In 1.12.2, doors that were in the detection range of a villager were added in xyz order. In 1.13, the order has changed. Existing iron farms will need to be rotated 90 degrees and mirrored to function again (in other words: completely torn down and rebuilt). This could seriously discourage upgrading to 1.13, and serves no conceivable purpose. This issue is distinct from, though related to, MC-131029
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@ [Mojang] Cory Scheviak
This bug affects advanced, large-scale iron farms that are capable of stacking multiple villages onto a spawning platform using redstone to move villagers around and validate doors in precise sequences. GruvaGuy made a video about iron farming that explains resettable iron farms at the end (cut to about 39:45).
A good example for testing (due to its more straightforward design) is GruvaGuy's Iron Casster which you can get a map download of here. Another such farm is the famous Iron Phoenix by TangoTek (though more complex and subject to error). Both these farms worked in 1.12 and should have worked in 1.13 too, were it not for this door detection order bug as well as the related detection range bug, MC-131029 (though the range change is not as detrimental as the change to detection order).
Though I'm not an expert on it myself, I believe the reason that these farms will need to be rebuilt because of the detection order change is that they rely on this very order to systematically validate and invalidate villages at specific locations at specific times. Village radius is expanded at specific points in time due to the detection of some doors, and villages are created and moved according to which doors are detected first. If the order of detection in different directions is changed, then the whole coordination of the farm is thrown out the window.
The importance of door detection order in these iron farms is evident by the fact that all tutorials for resettable iron farms stress that it is essential for the farm to be built in a particular position, facing North.
Resettable iron farms are extremely important to the technical community. Many survival servers also rely on these farms as a source of iron (including the well-known, technically-based SciCraft server). They are so advanced and resource-intensive to build in survival that many do not see them as exploits, rather, pieces of technological achievement that have been designed with such thought and time by their creators.
Phew, that was an essay! But if you do manage to fix this, be sure to record the importance of the villager door detection order code so that we don't get worried again next update!
Thanks for the attention towards this bug of ours, and I hope you guys can work it out!
Tested the Iron Casster in 18w30a, and I can confirm that this bug has definitely been fixed! But I also noticed the bug about golems spawning in air, and after testing it, found it to be a true. After accidentally making a duplicate report, I found the original report: MC-134889. I've attached a video of the bug to it too.
This seems like an odd use of a very arbitrary piece of code, the order of doors is not considered in any place in our codebase. Do you have a map download / video of what is the intended behavior and why things need to be rebuilt? Thanks!