vim-patch:8.0.1115: crash when using foldtextresult() recursively (#8972)

Problem:    Crash when using foldtextresult() recursively.
Solution:   Avoid recursive calls. (Yasuhiro Matsumoto, closes vim/vim#2098)
495b7dd213

src/nvim/eval.c

@@ -8872,9 +8872,14 @@ static void f_foldtextresult(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   char_u buf[FOLD_TEXT_LEN];
   foldinfo_T foldinfo;
   int fold_count;
+  static bool entered = false;
 
   rettv->v_type = VAR_STRING;
   rettv->vval.v_string = NULL;
+  if (entered) {
+    return;  // reject recursive use
+  }
+  entered = true;
   linenr_T lnum = tv_get_lnum(argvars);
   // Treat illegal types and illegal string values for {lnum} the same.
   if (lnum < 0) {
@@ -8888,6 +8893,8 @@ static void f_foldtextresult(typval_T *argvars, typval_T *rettv, FunPtr fptr)
     }
     rettv->vval.v_string = text;
   }
+
+  entered = false;
 }
 
 /*

src/nvim/testdir/test_fold.vim

@@ -278,6 +278,7 @@ func Test_move_folds_around_manual()
   call assert_equal(0, foldlevel(6))
   call assert_equal(9, foldclosedend(7))
   call assert_equal([-1, 2, 2, 2, 2, -1, 7, 7, 7, -1], map(range(1, line('$')), 'foldclosed(v:val)'))
+
   %d
   " Ensure moving around the edges still works.
   call setline(1, PrepIndent("a") + repeat(["a"], 3) + ["\ta"])
@@ -634,3 +635,16 @@ func Test_fold_move()
   set fdm& sw& fdl&
   enew!
 endfunc
+
+func Test_foldtext_recursive()
+  new
+  call setline(1, ['{{{', 'some text', '}}}'])
+  setlocal foldenable foldmethod=marker foldtext=foldtextresult(v\:foldstart)
+  " This was crashing because of endless recursion.
+  2foldclose
+  redraw
+  call assert_equal(1, foldlevel(2))
+  call assert_equal(1, foldclosed(2))
+  call assert_equal(3, foldclosedend(2))
+  bwipe!
+endfunc