Adjust serialization of VersionValue to make continuations compatible with versions older than 4.0.564.0 (#3179)

The serialization of the `VersionValue` was modified by #2943. However,
there was a flaw with the serialization, namely that it added a new
field. For background, there are certain planner objects that for space
reasons only get serialized fully the first time they are encountered
during a query (for example, `Type` definitions). When serializing the
new field in the `PVersionValue` message, if there were one of those
objects, we could end up inserting a value that if referenced would
later would only include the reference and not the data itself. That
meant that when _deserializing_ one of these continuations on older
versions, the older code would skip over the new field (not knowing what
it is for) and then encounter the reference for the very first time
somewhere else. Because it hadn't seen the full definition before, it
didn't know what to do and threw an error.

This addresses the issue by making the older serialization more closely
mirror the old code. In the future, once we no longer care about being
compatible with pre-4.0.564.0, we can drop the legacy mode, though we'll
have to keep the deserialization path for an additional cycle. I did
consider a slightly more invovled solution, where we adjust the
serialization path to avoid adding to the reference registry for this
field, and I got something that worked, but it ended up being a little
more fiddly than I'd like, so I went with this conceptually simpler
change. This does have the downside that the `Type` information gets
erased after a continuation, a disadvantage the other approach would not
have, but we have to be able to handle that anyway to correctly handle
upgrading continuations from older versions.

I've been able to validate locally that the updated continuations work,
and that the failing mixed-mode tests now pass.

This fixes #3178.

---------

Co-authored-by: Youssef Hatem <[email protected]>

Author: alecgrieser

fdb-record-layer-core/src/main/java/com/apple/foundationdb/record/query/plan/cascades/values/VersionValue.java

@@ -149,10 +149,23 @@ public boolean equals(final Object other) {
     @Nonnull
     @Override
     public PVersionValue toProto(@Nonnull final PlanSerializationContext serializationContext) {
-        return PVersionValue.newBuilder()
-                .setBaseAlias(getChildQuantifiedRecordValue().getAlias().getId())  // deprecated
-                .setChild(childValue.toValueProto(serializationContext))
-                .build();
+        if (childValue instanceof QuantifiedRecordValue) {
+            // Deprecated. This value previously required that the child be a QuantifiedRecordValue and only
+            // contained the alias. To preserve cross-version compatibility with versions older than 4.0.561.0,
+            // send messages without the full value.
+            // Note: it's tempting to also include the full value in the `child` field, but doing so can
+            // lead to mixed-mode errors. If there are types or plans referenced in the child, those can
+            // be registered with the serializationContext, which means they may only be included
+            // by reference elsewhere in the serialized plan. Older versions of the deserialization
+            // logic don't know about the field, and so they will trip over any dangling references
+            return PVersionValue.newBuilder()
+                    .setBaseAlias(getChildQuantifiedRecordValue().getAlias().getId())
+                    .build();
+        } else {
+            return PVersionValue.newBuilder()
+                    .setChild(childValue.toValueProto(serializationContext))
+                    .build();
+        }
     }
 
     @Nonnull