Fix Forge handshake taking extremely long time with many payloads

src/main/java/org/embeddedt/modernfix/common/mixin/perf/fix_handshake_stall/HandshakeHandlerMixin.java

@@ -0,0 +1,56 @@
+package org.embeddedt.modernfix.common.mixin.perf.fix_handshake_stall;
+
+import com.llamalad7.mixinextras.injector.wrapmethod.WrapMethod;
+import com.llamalad7.mixinextras.injector.wrapoperation.Operation;
+import com.llamalad7.mixinextras.injector.wrapoperation.WrapOperation;
+import net.minecraftforge.network.HandshakeHandler;
+import net.minecraftforge.network.NetworkRegistry;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Shadow;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Slice;
+
+import java.util.List;
+
+@Mixin(value = HandshakeHandler.class, remap = false)
+public class HandshakeHandlerMixin {
+    @Shadow
+    private int packetPosition;
+
+    @Shadow
+    private List<NetworkRegistry.LoginPayload> messageList;
+
+    @Shadow
+    private List<Integer> sentMessages;
+
+    /**
+     * @author embeddedt
+     * @reason Forge only sends one login payload per tick. It takes many seconds to send all the payloads at this rate.
+     * During this time, the game remains frozen on the chunk loading screen with almost zero CPU usage.
+     * To fix this, we re-tick the handshake handler until the packetPosition stops advancing or the handler indicates
+     * it no longer needs ticking.
+     */
+    @WrapMethod(method = "tickServer")
+    private boolean modernfix$retick(Operation<Boolean> original) {
+        boolean isDoneTicking;
+        int prevPacketPosition;
+        do {
+            prevPacketPosition = this.packetPosition;
+            isDoneTicking = original.call();
+        } while(!isDoneTicking && this.packetPosition > prevPacketPosition);
+        return isDoneTicking;
+    }
+
+    /**
+     * @author embeddedt
+     * @reason The original HandshakeHandler has an off-by-one error in its completion check. We patch this to prevent
+     * our optimization from potentially triggering it more often due to the timing change.
+     */
+    @WrapOperation(method = "tickServer", at = @At(value = "INVOKE", target = "Ljava/util/List;isEmpty()Z", ordinal = 0), slice = @Slice(from = @At(value = "INVOKE", target = "Ljava/util/List;removeIf(Ljava/util/function/Predicate;)Z", ordinal = 0)))
+    private boolean preventEarlyExit(List<?> instance, Operation<Boolean> original) {
+        if (instance != this.sentMessages) {
+            throw new AssertionError("Injector is misplaced");
+        }
+        return original.call(instance) && this.packetPosition >= this.messageList.size();
+    }
+}