[MC-101051] Entering Nether through portal resets all mobs

I built a guardian xp farm that sends guardians to the nether to kill them with a potion of harming. However I noticed that they sometimes despawned and couldn't figure out why.

I got someone to open the portals in the overworld whilst I stayed in spectator mode in the nether and watched the guardians come flooding in. However as soon as he went came through into the nether right above the guardians they all despawned. On further testing I noticed that ALL the mobs in the nether are reset when someone enters the nether. By reset I mean that all the mobs despawn and new ones respawn around the player. There were no other players in the nether during the testing.

This bug means that any farm that sends mobs to the nether will no longer work unless someone is on the other side to prevent them from despawning.

On further testing I don't think this issue affects single player worlds. Also when I went on the server and was the only person on-line the farm worked fine.

Related issues

duplicates

MC-97102 [Multiplayer] All hostile mobs in dimension despawn when player enters dimension

relates to

MC-89928 Portals not placing the player at correct coordinates causing them to take suffocation damage

Comments

kumasasa 2016-04-18T19:02:17Z

Are the mobs more than 32 blocks away from the entering portal ?
See http://minecraft.gamepedia.com/Spawn#Despawning

A mob that has had no player within 32 blocks of it for more than 30 seconds has a 1 in 800 chance of despawning on each game tick (1/20th of a second). Therefore, the half-life of monsters not within 32 blocks of a player is 27.73 seconds (after the initial 30 seconds have elapsed).

migrated 2016-04-18T19:12:46Z

No they are about 26 blocks away and like I said EVERY mob in the nether despawns in that area and new ones respawn around the player following normal mob spawning algorithms the instant a player enters the nether. The guy testing it with me hit a pigman then went back to the overworld (I remained in spectator mode). All the mobs remained in the nether in exactly the same position they were before he left. The moment he reentered the nether the pigman that was standing right by the portal despawned as did every other mob and new ones spawned. Then the pigmen were no longer agro.

migrated 2016-04-18T20:38:39Z

I have not tried to repro this, but I can imagine that MC-89928 could cause this.

migrated 2016-04-18T20:59:53Z

I don't think the bugs are related.

migrated 2016-04-18T21:02:13Z

Well they could be, due to the sheer distance between the old and correct coordianted, the mobs might already despawn from that.
Marking as related.

migrated 2016-04-18T23:04:33Z

I think one could determine if it's causal by seeing if it does not repro with a portal at 0,0 but does repro at a portal that's about 140 blocks from spawn (so the temporary bad coordinates would be > 128 blocks away for insta-despawn but also within render distance in a default-10-chunk server)

SunCat 2016-05-03T17:34:15Z

Dupe of MC-97102