Arrows with negative damage can crash the game on hitting an enemy

Can confirm.

For anyone testing this, the bow needs to be fully drawn.

Please attach a crash report.

Crash Report

Confirmed for

• 15w47c

Also when an arrow with negative damage is inside a chunk and the game tries to save the chunk it fails:

Saving chunks (15w47c)

[19:15:58] [Server thread/INFO]: Saving and pausing game...
[19:15:58] [Server thread/INFO]: Saving chunks for level 'New World'/Overworld
[19:15:58] [Server thread/ERROR]: Failed to save chunk
e: Saving entity NBT
	at ro.e(SourceFile:1302) ~[15w47c.jar:?]
	at ro.d(SourceFile:1243) ~[15w47c.jar:?]
	at ask.a(SourceFile:216) ~[15w47c.jar:?]
	at ask.a(SourceFile:102) [15w47c.jar:?]
	at lo.b(SourceFile:147) [15w47c.jar:?]
	at lo.a(SourceFile:166) [15w47c.jar:?]
	at lp.a(SourceFile:885) [15w47c.jar:?]
	at net.minecraft.server.MinecraftServer.a(SourceFile:370) [15w47c.jar:?]
	at bxx.a(SourceFile:213) [15w47c.jar:?]
	at bxx.C(SourceFile:144) [15w47c.jar:?]
	at net.minecraft.server.MinecraftServer.run(SourceFile:456) [15w47c.jar:?]
	at java.lang.Thread.run(Thread.java:745) [?:1.8.0_25]
Caused by: java.lang.NullPointerException
	at zr.b(SourceFile:429) ~[15w47c.jar:?]
	at aai.b(SourceFile:134) ~[15w47c.jar:?]
	at ro.e(SourceFile:1284) ~[15w47c.jar:?]
	... 11 more
[19:15:58] [Server thread/INFO]: Saving chunks for level 'New World'/Nether
[19:15:58] [Server thread/INFO]: Saving chunks for level 'New World'/The End

Tried in 1.12. Appears to be fixed, or something has changed, since my server didn't crash when I tried this on it.

Tried in 1.12. Appears to be fixed

Nope, not fixed as of 1.12.2, nor the latest 1.13 snapshot, 17w50a.

So I had this happen to me just the other day while I was trying to figure out raycast math. I wondered how critical arrow damage worked while writing a long comment on a youtube video since nowhere explained that damage, figured it out myself, and then experimented with negative values while I was at it. And boom, world permanently lost thanks to this bug (at least it wasn't terribly important). Well, at least until external editing programs update for 1.13 so I can delete the arrows from existence (if no fix comes).

Basically, if a critical arrow attempts to deal negative 4 base damage (e.g. not counting bonus crit damage) or beyond at the moment of contact (even if the victim is an entity that normally can't be damaged apparently, just crashed from trying it on a falling_block entity), you'll lose your world to permacrashes as long as the arrow exists (crash-strength arrows don't disappear on hit). I think the world progresses a handful of ticks every time you re-enter the world though, so if you're really persistent you might get back in eventually if it can despawn, not sure... If it hits a player in the snapshot though, you'll be able to get back into your world easily, which I think is due to a snapshot-only bug of projectiles not currently being able to hit entities from inside their hitbox... But even in the release version, you're able to get back into the world for a moment, so I think the arrow will eventually hit the ground/pass by the player. Apparently the world loads before the player does judging by my own crash reports (they seem to be just about the same as the one posted by the OP, but I'll link two of my last ones for the sake of evidence at the end of this comment, with the first one being from 1.12.2, second one from 17w50a), so I think that'd be the case. I mention that because if the arrow crashes from a non-player entity, the next time you attempt to load in, the crash report doesn't list the player as being in the world, because the world (and thus the entities) is loading pretty much instantly before the player even has a chance to appear (maybe you'd be able to escape the area that has the arrow if it was a really far-off entity that was hit though). Depending on the entity (and probably other situations), attempting to get back into that world can prevent the game from auto-closing on crash, and for such worlds, loading a different world first for some reason will allow it to crash-close on its own as normal. Otherwise, you need to use the task manager to close either the window or the process in the snapshot version, and for the release version, you need to end the process (unlike the snapshots, the window itself closes instantly via task manager, but the process will still be running).

I guess the moral of the story is to not mess with critical negative damage. Non-crit negative damage seems completely safe though, and negative damage arrows have a quite-interesting property of increasing the damage of the next hit the victim takes for 0.5 seconds by an amount equal to the negative damage. It's also a wonderful way (and the only way, I think?) to push players via knockback without harming them (I just wish 0 damage arrows could hit players like they can entities, so that there's no 0.5s "vulnerability" window... but I guess there's gonna be a vulnerability window no matter what since you can get hit right before the end of the 0.5s and then get hit again right away, but that in itself can be avoided/utilized with careful consideration). So only if possible, please don't get rid of negative damage mechanics if you fix this crash issue! 😃

All that aside, here's an example command of what can perma-crash your game:

WARNING: Do NOT risk doing this on a world you care about!

/summon arrow ~ ~2.5 ~ {Motion:[0.0,-1.0],damage:-4,crit:1}

While you might possibly recover just fine if you only hit yourself with it, if it hits an entity, well, don't say I didn't warn you! Seriously, use a test world if you're gonna run the command on anything, haha. Velocity times damage is how much the arrow will hurt (round it up (never down for arrows) to a full number if the result has decimals; also, motion on multiple axes complicates the velocity a bit, so I recommend a single axis for easy math). At a basically point-blank range like this, the air doesn't have time to slow it down, so it does precisely -4 damage, which is how much it takes (minimum) to crash as long as it's also a crit arrow.

Anyway, I've probably (over)written enough... These aforementioned crash reports are basically identical, but just in case:
1.12.2 report:

17w50a report:

Making an arrow with very high damage will crash the game when hitting a player. This can be used by any player with creative by getting this item and placing it above themselves:
give @s chicken_spawn_egg{EntityTag:{id:"minecraft:arrow",damage:1e100d,pickup:1,crit:1}}
I believe this to be a serious issue allowing players with creative to crash any Minecraft server

Can anyone reproduce this in recent versions?
In my testing it stopped happening precisely in the update from 1.14-prerelease5 to 1.14, although 1.14.2 is marked as affected.

This was indeed fixed in 1.14. For the crash for very high damage values, see MC-181313.
The affected version 1.14.2 came from MC-154177 which actually describes the issue for very high damage values. Removing that affected version and setting 1.14 as "Fix Version" is not easily (if at all) possibly, so I resolved this report as "Cannot Reproduce".

In 1.14 the code was changed to call Math.max(speed * damage, 0, which makes sure that the calculated damage value is always >= 0.