vim-patch:9.1.1447: completion: crash when backspacing with fuzzy completion

Problem:  completion: crash when backspacing with fuzzy completion
Solution: Don't dereference compl_first_match when it's NULL
          (zeertzjq).

related: neovim/neovim#34419
closes: vim/vim#17511

91782b4aeb

src/nvim/insexpand.c

@@ -1339,13 +1339,15 @@ static int cp_compare_nearest(const void *a, const void *b)
 /// Set fuzzy score.
 static void set_fuzzy_score(void)
 {
-  if (compl_leader.data != NULL && compl_leader.size > 0) {
+  if (!compl_first_match || compl_leader.data == NULL || compl_leader.size == 0) {
-    compl_T *comp = compl_first_match;
+    return;
-    do {
-      comp->cp_score = fuzzy_match_str(comp->cp_str.data, compl_leader.data);
-      comp = comp->cp_next;
-    } while (comp != NULL && !is_first_match(comp));
   }
+
+  compl_T *comp = compl_first_match;
+  do {
+    comp->cp_score = fuzzy_match_str(comp->cp_str.data, compl_leader.data);
+    comp = comp->cp_next;
+  } while (comp != NULL && !is_first_match(comp));
 }
 
 /// Sort completion matches, excluding the node that contains the leader.

test/old/testdir/test_ins_complete.vim

@@ -4835,4 +4835,29 @@ func Test_complete_unloaded_buf_refresh_always()
   delfunc TestComplete
 endfunc
 
+func Test_complete_fuzzy_omnifunc_backspace()
+  let g:do_complete = v:false
+  func Omni_test(findstart, base)
+    if a:findstart
+      let g:do_complete = !g:do_complete
+    endif
+    if g:do_complete
+      return a:findstart ? 0 : [#{word: a:base .. 'def'}, #{word: a:base .. 'ghi'}]
+    endif
+    return a:findstart ? -3 : {}
+  endfunc
+
+  new
+  redraw  " need this to prevent NULL dereference in Nvim
+  setlocal omnifunc=Omni_test
+  setlocal completeopt=menuone,fuzzy,noinsert
+  call setline(1, 'abc')
+  call feedkeys("A\<C-X>\<C-O>\<BS>\<Esc>0", 'tx!')
+  call assert_equal('ab', getline(1))
+
+  bwipe!
+  delfunc Omni_test
+  unlet g:do_complete
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab nofoldenable