Add persistent flight

When a player disconnects while flying in survival mode, they will
plummet to their deaths while reconnecting. These changes prevent that.
If a player is currently flying, their flight will not be disabled
unless the setGameMode method is called in the ServerPlayerEntity, which
is how the GameMode is usually set. Since the GameMode is initially set
using the interaction manager instead, players should be safe. :-)

src/main/java/party/_2a03/mc/mixin/MixinGameMode.java

@@ -0,0 +1,32 @@
+package party._2a03.mc.mixin;
+
+import net.minecraft.entity.player.PlayerAbilities;
+import net.minecraft.world.GameMode;
+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.Inject;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
+import party._2a03.mc.util.Config;
+
+@Mixin(GameMode.class)
+public class MixinGameMode {
+	private boolean flying;
+
+	@Inject(method = "setAbilities", at = @At("HEAD"))
+	public void checkFlight(PlayerAbilities abilities, CallbackInfo ci) {
+		this.flying = abilities.flying;
+	}
+
+	@Inject(method = "setAbilities", at = @At("TAIL"))
+	public void restoreFlight(PlayerAbilities abilities, CallbackInfo ci) {
+		if (!Config.getBool("persistentFlight"))
+			return;
+		if (abilities.flying)
+			return;
+		if (this.flying) {
+			abilities.allowFlying = true;
+			abilities.flying = true;
+		}
+	}
+}

src/main/java/party/_2a03/mc/mixin/MixinServerPlayerEntity.java

@@ -3,6 +3,7 @@ package party._2a03.mc.mixin;
 import net.minecraft.entity.player.PlayerEntity;
 import net.minecraft.server.network.ServerPlayerEntity;
 import net.minecraft.util.registry.RegistryKey;
+import net.minecraft.world.GameMode;
 import net.minecraft.world.World;
 import org.spongepowered.asm.mixin.injection.At;
 import org.spongepowered.asm.mixin.injection.Inject;
@@ -29,4 +30,12 @@ public abstract class MixinServerPlayerEntity extends PlayerEntity {
 			ci.cancel();
 		}
 	}
+
+	@Inject(method = "setGameMode", at = @At(value = "INVOKE", target = "Lnet/minecraft/server/network/ServerPlayerEntity;sendAbilitiesUpdate()V"))
+	public void disableFlightIfSafe(GameMode gameMode, CallbackInfo ci) {
+		if (gameMode != GameMode.CREATIVE && gameMode != GameMode.SPECTATOR) {
+			this.abilities.allowFlying = false;
+			this.abilities.flying = false;
+		}
+	}
 }

src/main/java/party/_2a03/mc/util/Config.java

@@ -35,6 +35,7 @@ public class Config {
 			json = new JSONObject();
 		}
 
+		setDefault("persistentFlight", true);
 		setDefault("disableTntExplosions", false);
 		setDefault("disableRespawnAnchorExplosions", false);
 		setDefault("iphubApiKey", "");

src/main/resources/minecraft-tweaks-2a03.mixins.json

@@ -3,6 +3,7 @@
 	"package": "party._2a03.mc.mixin",
 	"compatibilityLevel": "JAVA_8",
 	"server": [
+		"MixinGameMode",
 		"MixinGameModeCommand",
 		"MixinRespawnAnchorBlock",
 		"MixinServerPlayerEntity",