eval: Make sure that v:_null_dict does not crash dictwatcher*()

Ref #4615

2 files changed, 27 insertions, 0 deletions

diff --git a/src/nvim/eval/typval.c b/src/nvim/eval/typval.c
index 243e738c50..8c11f4bd8f 100644
--- a/src/nvim/eval/typval.c
+++ b/src/nvim/eval/typval.c
@@ -808,6 +808,9 @@ void tv_dict_watcher_add(dict_T *const dict, const char *const key_pattern,
                          const size_t key_pattern_len, Callback callback)
   FUNC_ATTR_NONNULL_ARG(2)
 {
+  if (dict == NULL) {
+    return;
+  }
   DictWatcher *const watcher = xmalloc(sizeof(DictWatcher));
   watcher->key_pattern = xmemdupz(key_pattern, key_pattern_len);
   watcher->key_pattern_len = key_pattern_len;
diff --git a/test/functional/ex_cmds/dict_notifications_spec.lua b/test/functional/ex_cmds/dict_notifications_spec.lua
index 8cc717483e..e3b4a1c504 100644
--- a/test/functional/ex_cmds/dict_notifications_spec.lua
+++ b/test/functional/ex_cmds/dict_notifications_spec.lua
@@ -244,6 +244,16 @@ describe('dictionary change notifications', function()
     end)
   end)
 
+  it('errors out when adding to v:_null_dict', function()
+    command([[
+    function! g:Watcher1(dict, key, value)
+      call rpcnotify(g:channel, '1', a:key, a:value)
+    endfunction
+    ]])
+    eq('Vim(call):E46: Cannot change read-only variable "dictwatcheradd() argument"',
+       exc_exec('call dictwatcheradd(v:_null_dict, "x", "g:Watcher1")'))
+  end)
+
   describe('errors', function()
     before_each(function()
       source([[
@@ -272,6 +282,20 @@ describe('dictionary change notifications', function()
       command('call dictwatcherdel(g:, "key", "g:InvalidCb")')
     end)
 
+    it('fails to remove watcher from v:_null_dict', function()
+      eq("Vim(call):Couldn't find a watcher matching key and callback",
+         exc_exec('call dictwatcherdel(v:_null_dict, "x", "g:Watcher2")'))
+    end)
+
+    --[[
+       [ it("fails to add/remove if the callback doesn't exist", function()
+       [   eq("Vim(call):Function g:InvalidCb doesn't exist",
+       [     exc_exec('call dictwatcheradd(g:, "key", "g:InvalidCb")'))
+       [   eq("Vim(call):Function g:InvalidCb doesn't exist",
+       [     exc_exec('call dictwatcherdel(g:, "key", "g:InvalidCb")'))
+       [ end)
+       ]]
+
     it('does not fail to replace a watcher function', function()
       source([[
       let g:key = 'v2'