message engine: only save non-empty message lists

Change-Id: I222a7f6aa81ccf40e5c7037b86c26e4c82f5155b

src/im/message_engine.cpp

@@ -305,34 +305,35 @@ MessageEngine::save_() const
                     payloads[p.first] = p.second;
                 peerRoot[to_hex_string(m.first)] = std::move(msg);
             }
+            if (peerRoot.size() == 0) continue;
             root[c.first] = std::move(peerRoot);
         }
+
         // Save asynchronously
         dht::ThreadPool::computation().run([path = savePath_,
                                             root = std::move(root),
-                                            accountID = account_.getAccountID(),
-                                            messageNum = messages_.size()] {
+                                            accountID = account_.getAccountID()] {
             std::lock_guard<std::mutex> lock(fileutils::getFileLock(path));
             try {
                 Json::StreamWriterBuilder wbuilder;
                 wbuilder["commentStyle"] = "None";
                 wbuilder["indentation"] = "";
-                const std::unique_ptr<Json::StreamWriter> writer(wbuilder.newStreamWriter());
+                std::unique_ptr<Json::StreamWriter> writer(wbuilder.newStreamWriter());
                 std::ofstream file;
                 file.exceptions(std::ifstream::failbit | std::ifstream::badbit);
                 fileutils::openStream(file, path, std::ios::trunc);
                 if (file.is_open())
                     writer->write(root, &file);
             } catch (const std::exception& e) {
-                JAMI_ERR("[Account %s] Couldn't save messages to %s: %s",
-                         accountID.c_str(),
-                         path.c_str(),
+                JAMI_ERROR("[Account {:s}] Couldn't save messages to {:s}: {:s}",
+                         accountID,
+                         path,
                          e.what());
             }
-            JAMI_DBG("[Account %s] saved %zu messages to %s",
-                     accountID.c_str(),
-                     messageNum,
-                     path.c_str());
+            JAMI_DEBUG("[Account {:s}] saved {:d} messages to {:s}",
+                     accountID,
+                     root.size(),
+                     path);
         });
     } catch (const std::exception& e) {
         JAMI_ERR("[Account %s] couldn't save messages to %s: %s",