vim-patch:8.2.4206: condition with many "(" causes a crash

Problem:    Condition with many "(" causes a crash.
Solution:   Limit recursion to 1000.

fe6fb267e6

Co-authored-by: Bram Moolenaar <Bram@vim.org>

src/nvim/eval.c

@@ -65,6 +65,7 @@ static char *e_nowhitespace
   = N_("E274: No white space allowed before parenthesis");
 static char *e_write2 = N_("E80: Error while writing: %s");
 static char *e_string_list_or_blob_required = N_("E1098: String, List or Blob required");
+static char e_expression_too_recursive_str[] = N_("E1169: Expression too recursive: %s");
 
 static char * const namespace_char = "abglstvw";
 
@@ -2911,6 +2912,7 @@ static int eval7(char **arg, typval_T *rettv, int evaluate, int want_string)
   const char *start_leader, *end_leader;
   int ret = OK;
   char *alias;
+  static int recurse = 0;
 
   // Initialise variable so that tv_clear() can't mistake this for a
   // string and free a string that isn't there.
@@ -2923,6 +2925,14 @@ static int eval7(char **arg, typval_T *rettv, int evaluate, int want_string)
   }
   end_leader = *arg;
 
+  // Limit recursion to 1000 levels.  At least at 10000 we run out of stack
+  // and crash.
+  if (recurse == 1000) {
+    semsg(_(e_expression_too_recursive_str), *arg);
+    return FAIL;
+  }
+  recurse++;
+
   switch (**arg) {
   // Number constant.
   case '0':
@@ -3127,6 +3137,8 @@ static int eval7(char **arg, typval_T *rettv, int evaluate, int want_string)
   if (ret == OK && evaluate && end_leader > start_leader) {
     ret = eval7_leader(rettv, (char *)start_leader, &end_leader);
   }
+
+  recurse--;
   return ret;
 }
 

src/nvim/testdir/test_eval_stuff.vim

@@ -367,6 +367,11 @@ func Test_curly_assignment()
   unlet g:gvar
 endfunc
 
+func Test_deep_recursion()
+  " this was running out of stack
+  call assert_fails("exe 'if ' .. repeat('(', 1002)", 'E1169: Expression too recursive: ((')
+endfunc
+
 " K_SPECIAL in the modified character used be escaped, which causes
 " double-escaping with feedkeys() or as the return value of an <expr> mapping,
 " and doesn't match what getchar() returns,