Backport new constant for Marshal.load(record.dump) forward compatibility

[kamipo]

The purpose of backporting #39612 is for Marshal.load(record.dump)
forward compatibility.

Actually we never said we would keep Mashal compatibility between two
versions of Rails, so we sometimes directly added and removed the
internal objects. Theoretically, removing objects will lose backward
compatibility and adding objects will lose forward compatibility.

Example:

• Rails 4.2/4.1 lost forward/backward compatibility due to entirely changed object layout
• Rails 5.0/4.2 lost backward compatibility due to removed MysqlDateTime f1a0fa9
• Rails 5.2/5.1 lost forward compatibility due to added SQLite3Integer SQLite3 valid integer value should be 8 bytes (64-bit signed integer) #28379
• Rails 5.2/5.1 lost forward/backward compatibility due to added Type::Json and removed MysqlJson/OID::Json Deprecate database specific JSON types #29666

I'm not sure how much we should be aware of Mashal.load compatibility.
Especially if we should keep perfectly forward compatibility, we almost
have no chance to improve object layout (includes ivar usage).

One possible idea to mitigate breaking forward compatibility is
backporting new constant to the latest stable branch.

[rafaelfranca]

I'm not sure we should backport changes to stable branches to keep backwards compatibly with Marshal. Why did you decided to do this? I don't remember us trying to keep Marshal compatibility, but we definitely need to know when YAML dump changes.

[rafaelfranca]

I remember now. I commented on #39612. I think it is fine to break Marshal.load compatibility, between minor versions of Rails. For us that we are always running on Rails master we can't invalidate caches based on the Rails version since it is always the same. I think the right solution for my problem is to use the git sha as the Rails version and not Rails backport commits to stable releases just to keep them compatible. I think you can revert this.

[kamipo]

Sure, I'll revert this.

[kamipo]

Reverted 60776ef.

[rafaelfranca]

Thank @kamipo. And sorry for the confusion.