Reverts third_party/protobuf: Update to HEAD (f52e188fe4)

third_party/protobuf/java/core/src/main/java/com/google/protobuf/LazyFieldLite.java

 // Protocol Buffers - Google's data interchange format
 // Copyright 2008 Google Inc.  All rights reserved.
 // https://developers.google.com/protocol-buffers/
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
@@ skipping 266 matching lines @@
     // in the case that the extension registries are not the same then we might in the future if we
     // need to serialze and parse a message again.
     if (this.value == null && other.value != null) {
       setValue(mergeValueAndBytes(other.value, this.delayedBytes, this.extensionRegistry));
       return;
     } else if (this.value != null && other.value == null) {
       setValue(mergeValueAndBytes(this.value, other.delayedBytes, other.extensionRegistry));
       return;
     }
 
-    // At this point we have two fully parsed messages.
-    setValue(this.value.toBuilder().mergeFrom(other.value).build());
+    // At this point we have two fully parsed messages. We can't merge directly from one to the
+    // other because only generated builder code contains methods to mergeFrom another parsed
+    // message. We have to serialize one instance and then merge the bytes into the other. This may
+    // drop extensions from one of the messages if one of the values had an extension set on it
+    // directly.
+    //
+    // To mitigate this we prefer serializing a message that has an extension registry, and
+    // therefore a chance that all extensions set on it are in that registry.
+    //
+    // NOTE: The check for other.extensionRegistry not being null must come first because at this
+    // point in time if other.extensionRegistry is not null then this.extensionRegistry will not be
+    // null either.
+    if (other.extensionRegistry != null) {
+      setValue(mergeValueAndBytes(this.value, other.toByteString(), other.extensionRegistry));
+      return;
+    } else if (this.extensionRegistry != null) {
+      setValue(mergeValueAndBytes(other.value, this.toByteString(), this.extensionRegistry));
+      return;
+    } else {
+      // All extensions from the other message will be dropped because we have no registry.
+      setValue(mergeValueAndBytes(this.value, other.toByteString(), EMPTY_REGISTRY));
+      return;
+    }
   }
   
   /**
    * Merges another instance's contents from a stream.
    *
    * <p>LazyField is not thread-safe for write access. Synchronizations are needed
    * under read/write situations.
    */
   public void mergeFrom(CodedInputStream input, ExtensionRegistryLite extensionRegistry)
       throws IOException {
@@ skipping 129 matching lines @@
 
   private static void checkArguments(ExtensionRegistryLite extensionRegistry, ByteString bytes) {
     if (extensionRegistry == null) {
       throw new NullPointerException("found null ExtensionRegistry");
     }
     if (bytes == null) {
       throw new NullPointerException("found null ByteString");
     }
   }
 }