Moving eventCast()

In a situation where you have an EventPtr that you want to place
somewhere as an `event_ptr_tt<SomeMoreSpecificEventType>` you have to
carefully check that the stored event is actually of
SomeMoreSpecificType and if it is, release() that event pointer,
downcast, and re-wrap it into that new event_ptr_tt - or, as can be seen
from the diff here, re-loadEvent() from JSON, which is simpler but
inefficient. To help clients, and the library, eventCast() can now
accept an rvalue smart pointer and do all the necessary things with it.

2 files changed, 44 insertions, 15 deletions

diff --git a/lib/connection.cpp b/lib/connection.cpp
index 81151135..6e04883e 100644
--- a/lib/connection.cpp
+++ b/lib/connection.cpp
@@ -977,22 +977,22 @@ void Connection::Private::consumeToDeviceEvents(Events&& toDeviceEvents)
     if (!toDeviceEvents.empty()) {
         qCDebug(E2EE) << "Consuming" << toDeviceEvents.size()
                       << "to-device events";
-        visitEach(toDeviceEvents, [this](const EncryptedEvent& event) {
-            if (event.algorithm() != OlmV1Curve25519AesSha2AlgoKey) {
-                qCDebug(E2EE) << "Unsupported algorithm" << event.id()
-                              << "for event" << event.algorithm();
-                return;
-            }
-            if (isKnownCurveKey(event.senderId(), event.senderKey())) {
-                handleEncryptedToDeviceEvent(event);
-                return;
+        for (auto&& tdEvt : toDeviceEvents)
+            if (auto&& event = eventCast<EncryptedEvent>(std::move(tdEvt))) {
+                if (event->algorithm() != OlmV1Curve25519AesSha2AlgoKey) {
+                    qCDebug(E2EE) << "Unsupported algorithm" << event->id()
+                                  << "for event" << event->algorithm();
+                    return;
+                }
+                if (isKnownCurveKey(event->senderId(), event->senderKey())) {
+                    handleEncryptedToDeviceEvent(*event);
+                    return;
+                }
+                trackedUsers += event->senderId();
+                outdatedUsers += event->senderId();
+                encryptionUpdateRequired = true;
+                pendingEncryptedEvents.push_back(std::move(event));
             }
-            trackedUsers += event.senderId();
-            outdatedUsers += event.senderId();
-            encryptionUpdateRequired = true;
-            pendingEncryptedEvents.push_back(
-                makeEvent<EncryptedEvent>(event.fullJson()));
-        });
     }
 #endif
 }
diff --git a/lib/events/event.h b/lib/events/event.h
index da6cf3c7..043d4f54 100644
--- a/lib/events/event.h
+++ b/lib/events/event.h
@@ -332,6 +332,13 @@ inline bool isUnknown(const Event& e)
     return e.type() == UnknownEventTypeId;
 }
 
+//! \brief Cast the event pointer down in a type-safe way
+//!
+//! Checks that the event \p eptr points to actually is of the requested type
+//! and returns a (plain) pointer to the event downcast to that type. \p eptr
+//! can be either "dumb" (BaseEventT*) or "smart" (`event_ptr_tt<>`). This
+//! overload doesn't affect the event ownership - if the original pointer owns
+//! the event it must outlive the downcast pointer to keep it from dangling.
 template <class EventT, typename BasePtrT>
 inline auto eventCast(const BasePtrT& eptr)
     -> decltype(static_cast<EventT*>(&*eptr))
@@ -341,6 +348,28 @@ inline auto eventCast(const BasePtrT& eptr)
                                            : nullptr;
 }
 
+//! \brief Cast the event pointer down in a type-safe way, with moving
+//!
+//! Checks that the event \p eptr points to actually is of the requested type;
+//! if (and only if) it is, releases the pointer, downcasts it to the requested
+//! event type and returns a new smart pointer wrapping the downcast one.
+//! Unlike the non-moving eventCast() overload, this one only accepts a smart
+//! pointer, and that smart pointer should be an rvalue (either a temporary,
+//! or as a result of std::move()). The ownership, respectively, is transferred
+//! to the new pointer; the original smart pointer is reset to nullptr, as is
+//! normal for `unique_ptr<>::release()`.
+//! \note If \p eptr's event type does not match \p EventT it retains ownership
+//!       after calling this overload; if it is a temporary, this normally
+//!       leads to the event getting deleted along with the end of
+//!       the temporary's lifetime.
+template <class EventT, typename BaseEventT>
+inline auto eventCast(event_ptr_tt<BaseEventT>&& eptr)
+{
+    return eptr && is<std::decay_t<EventT>>(*eptr)
+               ? event_ptr_tt<EventT>(static_cast<EventT*>(eptr.release()))
+               : nullptr;
+}
+
 namespace _impl {
     template <typename FnT, class BaseT>
     concept Invocable_With_Downcast =