init

.gitignore

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+# User-specific stuff
+.idea/
+
+*.iml
+*.ipr
+*.iws
+
+# IntelliJ
+out/
+# mpeltonen/sbt-idea plugin
+.idea_modules/
+
+# JIRA plugin
+atlassian-ide-plugin.xml
+
+# Compiled class file
+*.class
+
+# Log file
+*.log
+
+# BlueJ files
+*.ctxt
+
+# Package Files #
+*.jar
+*.war
+*.nar
+*.ear
+*.zip
+*.tar.gz
+*.rar
+
+# virtual machine crash logs, see http://www.java.com/en/download/help/error_hotspot.xml
+hs_err_pid*
+
+*~
+
+# temporary files which can be created if a process still has a handle open of a deleted file
+.fuse_hidden*
+
+# KDE directory preferences
+.directory
+
+# Linux trash folder which might appear on any partition or disk
+.Trash-*
+
+# .nfs files are created when an open file is removed but is still being accessed
+.nfs*
+
+# General
+.DS_Store
+.AppleDouble
+.LSOverride
+
+# Icon must end with two \r
+Icon
+
+# Thumbnails
+._*
+
+# Files that might appear in the root of a volume
+.DocumentRevisions-V100
+.fseventsd
+.Spotlight-V100
+.TemporaryItems
+.Trashes
+.VolumeIcon.icns
+.com.apple.timemachine.donotpresent
+
+# Directories potentially created on remote AFP share
+.AppleDB
+.AppleDesktop
+Network Trash Folder
+Temporary Items
+.apdisk
+
+# Windows thumbnail cache files
+Thumbs.db
+Thumbs.db:encryptable
+ehthumbs.db
+ehthumbs_vista.db
+
+# Dump file
+*.stackdump
+
+# Folder config file
+[Dd]esktop.ini
+
+# Recycle Bin used on file shares
+$RECYCLE.BIN/
+
+# Windows Installer files
+*.cab
+*.msi
+*.msix
+*.msm
+*.msp
+
+# Windows shortcuts
+*.lnk
+
+.gradle
+build/
+
+# Ignore Gradle GUI config
+gradle-app.setting
+
+# Cache of project
+.gradletasknamecache
+
+**/build/
+
+# Common working directory
+run/
+runs/
+
+# Avoid ignoring Gradle wrapper jar file (.jar files are usually ignored)
+!gradle-wrapper.jar

LICENSE.txt

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+Copyright (c) 2025 
+All rights reserved.

README.md

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+# MountTeleportFix
+1.21.4 Fabric mod for fixing MC-277807 bug.
+
+## Installation
+Simply drop the jar to `mods` folder (This mod is server side only). As well as the **Fabric API**.

