Fix LevelChunk capability attach crash.

Fix client chunks not having capability providers attached.
Add capability attach tests.

src/main/java/net/minecraftforge/common/capabilities/CapabilityProvider.java

@@ -30,6 +30,7 @@ import net.minecraft.core.Direction;
 import net.minecraft.world.level.chunk.LevelChunk;
 import net.minecraftforge.common.util.LazyOptional;
 import net.minecraftforge.event.ForgeEventFactory;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.function.Supplier;
 
@@ -83,10 +84,16 @@ public abstract class CapabilityProvider<B extends ICapabilityProviderImpl<B>> i
 
     private void doGatherCapabilities(@Nullable ICapabilityProvider parent)
     {
-        this.capabilities = ForgeEventFactory.gatherCapabilities(baseClass, this, parent);
+        this.capabilities = ForgeEventFactory.gatherCapabilities(baseClass, getProvider(), parent);
         this.initialized = true;
     }
 
+    @NotNull
+    B getProvider()
+    {
+        return (B)this;
+    }
+
     protected final @Nullable CapabilityDispatcher getCapabilities()
     {
         if (isLazy && !initialized)
@@ -196,14 +203,18 @@ public abstract class CapabilityProvider<B extends ICapabilityProviderImpl<B>> i
      */
     public static class AsField<B extends ICapabilityProviderImpl<B>> extends CapabilityProvider<B>
     {
-        public AsField(Class<B> baseClass)
+        private final B owner;
+
+        public AsField(Class<B> baseClass, B owner)
         {
             super(baseClass);
+            this.owner = owner;
         }
 
-        public AsField(Class<B> baseClass, boolean isLazy)
+        public AsField(Class<B> baseClass, B owner, boolean isLazy)
         {
             super(baseClass, isLazy);
+            this.owner = owner;
         }
 
         public void initInternal()
@@ -221,6 +232,13 @@ public abstract class CapabilityProvider<B extends ICapabilityProviderImpl<B>> i
         {
             deserializeCaps(tag);
         }
+
+        @Override
+        @NotNull
+        B getProvider()
+        {
+            return owner;
+        }
     };
 
 }

src/main/java/net/minecraftforge/common/capabilities/ICapabilityProviderImpl.java

@@ -1,3 +1,22 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2021.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
 package net.minecraftforge.common.capabilities;
 
 import javax.annotation.Nullable;