formalize the three states of player interaction events

src/main/java/net/fabricmc/fabric/events/PlayerInteractionEvent.java

@@ -32,10 +32,16 @@ import net.minecraft.world.World;
  *
  * These hook in BEFORE the spectator checks, so make sure to check for the player's game mode as well!
  *
+ * In general, the events return an ActionResult with the following side effects:
+ * - SUCCESS cancels further processing and, on the client, sends a packet to the server.
+ * - PASS falls back to further processing.
+ * - FAIL cancels further processing and does not send a packet to the server.
+ *
  * CURRENT LIMITATIONS:
  *
  * - INTERACT_BLOCK/INTERACT_ITEM do not expect the ItemStack instance in the player's held hand to change!
  *   If you must do that, consider returning an ActionResult.SUCCESS and re-emitting the event in some manner!
+ * - ATTACK_BLOCK does not let you control the packet sending process yet.
  */
 public final class PlayerInteractionEvent {
 	@FunctionalInterface

src/main/java/net/fabricmc/fabric/mixin/events/playerinteraction/MixinClientPlayerInteractionManager.java

@@ -26,6 +26,8 @@ import net.minecraft.client.world.ClientWorld;
 import net.minecraft.entity.Entity;
 import net.minecraft.entity.player.PlayerEntity;
 import net.minecraft.server.network.packet.PlayerInteractBlockServerPacket;
+import net.minecraft.server.network.packet.PlayerInteractEntityServerPacket;
+import net.minecraft.server.network.packet.PlayerInteractItemServerPacket;
 import net.minecraft.util.ActionResult;
 import net.minecraft.util.Hand;
 import net.minecraft.util.HitResult;
@@ -100,11 +102,14 @@ public class MixinClientPlayerInteractionManager {
 		}
 	}
 
-	@Inject(at = @At(value = "INVOKE", target = "Lnet/minecraft/entity/player/PlayerEntity;getStackInHand(Lnet/minecraft/util/Hand;)Lnet/minecraft/item/ItemStack;", ordinal = 0), method = "interactItem", cancellable = true)
+	@Inject(at = @At(value = "INVOKE", target = "Lnet/minecraft/client/network/ClientPlayNetworkHandler;sendPacket(Lnet/minecraft/network/Packet;)V", ordinal = 0), method = "interactItem", cancellable = true)
 	public void interactItem(PlayerEntity player, World world, Hand hand, CallbackInfoReturnable<ActionResult> info) {
 		for (PlayerInteractionEvent.Item handler : ((HandlerArray<PlayerInteractionEvent.Item>) PlayerInteractionEvent.INTERACT_ITEM).getBackingArray()) {
 			ActionResult result = handler.interact(player, world, hand);
 			if (result != ActionResult.PASS) {
+				if (result == ActionResult.SUCCESS) {
+					this.networkHandler.sendPacket(new PlayerInteractItemServerPacket(hand));
+				}
 				info.setReturnValue(result);
 				info.cancel();
 				return;
@@ -112,18 +117,21 @@ public class MixinClientPlayerInteractionManager {
 		}
 	}
 
-	@Inject(at = @At("HEAD"), method = "attackEntity", cancellable = true)
+	@Inject(at = @At(value = "INVOKE", target = "Lnet/minecraft/client/network/ClientPlayNetworkHandler;sendPacket(Lnet/minecraft/network/Packet;)V", ordinal = 0), method = "attackEntity", cancellable = true)
 	public void attackEntity(PlayerEntity player, Entity entity, CallbackInfo info) {
 		for (PlayerInteractionEvent.Entity handler : ((HandlerArray<PlayerInteractionEvent.Entity>) PlayerInteractionEvent.ATTACK_ENTITY).getBackingArray()) {
 			ActionResult result = handler.interact(player, player.getEntityWorld(), Hand.MAIN /* TODO */, entity);
 			if (result != ActionResult.PASS) {
+				if (result == ActionResult.SUCCESS) {
+					this.networkHandler.sendPacket(new PlayerInteractEntityServerPacket(entity));
+				}
 				info.cancel();
 				return;
 			}
 		}
 	}
 
-	@Inject(at = @At("HEAD"), method = "interactEntityAtLocation", cancellable = true)
+	@Inject(at = @At(value = "INVOKE", target = "Lnet/minecraft/client/network/ClientPlayNetworkHandler;sendPacket(Lnet/minecraft/network/Packet;)V", ordinal = 0), method = "interactEntityAtLocation", cancellable = true)
 	public void interactEntityAtLocation(PlayerEntity player, Entity entity, HitResult hitResult, Hand hand, CallbackInfoReturnable<ActionResult> info) {
 		// TODO: Remove double Vec3d creation?
 		Vec3d hitVec = new Vec3d(hitResult.pos.x - entity.x, hitResult.pos.y - entity.y, hitResult.pos.z - entity.z);
@@ -131,6 +139,9 @@ public class MixinClientPlayerInteractionManager {
 		for (PlayerInteractionEvent.EntityPositioned handler : ((HandlerArray<PlayerInteractionEvent.EntityPositioned>) PlayerInteractionEvent.INTERACT_ENTITY_POSITIONED).getBackingArray()) {
 			ActionResult result = handler.interact(player, player.getEntityWorld(), hand, entity, hitVec);
 			if (result != ActionResult.PASS) {
+				if (result == ActionResult.SUCCESS) {
+					this.networkHandler.sendPacket(new PlayerInteractEntityServerPacket(entity, hand, hitVec));
+				}
 				info.setReturnValue(result);
 				info.cancel();
 				return;