build.gradle

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+plugins {
+    id 'fabric-loom' version '1.10-SNAPSHOT'
+    id 'maven-publish'
+}
+
+version = project.mod_version
+group = project.maven_group
+
+base {
+    archivesName = project.archives_base_name
+}
+
+loom {
+    accessWidenerPath = file("src/main/resources/${project.archives_base_name}.accesswidener")
+}
+
+fabricApi {
+    configureDataGeneration {
+        client = true
+    }
+}
+
+repositories {
+    // Add repositories to retrieve artifacts from in here.
+    // You should only use this when depending on other mods because
+    // Loom adds the essential maven repositories to download Minecraft and libraries from automatically.
+    // See https://docs.gradle.org/current/userguide/declaring_repositories.html
+    // for more information about repositories.
+}
+
+dependencies {
+    // To change the versions see the gradle.properties file
+    minecraft "com.mojang:minecraft:${project.minecraft_version}"
+    mappings "net.fabricmc:yarn:${project.yarn_mappings}:v2"
+    modImplementation "net.fabricmc:fabric-loader:${project.loader_version}"
+
+    modImplementation "net.fabricmc.fabric-api:fabric-api:${project.fabric_version}"
+}
+
+processResources {
+    inputs.property "version", project.version
+    inputs.property "minecraft_version", project.minecraft_version
+    inputs.property "loader_version", project.loader_version
+    filteringCharset "UTF-8"
+
+    filesMatching("fabric.mod.json") {
+        expand "version": project.version,
+                "minecraft_version": project.minecraft_version,
+                "loader_version": project.loader_version
+    }
+}
+
+def targetJavaVersion = 21
+tasks.withType(JavaCompile).configureEach {
+    // ensure that the encoding is set to UTF-8, no matter what the system default is
+    // this fixes some edge cases with special characters not displaying correctly
+    // see http://yodaconditions.net/blog/fix-for-java-file-encoding-problems-with-gradle.html
+    // If Javadoc is generated, this must be specified in that task too.
+    it.options.encoding = "UTF-8"
+    if (targetJavaVersion >= 10 || JavaVersion.current().isJava10Compatible()) {
+        it.options.release.set(targetJavaVersion)
+    }
+}
+
+java {
+    def javaVersion = JavaVersion.toVersion(targetJavaVersion)
+    if (JavaVersion.current() < javaVersion) {
+        toolchain.languageVersion = JavaLanguageVersion.of(targetJavaVersion)
+    }
+    // Loom will automatically attach sourcesJar to a RemapSourcesJar task and to the "build" task
+    // if it is present.
+    // If you remove this line, sources will not be generated.
+    withSourcesJar()
+}
+
+jar {
+    from("LICENSE") {
+        rename { "${it}_${project.archivesBaseName}" }
+    }
+}
+
+// configure the maven publication
+publishing {
+    publications {
+        create("mavenJava", MavenPublication) {
+            artifactId = project.archives_base_name
+            from components.java
+        }
+    }
+
+    // See https://docs.gradle.org/current/userguide/publishing_maven.html for information on how to set up publishing.
+    repositories {
+        // Add repositories to publish to here.
+        // Notice: This block does NOT have the same function as the block in the top level.
+        // The repositories here will be used for publishing your artifact, not for
+        // retrieving dependencies.
+    }
+}

gradle.properties

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+# Done to increase the memory available to gradle.
+org.gradle.jvmargs=-Xmx1G
+# Fabric Properties
+# check these on https://modmuss50.me/fabric.html
+minecraft_version=1.21.4
+yarn_mappings=1.21.4+build.8
+loader_version=0.16.13
+# Mod Properties
+mod_version=1.0-SNAPSHOT
+maven_group=org.walruslab
+archives_base_name=mountteleportfix
+# Dependencies
+# check this on https://modmuss50.me/fabric.html
+fabric_version=0.119.2+1.21.4

gradle/wrapper/gradle-wrapper.properties

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+distributionUrl=https\://services.gradle.org/distributions/gradle-8.12-bin.zip

settings.gradle

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+pluginManagement {
+    repositories {
+        maven {
+            name = 'Fabric'
+            url = 'https://maven.fabricmc.net/'
+        }
+        gradlePluginPortal()
+    }
+}

src/main/java/org/walruslab/mountteleportfix/MountTeleportFix.java

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+package org.walruslab.mountteleportfix;
+
+import net.fabricmc.api.ModInitializer;
+
+public class MountTeleportFix implements ModInitializer {
+
+    @Override
+    public void onInitialize() {
+    }
+}

src/main/java/org/walruslab/mountteleportfix/client/MountTeleportFixDataGenerator.java

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+package org.walruslab.mountteleportfix.client;
+
+import net.fabricmc.fabric.api.datagen.v1.DataGeneratorEntrypoint;
+import net.fabricmc.fabric.api.datagen.v1.FabricDataGenerator;
+
+public class MountTeleportFixDataGenerator implements DataGeneratorEntrypoint {
+
+    @Override
+    public void onInitializeDataGenerator(FabricDataGenerator fabricDataGenerator) {
+        FabricDataGenerator.Pack pack = fabricDataGenerator.createPack();
+    }
+}

src/main/java/org/walruslab/mountteleportfix/mixin/ServerPlayerNetworkHandlerMixin.java

