fix(messages): capture execute("messages") with ext_messages (#34342)

Problem:  "msg_history_show" event is emitted when `msg_silent > 0`.
          E.g. when capturing its output with `execute()`, which also
          doesn't work with ext_messages.
Solution: Don't emit the "msg_history_show" event when `msg_silent > 0`.
          Call regular messaging functions when `redirecting()`, to
          execute `redir_write()` while ensuring the message itself
          is not emitted.

src/nvim/message.c

@@ -1187,10 +1187,12 @@ void ex_messages(exarg_T *eap)
   Array entries = ARRAY_DICT_INIT;
   MessageHistoryEntry *p = eap->skip ? msg_hist_temp : msg_hist_first;
   int skip = eap->addr_count ? (msg_hist_len - eap->line2) : 0;
-  while (p != NULL) {
+  for (; p != NULL; p = p->next) {
+    // Skip over count or temporary "g<" messages.
     if ((p->temp && !eap->skip) || skip-- > 0) {
-      // Skipping over count or temporary "g<" messages.
-    } else if (ui_has(kUIMessages)) {
+      continue;
+    }
+    if (ui_has(kUIMessages) && !msg_silent) {
       Array entry = ARRAY_DICT_INIT;
       ADD(entry, CSTR_TO_OBJ(p->kind));
       Array content = ARRAY_DICT_INIT;
@@ -1204,10 +1206,12 @@ void ex_messages(exarg_T *eap)
       }
       ADD(entry, ARRAY_OBJ(content));
       ADD(entries, ARRAY_OBJ(entry));
-    } else {
-      msg_multihl(p->msg, p->kind, false, false);
     }
-    p = p->next;
+    if (redirecting() || !ui_has(kUIMessages)) {
+      msg_silent += ui_has(kUIMessages);
+      msg_multihl(p->msg, p->kind, false, false);
+      msg_silent -= ui_has(kUIMessages);
+    }
   }
   if (kv_size(entries) > 0) {
     ui_call_msg_history_show(entries);

test/functional/ui/messages_spec.lua

@@ -1722,6 +1722,32 @@ stack traceback:
       },
     })
   end)
+
+  it('can capture execute("messages"))', function()
+    feed('Q')
+    screen:expect({
+      grid = [[
+        ^                         |
+        {1:~                        }|*4
+      ]],
+      messages = {
+        {
+          content = { { "E354: Invalid register name: '^@'", 9, 6 } },
+          history = true,
+          kind = 'emsg',
+        },
+      },
+    })
+    feed(':let msg = execute("messages")<CR>')
+    screen:expect({
+      grid = [[
+        ^                         |
+        {1:~                        }|*4
+      ]],
+      cmdline = { { abort = false } },
+    })
+    eq("E354: Invalid register name: '^@'", eval('msg'):gsub('\n', ''))
+  end)
 end)
 
 describe('ui/builtin messages', function()