What I expected:
Non-OP survival and adventure players would not be able to configure spawners using spawn eggs, or at least some means of preventing spawner configuration would exist.
What actually happened:
Spawn eggs can be used to configure spawners without any restrictions in Survival and Adventure. No gamerule or NBT tag affects this behavior either.
Why this is a valid issue:
This issue is valid on the same grounds
MC-2367 and MC-3066 were valid (and have since been fixed): a change to the game has broken adventure maps and minigames by adding an unfixable exploit, and the change cannot be avoided.
Any map where spawners and spawn eggs are accessible to players is affected, and such maps cannot be fixed to work properly on post-14w28 versions of Minecraft. Exploits range from game balance issues (easily disabling hostile spawners, cheaply obtaining spawner types which are intended to be expensive, or creation of spawners not intended to be found in the map), to game-breaking issues where a player's accident can render the map unplayable (this is especially true of maps where spawn eggs are used to cast spells).
Steps to Reproduce:
- Obtain a spawner with a command such as "/give @p minecraft:mob_spawner"
- Place it, and observe that it is a pig spawner.
- Use a (non-pig) spawn egg on it, and observe that it changes type.
- Make sure you have another spawn egg of a different type in your inventory.
- Switch to Survival or Adventure mode, and deop yourself (if applicable).
- Use the other spawn egg on the spawner. Observe that you can still change its type.