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+package org.walruslab.mountteleportfix.mixin;
+
+import net.minecraft.entity.MovementType;
+import net.minecraft.entity.effect.StatusEffects;
+import net.minecraft.network.NetworkThreadUtils;
+import net.minecraft.network.packet.c2s.play.PlayerMoveC2SPacket;
+import net.minecraft.server.network.ServerPlayNetworkHandler;
+import net.minecraft.server.world.ServerWorld;
+import net.minecraft.text.Text;
+import net.minecraft.util.math.Box;
+import net.minecraft.util.math.MathHelper;
+import net.minecraft.util.math.Vec3d;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Overwrite;
+
+@Mixin(ServerPlayNetworkHandler.class)
+public class ServerPlayerNetworkHandlerMixin {
+    /**
+     * @author MincoMK
+     * @reason Fix MC-277807
+     */
+    @Overwrite
+    public void onPlayerMove(PlayerMoveC2SPacket packet) {
+        ServerPlayNetworkHandler self = (ServerPlayNetworkHandler) (Object) this;
+
+        NetworkThreadUtils.forceMainThread(packet, self, self.player.getServerWorld());
+        if (ServerPlayNetworkHandler.isMovementInvalid(packet.getX(0.0), packet.getY(0.0), packet.getZ(0.0), packet.getYaw(0.0F), packet.getPitch(0.0F))) {
+            self.disconnect(Text.translatable("multiplayer.disconnect.invalid_player_movement"));
+        } else {
+            ServerWorld serverWorld = self.player.getServerWorld();
+            if (!self.player.notInAnyWorld) {
+                if (self.ticks == 0) {
+                    self.syncWithPlayerPosition();
+                }
+
+                if (self.player.isLoaded()) {
+                    float f = MathHelper.wrapDegrees(packet.getYaw(self.player.getYaw()));
+                    float g = MathHelper.wrapDegrees(packet.getPitch(self.player.getPitch()));
+                    if (self.handlePendingTeleport()) {
+                        self.player.setAngles(f, g);
+                    } else {
+                        double d = ServerPlayNetworkHandler.clampHorizontal(packet.getX(self.player.getX()));
+                        double e = ServerPlayNetworkHandler.clampVertical(packet.getY(self.player.getY()));
+                        double h = ServerPlayNetworkHandler.clampHorizontal(packet.getZ(self.player.getZ()));
+                        if (self.player.hasVehicle()) {
+                            self.player.updatePositionAndAngles(self.player.getX(), self.player.getY(), self.player.getZ(), f, g);
+                            self.player.getServerWorld().getChunkManager().updatePosition(self.player);
+                        } else {
+                            double i = self.player.getX();
+                            double j = self.player.getY();
+                            double k = self.player.getZ();
+                            double l = d - self.lastTickX;
+                            double m = e - self.lastTickY;
+                            double n = h - self.lastTickZ;
+                            double o = self.player.getVelocity().lengthSquared();
+                            double p = l * l + m * m + n * n;
+                            if (self.player.isSleeping()) {
+                                if (p > 1.0) {
+                                    self.requestTeleport(self.player.getX(), self.player.getY(), self.player.getZ(), f, g);
+                                }
+                            } else {
+                                boolean bl = self.player.isGliding();
+                                if (serverWorld.getTickManager().shouldTick()) {
+                                    self.movePacketsCount++;
+                                    int q = self.movePacketsCount - self.lastTickMovePacketsCount;
+                                    if (q > 5) {
+                                        ServerPlayNetworkHandler.LOGGER.debug("{} is sending move packets too frequently ({} packets since last tick)", self.player.getName().getString(), q);
+                                        q = 1;
+                                    }
+
+                                    if (self.shouldCheckMovement(bl)) {
+                                        float r = bl ? 300.0F : 100.0F;
+                                        if (p - o > r * q) {
+                                            ServerPlayNetworkHandler.LOGGER.warn("{} moved too quickly! {},{},{}", self.player.getName().getString(), l, m, n);
+                                            self.requestTeleport(self.player.getX(), self.player.getY(), self.player.getZ(), self.player.getYaw(), self.player.getPitch());
+                                            return;
