vim-patch:9.2.0996: debugger: crash when evaluating a variable in a :def function frame (#41460)

Problem:  In the debugger ">up" and ">frame" select an older function call
          frame, but get_funccal_local_ht() and the related functions check
          current_funccal while returning a dictionary of the frame that
          get_funccal() selected.  A :def function keeps its local variables
          on the vim9 stack, its funccall_T has no l: and a: dictionaries and
          is allocated cleared, so with such a frame selected the returned
          hashtab has a NULL ht_array and evaluating any variable name at the
          debug prompt crashes in hash_lookup().
Solution: Check the funccal that is actually used and return NULL when it has
          no l: variables, so that the variable is reported as undefined
          instead.

closes: vim/vim#21111

5ad47b07af

Co-authored-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/nvim/eval/userfunc.c

@@ -3944,14 +3944,29 @@ funccall_T *get_funccal(void)
   return funccal;
 }
 
+/// Get the function call environment to use for the l: and a: variables, based
+/// on the backtrace debug level.
+/// Returns NULL if there is no current funccal.
+static funccall_T *get_funccal_for_vars(void)
+{
+  funccall_T *funccal = NULL;
+
+  if (current_funccal == NULL) {
+    return NULL;
+  }
+  funccal = get_funccal();
+  if (funccal == NULL || funccal->fc_l_vars.dv_refcount == 0) {
+    return NULL;
+  }
+  return funccal;
+}
+
 /// @return  dict used for local variables in the current funccal or
 ///          NULL if there is no current funccal.
 dict_T *get_funccal_local_dict(void)
 {
-  if (current_funccal == NULL || current_funccal->fc_l_vars.dv_refcount == 0) {
-    return NULL;
-  }
-  return &get_funccal()->fc_l_vars;
+  funccall_T *funccal = get_funccal_for_vars();
+  return funccal == NULL ? NULL : &funccal->fc_l_vars;
 }
 
 /// @return  hashtable used for local variables in the current funccal or
@@ -3966,20 +3981,16 @@ hashtab_T *get_funccal_local_ht(void)
 ///           NULL if there is no current funccal.
 dictitem_T *get_funccal_local_var(void)
 {
-  if (current_funccal == NULL || current_funccal->fc_l_vars.dv_refcount == 0) {
-    return NULL;
-  }
-  return (dictitem_T *)&get_funccal()->fc_l_vars_var;
+  funccall_T *funccal = get_funccal_for_vars();
+  return funccal == NULL ? NULL : (dictitem_T *)&funccal->fc_l_vars_var;
 }
 
 /// @return  the dict used for argument in the current funccal or
 ///          NULL if there is no current funccal.
 dict_T *get_funccal_args_dict(void)
 {
-  if (current_funccal == NULL || current_funccal->fc_l_vars.dv_refcount == 0) {
-    return NULL;
-  }
-  return &get_funccal()->fc_l_avars;
+  funccall_T *funccal = get_funccal_for_vars();
+  return funccal == NULL ? NULL : &funccal->fc_l_avars;
 }
 
 /// @return  the hashtable used for argument in the current funccal or
@@ -3994,10 +4005,8 @@ hashtab_T *get_funccal_args_ht(void)
 ///          NULL if there is no current funccal.
 dictitem_T *get_funccal_args_var(void)
 {
-  if (current_funccal == NULL || current_funccal->fc_l_vars.dv_refcount == 0) {
-    return NULL;
-  }
-  return (dictitem_T *)&get_funccal()->fc_l_avars_var;
+  funccall_T *funccal = get_funccal_for_vars();
+  return funccal == NULL ? NULL : (dictitem_T *)&funccal->fc_l_avars_var;
 }
 
 /// List function variables, if there is a function.

test/old/testdir/test_debugger.vim

@@ -1107,6 +1107,51 @@ func Test_debug_def_and_legacy_function()
   call StopVimInTerminal(buf)
 endfunc
 
+" Test for using "up" in the debugger to select the frame of a :def function
+" and then evaluating a variable name
+func Test_debug_backtrace_up_def_function()
+  CheckRunVimInTerminal
+  CheckCWD
+  let file =<< trim END
+    vim9script
+    def g:DefFunc()
+      var defvar = 'in def'
+      g:LegacyFunc('arg')
+    enddef
+
+    func g:LegacyFunc(text)
+      let legacyvar = 'in legacy'
+      echo legacyvar
+    endfunc
+
+    breakadd func 2 g:LegacyFunc
+  END
+  call writefile(file, 'XtestDebugUp.vim', 'D')
+
+  let buf = RunVimInTerminal('-S XtestDebugUp.vim', {})
+
+  call s:RunDbgCmd(buf, ':call DefFunc()', ['line 2: echo legacyvar'])
+  call s:RunDbgCmd(buf, 'echo legacyvar', ['in legacy'])
+  call s:RunDbgCmd(buf, 'echo l:legacyvar', ['in legacy'])
+  call s:RunDbgCmd(buf, 'echo a:text', ['arg'])
+
+  " Move up to the frame of the :def function, the local variables of the
+  " legacy function are not available there.  This used to crash.
+  call s:RunDbgCmd(buf, 'up')
+  call s:RunDbgCmd(buf, 'echo legacyvar',
+        \ ['E121: Undefined variable: legacyvar'])
+  call s:RunDbgCmd(buf, 'echo l:legacyvar',
+        \ ['E121: Undefined variable: l:legacyvar'])
+  call s:RunDbgCmd(buf, 'echo a:text', ['E121: Undefined variable: a:text'])
+
+  " Back in the frame of the legacy function the variables are found again.
+  call s:RunDbgCmd(buf, 'down')
+  call s:RunDbgCmd(buf, 'echo legacyvar', ['in legacy'])
+
+  call s:RunDbgCmd(buf, 'cont')
+  call StopVimInTerminal(buf)
+endfunc
+
 func Test_debug_def_function()
   CheckRunVimInTerminal
   CheckCWD