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  1. Minecraft: Java Edition
  2. MC-73207

Minecraft server list displays "Can't connect to server" on startup



    • Icon: Bug Bug
    • Resolution: Fixed
    • Minecraft 18w02a
    • Minecraft 1.8, Minecraft 1.9, Minecraft 1.10.2, Minecraft 16w41a, Minecraft 16w42a, Minecraft 1.11.2, Minecraft 1.12.2, Minecraft 17w50a
    • Confirmed


      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 MWisBest

      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.




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            xXDeadlyBlazeXx SpongerDanger
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