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.

3 files changed, 105 insertions, 69 deletions

diff --git a/src/Cache.cpp b/src/Cache.cpp
index e090e40d..2784cf50 100644
--- a/src/Cache.cpp
+++ b/src/Cache.cpp
@@ -913,6 +913,29 @@ Cache::getMegolmSessionData(const MegolmSessionIndex &index)
 //
 
 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,
                       uint64_t timestamp)
diff --git a/src/Cache_p.h b/src/Cache_p.h
index 1694adb7..742e4aab 100644
--- a/src/Cache_p.h
+++ b/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);
diff --git a/src/encryption/Olm.cpp b/src/encryption/Olm.cpp
index 7ada2f92..a9d5b1c2 100644
--- a/src/encryption/Olm.cpp
+++ b/src/encryption/Olm.cpp
@@ -1299,78 +1299,83 @@ send_encrypted_to_device_messages(const std::map<std::string, std::vector<std::s
 
     auto our_curve = olm::client()->identity_keys().curve25519;
 
-    for (const auto &[user, devices] : targets) {
-        auto deviceKeys = cache::client()->userKeys(user);
+    {
+        auto currentTime = QDateTime::currentSecsSinceEpoch();
+        std::vector<std::pair<std::string, mtx::crypto::OlmSessionPtr>> sessionsToPersist;
 
-        // no keys for user, query them
-        if (!deviceKeys) {
-            keysToQuery[user] = devices;
-            continue;
-        }
+        for (const auto &[user, devices] : targets) {
+            auto deviceKeys = cache::client()->userKeys(user);
 
-        auto deviceTargets = devices;
-        if (devices.empty()) {
-            deviceTargets.clear();
-            deviceTargets.reserve(deviceKeys->device_keys.size());
-            for (const auto &[device, keys] : deviceKeys->device_keys) {
-                (void)keys;
-                deviceTargets.push_back(device);
+            // no keys for user, query them
+            if (!deviceKeys) {
+                keysToQuery[user] = devices;
+                continue;
             }
-        }
 
-        for (const auto &device : deviceTargets) {
-            if (!deviceKeys->device_keys.count(device)) {
-                keysToQuery[user] = {};
-                break;
+            auto deviceTargets = devices;
+            if (devices.empty()) {
+                deviceTargets.clear();
+                deviceTargets.reserve(deviceKeys->device_keys.size());
+                for (const auto &[device, keys] : deviceKeys->device_keys) {
+                    (void)keys;
+                    deviceTargets.push_back(device);
+                }
             }
 
-            auto d = deviceKeys->device_keys.at(device);
+            for (const auto &device : deviceTargets) {
+                if (!deviceKeys->device_keys.count(device)) {
+                    keysToQuery[user] = {};
+                    break;
+                }
 
-            if (!d.keys.count("curve25519:" + device) || !d.keys.count("ed25519:" + device)) {
-                nhlog::crypto()->warn("Skipping device {} since it has no keys!", device);
-                continue;
-            }
+                const auto &d = deviceKeys->device_keys.at(device);
 
-            auto device_curve = d.keys.at("curve25519:" + device);
-            if (device_curve == our_curve) {
-                nhlog::crypto()->warn("Skipping our own device, since sending "
-                                      "ourselves olm messages makes no sense.");
-                continue;
-            }
+                if (!d.keys.count("curve25519:" + device) || !d.keys.count("ed25519:" + device)) {
+                    nhlog::crypto()->warn("Skipping device {} since it has no keys!", device);
+                    continue;
+                }
 
-            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);
-                    pks[user][device].curve25519       = d.keys.at("curve25519:" + device);
+                auto device_curve = d.keys.at("curve25519:" + device);
+                if (device_curve == our_curve) {
+                    nhlog::crypto()->warn("Skipping our own device, since sending "
+                                          "ourselves olm messages makes no sense.");
+                    continue;
+                }
 
-                    rateLimit.insert(QPair(user, device), currentTime);
-                } else {
-                    nhlog::crypto()->warn("Not creating new session with {}:{} "
-                                          "because of rate limit",
-                                          user,
-                                          device);
+                auto session = cache::getLatestOlmSession(device_curve);
+                if (!session || force_new_session) {
+                    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);
+                        pks[user][device].curve25519       = d.keys.at("curve25519:" + device);
+
+                        rateLimit.insert(QPair(user, device), currentTime);
+                    } else {
+                        nhlog::crypto()->warn("Not creating new session with {}:{} "
+                                              "because of rate limit",
+                                              user,
+                                              device);
+                    }
+                    continue;
                 }
-                continue;
-            }
 
-            messages[mtx::identifiers::parse<mtx::identifiers::User>(user)][device] =
-              olm::client()
-                ->create_olm_encrypted_content(session->get(),
-                                               ev_json,
-                                               UserId(user),
-                                               d.keys.at("ed25519:" + device),
-                                               device_curve)
-                .get<mtx::events::msg::OlmEncrypted>();
+                messages[mtx::identifiers::parse<mtx::identifiers::User>(user)][device] =
+                  olm::client()
+                    ->create_olm_encrypted_content(session->get(),
+                                                   ev_json,
+                                                   UserId(user),
+                                                   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: {}",
-                                       mtx::crypto::session_id(session->get()));
-                cache::saveOlmSession(d.keys.at("curve25519:" + device),
-                                      std::move(*session),
-                                      QDateTime::currentMSecsSinceEpoch());
+                nhlog::crypto()->debug("Updated olm sessions: {}", sessionsToPersist.size());
+                cache::client()->saveOlmSessions(std::move(sessionsToPersist), currentTime);
             } 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) {
@@ -1440,21 +1448,24 @@ 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>();
 
-                    try {
-                        nhlog::crypto()->debug("Updated olm session: {}",
-                                               mtx::crypto::session_id(session.get()));
-                        cache::saveOlmSession(
-                          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) {
-                        nhlog::crypto()->critical("failed to pickle outbound olm session: {}",
-                                                  e.what());
-                    }
+                    sessionsToPersist.emplace_back(id_key, std::move(session));
                 }
                 nhlog::net()->info("send_to_device: {}", user_id);
             }
 
+            if (!sessionsToPersist.empty()) {
+                try {
+                    nhlog::crypto()->debug("Updated (new) olm sessions: {}",
+                                           sessionsToPersist.size());
+                    cache::client()->saveOlmSessions(std::move(sessionsToPersist), currentTime);
+                } catch (const lmdb::error &e) {
+                    nhlog::db()->critical("failed to save outbound olm session: {}", e.what());
+                } catch (const mtx::crypto::olm_exception &e) {
+                    nhlog::crypto()->critical("failed to pickle outbound olm session: {}",
+                                              e.what());
+                }
+            }
+
             if (!messages.empty())
                 http::client()->send_to_device<mtx::events::msg::OlmEncrypted>(
                   http::client()->generate_txn_id(), messages, [](mtx::http::RequestErr err) {