_pattern has been reset.
+    // The following won't work then, bail out.
+    if (compl_pattern.data == NULL) {
+      break;
+    }
+
     if (may_advance_cpt_idx) {
       if (!advance_cpt_sources_index_safe()) {
         break;

test/old/testdir/test_ins_complete.vim

@@ -3290,6 +3290,35 @@ func Test_tagfunc_wipes_out_buffer()
   bwipe!
 endfunc
 
+func s:TagfuncComplete(t,f,o)
+  call complete(1, ['ddd', 'eee', 'fff'])
+  return []
+endfunc
+
+" 'tagfunc' calling complete() should not cause hang or E684.
+func Test_tagfunc_calls_complete()
+  new
+  call setline(1, ['aaa', 'bbb', 'ccc'])
+  setlocal tagfunc=s:TagfuncComplete
+  setlocal completeopt=menu,noselect
+
+  let v:errmsg = ''
+
+  " This used to hang.
+  setlocal complete=.,t
+  call feedkeys("Go\<C-N>\<C-E>\<Esc>", 'tx')
+  call assert_equal('', getline('.'))
+  call assert_equal('', v:errmsg)
+
+  " This used to cause E684.
+  setlocal complete=t,.
+  call feedkeys("cc\<C-N>\<C-E>\<Esc>", 'tx')
+  call assert_equal('', getline('.'))
+  call assert_equal('', v:errmsg)
+
+  bwipe!
+endfunc
+
 func Test_ins_complete_popup_position()
   CheckScreendump