vim-patch:8.2.0908: crash when changing the function table while listing it

Problem:    Crash when changing the function table while listing it.
Solution:   Bail out when the function table changes. (closes vim/vim#6209)

3fffa97159

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

src/nvim/eval/userfunc.c

@@ -1877,6 +1877,36 @@ theend:
 
 #define MAX_FUNC_NESTING 50
 
+/// List functions.
+///
+/// @param regmatch  When NULL, all of them.
+///                  Otherwise functions matching "regmatch".
+static void list_functions(regmatch_T *regmatch)
+{
+  const size_t used = func_hashtab.ht_used;
+  size_t todo = used;
+  const hashitem_T *const ht_array = func_hashtab.ht_array;
+
+  for (const hashitem_T *hi = ht_array; todo > 0 && !got_int; hi++) {
+    if (!HASHITEM_EMPTY(hi)) {
+      ufunc_T *fp = HI2UF(hi);
+      todo--;
+      if ((fp->uf_flags & FC_DEAD) == 0
+          && (regmatch == NULL
+              ? (!message_filtered((char *)fp->uf_name)
+                 && !func_name_refcount(fp->uf_name))
+              : (!isdigit(*fp->uf_name)
+                 && vim_regexec(regmatch, (char *)fp->uf_name, 0)))) {
+        list_func_head(fp, false, false);
+        if (used != func_hashtab.ht_used || ht_array != func_hashtab.ht_array) {
+          emsg(_("E454: function list was modified"));
+          return;
+        }
+      }
+    }
+  }
+}
+
 /// ":function"
 void ex_function(exarg_T *eap)
 {
@@ -1903,7 +1933,6 @@ void ex_function(exarg_T *eap)
   static int func_nr = 0;           // number for nameless function
   int paren;
   hashtab_T *ht;
-  int todo;
   hashitem_T *hi;
   linenr_T sourcing_lnum_off;
   linenr_T sourcing_lnum_top;
@@ -1916,19 +1945,7 @@ void ex_function(exarg_T *eap)
   // ":function" without argument: list functions.
   if (ends_excmd(*eap->arg)) {
     if (!eap->skip) {
-      todo = (int)func_hashtab.ht_used;
+      list_functions(NULL);
-      for (hi = func_hashtab.ht_array; todo > 0 && !got_int; hi++) {
-        if (!HASHITEM_EMPTY(hi)) {
-          todo--;
-          fp = HI2UF(hi);
-          if (message_filtered((char *)fp->uf_name)) {
-            continue;
-          }
-          if (!func_name_refcount(fp->uf_name)) {
-            list_func_head(fp, false, false);
-          }
-        }
-      }
     }
     eap->nextcmd = check_nextcmd(eap->arg);
     return;
@@ -1946,18 +1963,7 @@ void ex_function(exarg_T *eap)
       *p = c;
       if (regmatch.regprog != NULL) {
         regmatch.rm_ic = p_ic;
-
+        list_functions(&regmatch);
-        todo = (int)func_hashtab.ht_used;
-        for (hi = func_hashtab.ht_array; todo > 0 && !got_int; hi++) {
-          if (!HASHITEM_EMPTY(hi)) {
-            todo--;
-            fp = HI2UF(hi);
-            if (!isdigit(*fp->uf_name)
-                && vim_regexec(&regmatch, (char *)fp->uf_name, 0)) {
-              list_func_head(fp, false, false);
-            }
-          }
-        }
         vim_regfree(regmatch.regprog);
       }
     }

src/nvim/testdir/test_timers.vim

@@ -3,6 +3,7 @@
 source check.vim
 CheckFeature timers
 
+source screendump.vim
 source shared.vim
 source term_util.vim
 source load.vim
@@ -408,6 +409,30 @@ func Test_timer_invalid_callback()
   call assert_fails('call timer_start(0, "0")', 'E921')
 endfunc
 
+func Test_timer_changing_function_list()
+  CheckRunVimInTerminal
+
+  " Create a large number of functions.  Should get the "more" prompt.
+  " The typing "G" triggers the timer, which changes the function table.
+  let lines =<< trim END
+    for func in map(range(1,99), "'Func' .. v:val")
+      exe "func " .. func .. "()"
+      endfunc
+    endfor
+    au CmdlineLeave : call timer_start(0, {-> 0})
+  END
+  call writefile(lines, 'XTest_timerchange')
+  let buf = RunVimInTerminal('-S XTest_timerchange', #{rows: 10})
+  call term_sendkeys(buf, ":fu\<CR>")
+  call WaitForAssert({-> assert_match('-- More --', term_getline(buf, 10))})
+  call term_sendkeys(buf, "G")
+  call WaitForAssert({-> assert_match('E454', term_getline(buf, 9))})
+  call term_sendkeys(buf, "\<Esc>")
+
+  call StopVimInTerminal(buf)
+  call delete('XTest_timerchange')
+endfunc
+
 func Test_timer_using_win_execute_undo_sync()
   let bufnr1 = bufnr()
   new