Kyle Repinski
- MWisBest
- mwisbest
- America/Chicago
- Yes
- No
MC-119126
Why was this one marked as a duplicate? As Pokechu22 said, this has an actual analysis. The others should be forward-duplicated to this, not the other way around.
The bug
Whenever I start my game and go onto the Multiplayer server list, the servers all say "Can't connect to server" but when I refresh the page, it says the normal servers MOTD in the listing.
Code analysis (MCP names), comments, and additional note by Kyle Repinski
I've analyzed the issue and found it's a race condition with the multi-threaded server pinger code. The server pinging code (net.minecraft.client.network.ServerPinger) is run in multiple threads, and each thread calls net.minecraft.network.NetworkManager.createNetworkManagerAndConnect(). This function uses "lazy loaders" (net.minecraft.util.LazyLoadBase) to initialize some static variables. The issue lies in net.minecraft.util.LazyLoadBase.getValue(). This function is not thread-safe. Below you'll find the code in question with comments explaining exactly what is happening and how to fix it.
package net.minecraft.util; public abstract class LazyLoadBase<T> { private T value; private boolean isLoaded; public T getValue() // NOT THREAD-SAFE. SIMPLE FIX: declare the method synchronized { if (!this.isLoaded) // Thread 1: Condition is true; Thread 2: Condition is false { this.isLoaded = true; // Thread 1: SETS isLoaded to true, BUT VALUE IS STILL NULL this.value = this.load(); } return this.value; // Thread 2: RETURNS NULL because this.load() is not finished in Thread 1 } protected abstract T load(); }
Note that depending on the speed of the computer, this bug might not occur. If the first thread finishes quickly enough everything will appear normal.
Really annoying that this hasn't been fixed. It would take less than a minute with the information I provided.