Failure to upgrade worlds from 1.21.11 to 26.1.1

[Astralchroma]

Stack trace

Starting org.bukkit.craftbukkit.Main
[23:11:51 INFO]: [bootstrap] Running Java 25 (OpenJDK 64-Bit Server VM 25.0.2; Arch Linux null) on Linux 6.19.10-arch1-1 (amd64)
[23:11:51 INFO]: [bootstrap] Loading Paper 26.1.1-8-dev/26.1@52f7e24 (2026-04-03T21:37:22Z) for Minecraft 26.1.1
[23:11:51 INFO]: [PluginInitializerManager] Initializing plugins...
[23:11:51 INFO]: [PluginInitializerManager] Initialized 1 plugin
[23:11:51 INFO]: [PluginInitializerManager] Paper plugins (1):
 - Solarium (0.1.0-alpha.4)
[23:11:52 INFO]: [Solarium] Bootstrap completed in 48 ms
WARNING: A terminally deprecated method in sun.misc.Unsafe has been called
WARNING: sun.misc.Unsafe::objectFieldOffset has been called by org.joml.MemUtil$MemUtilUnsafe (file:/data/solarium/plugin/run/libraries/org/joml/joml/1.10.8/joml-1.10.8.jar)
WARNING: Please consider reporting this to the maintainers of class org.joml.MemUtil$MemUtilUnsafe
WARNING: sun.misc.Unsafe::objectFieldOffset will be removed in a future release
[23:11:55 INFO]: Environment: Environment[sessionHost=https://sessionserver.mojang.com, servicesHost=https://api.minecraftservices.com, profilesHost=https://api.mojang.com, name=PROD]
[23:11:55 INFO]: Starting upgrade for world "world"
[23:11:55 ERROR]: Failed to start the minecraft server
java.lang.UnsupportedOperationException: Path not associated with default file system.
	at java.base/java.nio.file.Path.toFile(Path.java:837) ~[?:?]
	at net.minecraft.world.level.chunk.storage.RegionFileStorage.isChunkDataFolder(RegionFileStorage.java:31) ~[paper-26.1.1.jar:26.1.1-8-52f7e24]
	at net.minecraft.world.level.chunk.storage.RegionFileStorage.<init>(RegionFileStorage.java:277) ~[paper-26.1.1.jar:26.1.1-8-52f7e24]
	at net.minecraft.world.level.chunk.storage.IOWorker.<init>(IOWorker.java:40) ~[paper-26.1.1.jar:26.1.1-8-52f7e24]
	at net.minecraft.world.level.chunk.storage.SimpleRegionStorage.<init>(SimpleRegionStorage.java:39) ~[paper-26.1.1.jar:26.1.1-8-52f7e24]
	at net.minecraft.util.filefix.access.ChunkNbt.<init>(ChunkNbt.java:20) ~[paper-26.1.1.jar:26.1.1-8-52f7e24]
	at net.minecraft.util.filefix.access.FileResourceTypes.lambda$chunk$0(FileResourceTypes.java:20) ~[paper-26.1.1.jar:26.1.1-8-52f7e24]
	at net.minecraft.util.filefix.access.FileResourceType.create(FileResourceType.java:13) ~[paper-26.1.1.jar:26.1.1-8-52f7e24]

Plugin and Datapack List

Visible in log, and also not relevant.

Actions to reproduce (if known)

No response

Paper version

Visible in log.

Other

Putting it through a debugger it seemed to think the filesystem root was the server, traced it up and I think I found the cuprit in net.minecraft.util.filefix.FileFixerUpper on line 197, weird call to fs.rootPath() which seems to get passed down the call chain into the migrator.

[electronicboy]

that rootPath is being called on their custom filesystem, not really a direct concern; We need more stable reproduction info

[electronicboy]

Could not reproduce on a fresh 1.21.11 vanilla server jar > 26.1, blew up trying to upgrade when ran in my usual setups (macOS)

[Astralchroma]

The rootPath() call gets passed down until RegionFileStorage.isChunkDataFolder() which obviously failed. Still not exactly sure why the rootPath() call is done or where it gets / from, at first I a bit too confidently thought it had something to do with subvolumes but that doesn't appear to be the case.

[uRyanxD]

I'm not sure if it's related, I have a test server on 1.21.11 and I was migrating to 26.1.1 now, the server simply shutdown when starting the world conversion

[19:34:36 INFO]: [bootstrap] Running Java 25 (OpenJDK 64-Bit Server VM 25.0.2+10-LTS; Eclipse Adoptium Temurin-25.0.2+10) on Windows 10 10.0 (amd64)
[19:34:36 INFO]: [bootstrap] Loading Paper 26.1.1-10-dev/26.1@ac42a07 (2026-04-03T22:20:25Z) for Minecraft 26.1.1
[19:34:37 ​​INFO]: [PluginInitializerManager] Initializing plugins...
[19:34:37 ​​INFO]: [PluginInitializerManager] Initialized 0 plugins
WARNING: A terminally deprecated method in sun.misc.Unsafe has been called
WARNING: sun.misc.Unsafe::objectFieldOffset has been called by org.joml.MemUtil$MemUtilUnsafe (file:/C:/Users/Usuario/Desktop/Minecraft%20Server's/1.21.11/libraries/org/joml/joml/1.10.8/joml-1.10.8.jar)
WARNING: Please consider reporting this to the maintainers of class org.joml.MemUtil$MemUtilUnsafe
WARNING: sun.misc.Unsafe::objectFieldOffset will be removed in a future release
[19:34:42 INFO]: Environment: Environment[sessionHost=https://sessionserver.mojang.com, servicesHost=https://api.minecraftservices.com, profilesHost=https://api.mojang.com, name=PROD]
[19:34:42 INFO]: Starting upgrade for world "world"
Press any key to continue. . .

[Spottedleaf]

Resolved by 14357cc

[Astralchroma]

lmao, and here I was still trying to figure out where the hell it was getting / from. insanely fast, thanks lol

[Astralchroma]

Can confirm the fix is working.