+                                        }
+                                    }
+                                }
+
+                                Box box = self.player.getBoundingBox();
+                                l = d - self.updatedX;
+                                m = e - self.updatedY;
+                                n = h - self.updatedZ;
+                                boolean bl2 = m > 0.0;
+                                if (self.player.isOnGround() && !packet.isOnGround() && bl2) {
+                                    self.player.jump();
+                                }
+
+                                boolean bl3 = self.player.groundCollision;
+                                self.player.move(MovementType.PLAYER, new Vec3d(l, m, n));
+                                l = d - self.player.getX();
+                                m = e - self.player.getY();
+                                if (m > -0.5 || m < 0.5) {
+                                    m = 0.0;
+                                }
+
+                                n = h - self.player.getZ();
+                                p = l * l + m * m + n * n;
+                                boolean bl4 = false;
+                                if (!self.player.isInTeleportationState() && p > 0.0625 && !self.player.isSleeping() && !self.player.isCreative() && !self.player.isSpectator()) {
+                                    bl4 = true;
+                                    ServerPlayNetworkHandler.LOGGER.warn("{} moved wrongly!", self.player.getName().getString());
+                                }
+
+                                if (self.player.noClip
+                                        || self.player.isSleeping()
+                                        || (!bl4 || !serverWorld.isSpaceEmpty(self.player, box)) && !self.isPlayerNotCollidingWithBlocks(serverWorld, box, d, e, h)) {
+                                    self.player.updatePositionAndAngles(d, e, h, f, g);
+                                    boolean bl5 = self.player.isUsingRiptide();
+                                    self.floating = m >= -0.03125
+                                            && !bl3
+                                            && !self.player.isSpectator()
+                                            && !self.server.isFlightEnabled()
+                                            && !self.player.getAbilities().allowFlying
+                                            && !self.player.hasStatusEffect(StatusEffects.LEVITATION)
+                                            && !bl
+                                            && !bl5
+                                            && self.isEntityOnAir(self.player);
+                                    self.player.getServerWorld().getChunkManager().updatePosition(self.player);
+                                    Vec3d vec3d = new Vec3d(self.player.getX() - i, self.player.getY() - j, self.player.getZ() - k);
+                                    self.player.setMovement(packet.isOnGround(), packet.horizontalCollision(), vec3d);
+                                    self.player.handleFall(vec3d.x, vec3d.y, vec3d.z, packet.isOnGround());
+                                    self.handleMovement(vec3d);
+                                    if (bl2) {
+                                        self.player.onLanding();
+                                    }
+
+                                    if (packet.isOnGround() || self.player.hasLandedInFluid() || self.player.isClimbing() || self.player.isSpectator() || bl || bl5) {
+                                        self.player.tryClearCurrentExplosion();
+                                    }
+
+                                    self.player.increaseTravelMotionStats(self.player.getX() - i, self.player.getY() - j, self.player.getZ() - k);
+                                    self.updatedX = self.player.getX();
+                                    self.updatedY = self.player.getY();
+                                    self.updatedZ = self.player.getZ();
+                                } else {
+                                    self.requestTeleport(i, j, k, f, g);
+                                    self.player.handleFall(self.player.getX() - i, self.player.getY() - j, self.player.getZ() - k, packet.isOnGround());
+
+                                    if (!self.player.queuedCollisionChecks.isEmpty()) {
+                                        self.player.queuedCollisionChecks.removeLast();
+                                    }
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
+}

