Speedup sending encrypted messages after metasync was reenabled

Calling fsync everytime we save to the db is slow, which is actually
fairly noticeable in some larger E2EE rooms. Speed that up slightly by
batching the olm session persisting.

src/Cache.cpp

@@ -912,6 +912,29 @@ Cache::getMegolmSessionData(const MegolmSessionIndex &index)
 // OLM sessions.
 //
 
+void
+Cache::saveOlmSessions(std::vector<std::pair<std::string, mtx::crypto::OlmSessionPtr>> sessions,
+                       uint64_t timestamp)
+{
+    using namespace mtx::crypto;
+
+    auto txn = lmdb::txn::begin(env_);
+    for (const auto &[curve25519, session] : sessions) {
+        auto db = getOlmSessionsDb(txn, curve25519);
+
+        const auto pickled    = pickle<SessionObject>(session.get(), pickle_secret_);
+        const auto session_id = mtx::crypto::session_id(session.get());
+
+        StoredOlmSession stored_session;
+        stored_session.pickled_session = pickled;
+        stored_session.last_message_ts = timestamp;
+
+        db.put(txn, session_id, nlohmann::json(stored_session).dump());
+    }
+
+    txn.commit();
+}
+
 void
 Cache::saveOlmSession(const std::string &curve25519,
                       mtx::crypto::OlmSessionPtr session,

src/Cache_p.h

@@ -277,6 +277,8 @@ public:
     void saveOlmSession(const std::string &curve25519,
                         mtx::crypto::OlmSessionPtr session,
                         uint64_t timestamp);
+    void saveOlmSessions(std::vector<std::pair<std::string, mtx::crypto::OlmSessionPtr>> sessions,
+                         uint64_t timestamp);
     std::vector<std::string> getOlmSessions(const std::string &curve25519);
     std::optional<mtx::crypto::OlmSessionPtr>
     getOlmSession(const std::string &curve25519, const std::string &session_id);

src/encryption/Olm.cpp

@@ -1299,6 +1299,10 @@ send_encrypted_to_device_messages(const std::map<std::string, std::vector<std::s
 
     auto our_curve = olm::client()->identity_keys().curve25519;
 
+    {
+        auto currentTime = QDateTime::currentSecsSinceEpoch();
+        std::vector<std::pair<std::string, mtx::crypto::OlmSessionPtr>> sessionsToPersist;
+
         for (const auto &[user, devices] : targets) {
             auto deviceKeys = cache::client()->userKeys(user);
 
@@ -1324,7 +1328,7 @@ send_encrypted_to_device_messages(const std::map<std::string, std::vector<std::s
                     break;
                 }
 
-            auto d = deviceKeys->device_keys.at(device);
+                const auto &d = deviceKeys->device_keys.at(device);
 
                 if (!d.keys.count("curve25519:" + device) || !d.keys.count("ed25519:" + device)) {
                     nhlog::crypto()->warn("Skipping device {} since it has no keys!", device);
@@ -1340,7 +1344,6 @@ send_encrypted_to_device_messages(const std::map<std::string, std::vector<std::s
 
                 auto session = cache::getLatestOlmSession(device_curve);
                 if (!session || force_new_session) {
-                auto currentTime = QDateTime::currentSecsSinceEpoch();
                     if (rateLimit.value(QPair(user, device)) + 60 * 60 * 10 < currentTime) {
                         claims.one_time_keys[user][device] = mtx::crypto::SIGNED_CURVE25519;
                         pks[user][device].ed25519          = d.keys.at("ed25519:" + device);
@@ -1364,13 +1367,15 @@ send_encrypted_to_device_messages(const std::map<std::string, std::vector<std::s
                                                    d.keys.at("ed25519:" + device),
                                                    device_curve)
                     .get<mtx::events::msg::OlmEncrypted>();
+                sessionsToPersist.emplace_back(d.keys.at("curve25519:" + device),
+                                               std::move(*session));
+            }
+        }
 
+        if (!sessionsToPersist.empty()) {
             try {
-                nhlog::crypto()->debug("Updated olm session: {}",
+                nhlog::crypto()->debug("Updated olm sessions: {}", sessionsToPersist.size());
-                                       mtx::crypto::session_id(session->get()));
+                cache::client()->saveOlmSessions(std::move(sessionsToPersist), currentTime);
-                cache::saveOlmSession(d.keys.at("curve25519:" + device),
-                                      std::move(*session),
-                                      QDateTime::currentMSecsSinceEpoch());
             } catch (const lmdb::error &e) {
                 nhlog::db()->critical("failed to save outbound olm session: {}", e.what());
             } catch (const mtx::crypto::olm_exception &e) {
@@ -1395,6 +1400,9 @@ send_encrypted_to_device_messages(const std::map<std::string, std::vector<std::s
                                          mtx::http::RequestErr) {
             std::map<mtx::identifiers::User, std::map<std::string, mtx::events::msg::OlmEncrypted>>
               messages;
+            auto currentTime = QDateTime::currentSecsSinceEpoch();
+            std::vector<std::pair<std::string, mtx::crypto::OlmSessionPtr>> sessionsToPersist;
+
             for (const auto &[user_id, retrieved_devices] : res.one_time_keys) {
                 nhlog::net()->debug("claimed keys for {}", user_id);
                 if (retrieved_devices.size() == 0) {
@@ -1440,11 +1448,16 @@ send_encrypted_to_device_messages(const std::map<std::string, std::vector<std::s
                           session.get(), ev_json, UserId(user_id), sign_key, id_key)
                         .get<mtx::events::msg::OlmEncrypted>();
 
+                    sessionsToPersist.emplace_back(id_key, std::move(session));
+                }
+                nhlog::net()->info("send_to_device: {}", user_id);
+            }
+
+            if (!sessionsToPersist.empty()) {
                 try {
-                        nhlog::crypto()->debug("Updated olm session: {}",
+                    nhlog::crypto()->debug("Updated (new) olm sessions: {}",
-                                               mtx::crypto::session_id(session.get()));
+                                           sessionsToPersist.size());
-                        cache::saveOlmSession(
+                    cache::client()->saveOlmSessions(std::move(sessionsToPersist), currentTime);
-                          id_key, std::move(session), QDateTime::currentMSecsSinceEpoch());
                 } catch (const lmdb::error &e) {
                     nhlog::db()->critical("failed to save outbound olm session: {}", e.what());
                 } catch (const mtx::crypto::olm_exception &e) {
@@ -1452,8 +1465,6 @@ send_encrypted_to_device_messages(const std::map<std::string, std::vector<std::s
                                               e.what());
                 }
             }
-                nhlog::net()->info("send_to_device: {}", user_id);
-            }
 
             if (!messages.empty())
                 http::client()->send_to_device<mtx::events::msg::OlmEncrypted>(