[Bug-Critical][Reproducible] Any Level Blueprint Multicasts with any Params Crash Unreal 4.16
Level Blueprints can no longer make any Multicast calls with any parameters in 4.16. Doing so will crash the engine in the following way:
Steps to reproduce from Blank Project:
1: Create a new empty 4.16 project with 2 levels. Call them LevelA and LevelB.
2: In LevelA, create a level blueprint that calls a reliable Multicast elsewhere in the same Level Blueprint. Make sure the Multicast has a parameter. 3: Enable Seamless Travel (Subclass the GameMode with a TestGameMode class with Seamless travel set to True. Make sure the Maps & Modes in the project settings are set to use your new custom game mode).
TO REPRODUCE CRASH:
If the multicast call was made on the server in the Level Blueprint (step 5), it should now hard-crash to desktop with the following error in the Log:
This exact same issue also breaks Level Streaming, but in different, weird way: For example, in the case above you could Stream in LevelA (through blueprint nodes), trigger the multicast, unload LevelA, and then stream in LevelA again. Rather than crashing this time, a rather bizarre broken state is reached: LevelA will stream-in just fine, but all direct references to objects in the Level Blueprint (like the PlayerStart) in the example above will fail. The log will be filled with Warnings that it failed to find these objects, and all scripts will treat these objects as though they are all replaced with None. In most cases, this basically renders the Level Blueprint completely useless as it will fail to obtain a proper reference to any object in the Level. This almost deserves its own separate bug report to fully explain, but the cause is exactly the same, so it's likely the same Engine bug just manifesting in a new way.
Sadly, my multiplayer game uses a lot of Level-streaming and Seamless ServerTravel. We're having to hunt down and rip out any Level Blueprints that have a Multicast call with a parameter, and move them into replicated dummy actors. Needless to say, it's kind of a disaster.
asked Jun 25 '17 at 05:03 PM in Bug Reports
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