Send only the written bytes of FML proxy packets

src/main/java/com/mitchej123/hodgepodge/config/FixesConfig.java

@@ -40,6 +40,10 @@ public class FixesConfig {
     @Config.DefaultBoolean(true)
     public static boolean fixBogusIntegratedServerNPEs;
 
+    @Config.Comment("Send only the written bytes of FML proxy packets. FML sends the whole backing array of a packet's buffer, so mods that pass an untrimmed buffer also send its unused capacity as zeros (up to the next power of two, at least 256 bytes per packet).")
+    @Config.DefaultBoolean(true)
+    public static boolean fixProxyPacketPadding;
+
     @Config.Comment("Patch Forge's @Optional.Interface processor to also strip Signature attribute entries (fixes TypeNotPresentException from Class.getGenericInterfaces when an optional interface's mod is missing)")
     @Config.DefaultBoolean(true)
     public static boolean fixForgeOptionalInterfaceSignature;

src/main/java/com/mitchej123/hodgepodge/mixins/Mixins.java

@@ -465,6 +465,10 @@ public enum Mixins implements IMixins {
                     "minecraft.NetworkManagerAccessor")
             .setApplyIf(() -> FixesConfig.fixBogusIntegratedServerNPEs)
             .setPhase(Phase.EARLY)),
+    FIX_PROXY_PACKET_PADDING(new MixinBuilder("Send only the written bytes of FML proxy packets, not their buffer's unused capacity")
+            .addCommonMixins("fml.MixinFMLProxyPacket_TrimPayload")
+            .setApplyIf(() -> FixesConfig.fixProxyPacketPadding)
+            .setPhase(Phase.EARLY)),
     FIX_LAG_ON_INVENTORY_SYNC(new MixinBuilder("Fix inventory sync lag")
             .addCommonMixins("minecraft.MixinInventoryCrafting")
             .setApplyIf(() -> FixesConfig.fixInventorySyncLag)

src/main/java/com/mitchej123/hodgepodge/mixins/early/fml/MixinFMLProxyPacket_TrimPayload.java

@@ -0,0 +1,23 @@
+package com.mitchej123.hodgepodge.mixins.early.fml;
+
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Redirect;
+
+import com.mitchej123.hodgepodge.mixins.hooks.ProxyPacketPayload;
+
+import cpw.mods.fml.common.network.internal.FMLProxyPacket;
+import io.netty.buffer.ByteBuf;
+
+@Mixin(FMLProxyPacket.class)
+public class MixinFMLProxyPacket_TrimPayload {
+
+    @Redirect(
+            method = { "toC17Packet", "toS3FPacket" },
+            at = @At(value = "INVOKE", target = "Lio/netty/buffer/ByteBuf;array()[B", remap = false),
+            remap = false,
+            expect = 2)
+    private byte[] hodgepodge$sendWrittenBytesOnly(ByteBuf payload) {
+        return ProxyPacketPayload.withoutUnusedCapacity(payload);
+    }
+}

src/main/java/com/mitchej123/hodgepodge/mixins/hooks/ProxyPacketPayload.java

@@ -0,0 +1,35 @@
+package com.mitchej123.hodgepodge.mixins.hooks;
+
+import java.util.Arrays;
+
+import io.netty.buffer.ByteBuf;
+
+/**
+ * FMLProxyPacket sends the whole backing array of its payload buffer. Mods that hand it an untrimmed buffer (such as a
+ * plain {@code Unpooled.buffer()}) send the buffer's unused capacity as trailing zeros: up to the next power of two, at
+ * least 256 bytes per packet.
+ */
+public final class ProxyPacketPayload {
+
+    private ProxyPacketPayload() {}
+
+    /**
+     * The payload's backing array without its unused capacity, the zeros after the written bytes. Everything up to the
+     * writer index is kept as before, including bytes before the reader index. The array is returned unchanged if
+     * nothing follows the written bytes, if anything after them is not zero, or if nothing was written: a client to
+     * server payload of length 0 arrives as null and fails in FML, so empty payloads keep their zeros.
+     */
+    public static byte[] withoutUnusedCapacity(ByteBuf payload) {
+        final byte[] array = payload.array();
+        final int end = payload.arrayOffset() + payload.writerIndex();
+        if (end <= 0 || end >= array.length) {
+            return array;
+        }
+        for (int i = end; i < array.length; i++) {
+            if (array[i] != 0) {
+                return array;
+            }
+        }
+        return Arrays.copyOf(array, end);
+    }
+}