[MC-193310] Nether/Overworld Portal Synchronization Broken

Between 1.15.2 and 1.16.1, portal creation/synchronization broke.

When first exiting the nether, a new portal will be generated if one does not exist in that area.
When returning to the nether, the correct portal will be located and used.
When re-using that same portal, it appears to (usually [1]) bring you back to the same portal.

If the overworld portal is moved just 2-3 blocks, the bug becomes apparent;
After moving the portal a few blocks and going to the nether, all is still fine.
When returning to the overworld, the player is dropped off at the original portal location with no portal being generated.
The player can then use their overworld portal again to get correctly to the nether.
Every return to the overworld will result in being returned to the phantom portal.

[1] - In some cases, a player will seem to be randomly sent to one of two phantom portal locations.

This has been reliably reproduced on MCv1.16.1 in multiple chunks, including chunks that are part of regions which were deleted and re-generated.

Linked issues

is duplicated by 1

MC-193321 MC-193310 closed without resolutions Resolved

Comments 27

galaxy_2alex 2020-07-02T21:14:01Z

regions which were deleted and re-generated.

Manually editing world files like that is not supported and can cause unexpected behaviour.

Bumbu 2020-07-02T21:28:52Z

I'm also experiencing this problem after updating from 1.15.2 to 1.16.1.

Using the same seed on a fresh 1.16.1 map and the same portal location, I could not reproduce this in a local or server game.

I have also seen one location where the player was randomly dropped between one of two phantom portal locations. The two locations did not change, but which one we are sent to does.

One user on IRC also experiencing this issue suggested there may be "something hinky happens with the nbt at some point."

Mike 2020-07-02T21:34:40Z

Manually editing world files like that is not supported and can cause unexpected behaviour.

Perhaps, but it's a very well-documented and trivial process.

Stick coordinates into https://dinnerbone.com/minecraft/tools/coordinates/
Turn the server off
Delete the relevant file
Turn the server on

I don't know what sort of unexpected behavior you would expect to see from that.

Mike 2020-07-02T21:35:44Z

Why was this closed without any resolution?

galaxy_2alex 2020-07-02T21:38:46Z

The manual editing of a world save is not supported, and can cause issues like this.

17 more comments

Mike 2020-07-02T21:41:25Z

Editing the world is NOT what caused this.

Deleting regions was an attempt to debug the problem. Deleting regions was done in a clone.

This is reliably reproducible in a world that has not been manually modified.

Mike 2020-07-02T21:44:45Z

There are three different server admins that I'm aware of, plus myself, who are experiencing this issue only after jumping to 1.16 from 1.15.

galaxy_2alex 2020-07-02T21:46:40Z

What server software are you using?

Mike 2020-07-02T22:51:37Z

I'm using vanilla (also meaning, no plugins), with msm as a process manager. At least one of the other server admins is seeing this on both vanilla and spigot.

Nick Bastin 2020-07-02T23:01:38Z

I have reproduced this with 2 portals in vanilla 1.16, upgraded from 1.15.2 from a world that was started in 1.14. Most portals are ok, but we have 2 that create phantom portals when you go through them after you try to move them (the new locations are fine from the nether to the overworld, but when you try to come back to the nether you appear at the old location, with no generated portal).

Bumbu 2020-07-02T23:14:47Z

I first saw this in a map running spigot. I moved over to a vanilla server, checked all portals, and found the error in one of those portals.

... It's rather appalling that you just defaulted to marking this Invalid/Resolved without gathering any details. This seems to be a common trend for issues you're involved in. :/

galaxy_2alex 2020-07-02T23:49:07Z

Does the wiki page explain the behaviour? Please note that all behaviour described on the page is currently accurate and intentional.
https://minecraft.gamepedia.com/Nether_portal#Portal_linkage_between_Overworld_and_Nether

Mike 2020-07-02T23:51:57Z

It absolutely does not even come close to explaining the issue. It appears you have not read our problem statements.

Nick Bastin 2020-07-03T01:17:49Z

I can't seem to find the text on the wiki that says "your portal can dump you in a location where there is no portal, and it will not generate a portal".

Chandler 2020-07-03T01:35:59Z

Was the portal broken after/while entering it? MC-175067

Mike 2020-07-03T01:53:13Z

This appears to be an entirely different bug.