src/main/resources/fabric.mod.json

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+{
+  "schemaVersion": 1,
+  "id": "mountteleportfix",
+  "version": "${version}",
+  "name": "MountTeleportFix",
+  "description": "",
+  "authors": [],
+  "contact": {},
+  "license": "All-Rights-Reserved",
+  "icon": "assets/mountteleportfix/icon.png",
+  "environment": "server",
+  "accessWidener": "mountteleportfix.accesswidener",
+  "entrypoints": {
+    "fabric-datagen": [
+      "org.walruslab.mountteleportfix.client.MountTeleportFixDataGenerator"
+    ],
+    "main": [
+      "org.walruslab.mountteleportfix.MountTeleportFix"
+    ]
+  },
+  "mixins": [
+    "mountteleportfix.mixins.json"
+  ],
+  "depends": {
+    "fabricloader": ">=${loader_version}",
+    "fabric": "*",
+    "minecraft": "${minecraft_version}"
+  }
+}

src/main/resources/mountteleportfix.accesswidener

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+accessWidener v2 named
+
+accessible method net/minecraft/server/network/ServerPlayNetworkHandler handlePendingTeleport ()Z
+accessible method net/minecraft/server/network/ServerPlayNetworkHandler isMovementInvalid (DDDFF)Z
+accessible method net/minecraft/server/network/ServerPlayNetworkHandler clampHorizontal (D)D
+accessible method net/minecraft/server/network/ServerPlayNetworkHandler clampVertical (D)D
+accessible field net/minecraft/server/network/ServerPlayNetworkHandler lastTickX D
+accessible field net/minecraft/server/network/ServerPlayNetworkHandler lastTickY D
+accessible field net/minecraft/server/network/ServerPlayNetworkHandler lastTickZ D
+accessible field net/minecraft/server/network/ServerPlayNetworkHandler movePacketsCount I
+accessible field net/minecraft/server/network/ServerPlayNetworkHandler lastTickMovePacketsCount I
+accessible field net/minecraft/server/network/ServerPlayNetworkHandler LOGGER Lorg/slf4j/Logger;
+accessible method net/minecraft/server/network/ServerPlayNetworkHandler shouldCheckMovement (Z)Z
+accessible field net/minecraft/server/network/ServerPlayNetworkHandler updatedX D
+accessible field net/minecraft/server/network/ServerPlayNetworkHandler updatedY D
+accessible field net/minecraft/server/network/ServerPlayNetworkHandler updatedZ D
+accessible field net/minecraft/server/network/ServerPlayNetworkHandler floating Z
+accessible field net/minecraft/server/network/ServerCommonNetworkHandler server Lnet/minecraft/server/MinecraftServer;
+accessible method net/minecraft/server/network/ServerPlayNetworkHandler handleMovement (Lnet/minecraft/util/math/Vec3d;)V
+accessible method net/minecraft/server/network/ServerPlayNetworkHandler isEntityOnAir (Lnet/minecraft/entity/Entity;)Z
+accessible method net/minecraft/server/network/ServerPlayNetworkHandler isPlayerNotCollidingWithBlocks (Lnet/minecraft/world/WorldView;Lnet/minecraft/util/math/Box;DDD)Z
+accessible field net/minecraft/server/network/ServerPlayNetworkHandler ticks I
+accessible field net/minecraft/entity/Entity queuedCollisionChecks Ljava/util/List;

src/main/resources/mountteleportfix.mixins.json

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+{
+  "required": true,
+  "minVersion": "0.8",
+  "package": "org.walruslab.mountteleportfix.mixin",
+  "compatibilityLevel": "JAVA_21",
+  "mixins": [
+    "ServerPlayerNetworkHandlerMixin"
+  ],
+  "client": [
+  ],
+  "injectors": {
+    "defaultRequire": 1
+  }
+}