e9aceca1de
One of the easiest things to mess up with writing a custom peripheral is
handling attached peripherals. IPeripheral.{attach,detach} are called
from multiple threads, so naive implementations that just store
computers in a set/list will at some point throw a CME.
Historically I've suggested using a concurrent collection (i.e.
ConcurrentHashMap). While this solves the problems of CMEs, it still has
some flaws. If a computer is detached while iterating over the
collection, the iterator will still yield the now-detached peripheral,
causing usages of that computer (e.g. queueEvent) to throw an exception.
The only fix here is to use a lock when updating and iterating over the
collection. This does come with some risks, but I think they are not too
serious:
- Lock contention: Contention is relatively rare in general (as
peripheral attach/detach is not especially frequent). If we do see
contention, both iteration and update actions are cheap, so I would
not expect the other thread to be blocked for a significant time.
- Deadlocks: One could imagine an implementation if IComputerAccess
that holds a lock both when detaching a peripheral and inside
queueEvent.
If we queue an event on one thread, and try to detach on the other,
we could see a deadlock:
Thread 1 | Thread 2
----------------------------------------------------------
AttachedComputerSet.queueEvent | MyModem.detach
(take lock #1) | (take lock #2)
-> MyModem.queueEvent | AttachedComputerSet.remove
(wait on lock #2) | (wait on lock #1)
Such code would have been broken already (some peripherals already
use locks), so I'm fairly sure we've fixed this in CC. But definitely
something to watch out for.
Anyway, the long and short of it:
- Add a new AttachedComputerSet that can be used to track the computers
attached to a peripheral. We also mention this in the attach/detach
docs, to hopefully make it a little more obvoius.
- Update speakers and monitors to use this new class.
CC: Tweaked is a mod for Minecraft which adds programmable computers, turtles and more to the game. A fork of the much-beloved ComputerCraft, it continues its legacy with improved performance and stability, along with a wealth of new features.
CC: Tweaked can be installed from CurseForge or Modrinth. It runs on both Minecraft Forge and Fabric.
Contributing
Any contribution is welcome, be that using the mod, reporting bugs or contributing code. If you want to get started developing the mod, check out the instructions here.
Community
If you need help getting started with CC: Tweaked, want to show off your latest project, or just want to chat about
ComputerCraft, do check out our GitHub discussions page! There's also a fairly populated,
albeit quiet IRC channel on EsperNet, if that's more your cup of tea. You can join #computercraft through your
desktop client, or online using KiwiIRC.
We also host fairly comprehensive documentation at tweaked.cc.
Using
CC: Tweaked is hosted on my maven repo, and so is relatively simple to depend on. You may wish to add a soft (or hard)
dependency in your mods.toml file, with the appropriate version bounds, to ensure that API functionality you depend
on is present.
repositories {
maven {
url "https://maven.squiddev.cc"
content {
includeGroup("cc.tweaked")
}
}
}
dependencies {
// Vanilla (i.e. for multi-loader systems)
compileOnly("cc.tweaked:cc-tweaked-$mcVersion-common-api:$cctVersion")
// Forge Gradle
compileOnly("cc.tweaked:cc-tweaked-$mcVersion-core-api:$cctVersion")
compileOnly(fg.deobf("cc.tweaked:cc-tweaked-$mcVersion-forge-api:$cctVersion"))
runtimeOnly(fg.deobf("cc.tweaked:cc-tweaked-$mcVersion-forge:$cctVersion"))
// Fabric Loom
modCompileOnly("cc.tweaked:cc-tweaked-$mcVersion-fabric-api:$cctVersion")
modRuntimeOnly("cc.tweaked:cc-tweaked-$mcVersion-fabric:$cctVersion")
}
When using ForgeGradle, you may also need to add the following:
minecraft {
runs {
configureEach {
property 'mixin.env.remapRefMap', 'true'
property 'mixin.env.refMapRemappingFile', "${buildDir}/createSrgToMcp/output.srg"
}
}
}
You should also be careful to only use classes within the dan200.computercraft.api package. Non-API classes are
subject to change at any point. If you depend on functionality outside the API (or need to mixin to CC:T), please file
an issue to let me know!
We bundle the API sources with the jar, so documentation should be easily viewable within your editor. Alternatively, the generated documentation can be browsed online.