To simplify the procedure described in the issue description-

Go through a portal in the nether [portal N1]
Let it generate a portal in the overworld [portal O1]
Relocate that portal just a few blocks [destroy O1, create O2]
Go through portal O2 (wind up at N1)
Return through portal N1
Wind up at O1, with no portal generated and O2 still intact and nearby

Bugs:

Player is not sent to the correct portal
Player is sent to a portal that does not exist
No portal is generated

Further:

In one case, going through N1 sent player to two different phantom portal locations. The locations were consistent, but they phantom portal selected was random and varied by about 10 blocks.

TorteShui 2020-07-03T10:56:51Z

I think, MC now caches portal locations, and it seems that the cache did not get invalidated when destroying portal O1, so any attempt to go through N1 would always dump you at the old O1 location because it is still in the cache. If that's the case, it sounds like an actual bug and the ticket should be reopened.

Adrian Östergård 2020-07-08T07:09:07Z

Please provide a world download.

Erik Broes 2020-07-15T16:56:44Z

And if possible accurate steps to reproduce any issues you have experienced.

Erik Broes 2020-07-15T16:59:31Z

The portals are in no way 'connected' and stored, each time something teleports it finds a suitable portal to go to. If there are phantom portals, it means the POI blocks it uses for lookup are still there. This could indicate a problem with removing portals and for some reason not removing their POI data (we use to find them quickly). Reproduction steps would be greatly appreciated.

Mike 2020-07-16T14:16:26Z

1) Why was this marked as resolved when it's absurdly obvious nothing was resolved? "Awaiting Response" is not a resolution.

2) I am not able to upload my map to this ticket. I am getting an error about the file size being too large.

3) Galaxy_2Alex has made it clear that removing bits of the map so that it can be uploaded is unacceptable.

4) I give up... it's clear that the only thing mods/mojang care about is closing tickets. Y'all seem to not give a crap about actually fixing bugs. If you can't even pretend to care, then I'm just going to move along and do what I can to support open source competitors.

galaxy_2alex 2020-07-16T15:55:42Z

1) This is the workflow of the bug tracker.
2) Please use a third party website to upload the world, and provide a link for us to download.
4) We are trying our best to reproduce this, however, have been unable to do so, and need more clarification. I do accept that my behaviour in the beginning on this report was not correct, and I do apologise. Please also do note that only a handful of people currently claim to experience this issue, all of them in this report, but no one else has reported any wrong behaviour like this, so of course there is more scrutiny.

Erik Broes 2020-08-10T09:54:10Z

I'm unable to reproduce the issue, if anyone has reproduction steps or a world on which this happens which has not been edited please comment on the ticket.

Nick Bastin 2020-08-11T16:17:21Z

I have managed to figure this out after tinkering with our world for a few weeks on and off. It is not a game bug, but rather a filesystem data corruption issue, although it would be nice if there was some kind of server command / utility to fix the problem.

The issue (for us, at least) is that at some point in the past (during 1.14, actually, the only reason we found it in 1.16 is we were moving a lot of portals around) a few poi files got out of sync with their region files (probably actually a bunch due to an L2 arc panic, but the nether portal ones manifest in an obvious way to users), such that nether_portal locations in the poi existed, but the portals weren't actually there in the region. Once that happens, there's no way to get rid of those portal locations in the POI file from the game, because making a new portal even at the same location just makes new entries, and deletes those entries when you remove it, but you're still left with the stale ones (and they get used when you use a portal, and don't generate a portal when you come through on the other side since it thinks one is already there, thus creating this "phantom portal" experience).

I managed to fix one up by editing the POI mca with NBTExplorer, although I don't claim to understand enough about the format of that file to be sure I didn't create new problems (I just deleted the chunk branches entirely, rather than trying to edit their values). Are these files safe to delete and just have the game regenerate them (which would obviously be the best solution right now from a server admin point of view)?

EDIT: Just to be clear, the files themselves are not corrupt individually, they're just in a state where there is a logical de-sync due to one of them being written and one of them not at some point in the past.

[Mod] turbo 2024-05-26T18:09:44Z

If this still happens to anyone in 1.20.6 or later, please provide a world download and/or reliable steps to reproduce this problem.

nintos64 2024-07-05T18:29:15Z

i tried moving the portal a few times and would be located back to the original portal the first few times. But once in the nether for a longer period of time the portal would not work upon return. It would relocate me to a random cave once again... every time it does this the a new portal is generated.

nintos64 2024-07-05T18:35:18Z

I just downloaded MC on my new laptop and started a new world with 0 mods.