Ruby: Implement Write Barriers

[casperisfine]

Write barrier protected objects are allowed to be promoted to the old generation, which means they only get marked on major GC.

The downside is that the RB_BJ_WRITE macro MUST be used to set references, otherwise the referenced object may be garbaged collected.

But the *Descriptor classes and Arena have very few references and are only set in a few places, so it's relatively easy to implement.

cc @peterzhu2118

[casperisfine]

I just noticed the "Message" struct was also from protobuf, and easy to add write barriers for.

For context, here's we have a couple thousands of these instances in our heap that need to be mark on every minor GC:

 ["DATA(Google::Protobuf::Internal::Arena)", 1268],
 ["DATA(Message)", 1268],
...
 ["DATA(Google::Protobuf::EnumDescriptor)", 77],
 ["DATA(Google::Protobuf::FileDescriptor)", 68],

[casperisfine]

@JasonLunn @haberman sorry for the ping, but is this on your radar? We have about 3.5k of these objects on our heap, and we'd love if we didn't have to mark them on every minor GC.

[haberman]

It looks like there is an assertion failure on Ruby 2.7:

....ruby: ruby/ext/google/protobuf_c/protobuf.c:404: void ObjectCache_Add(const void *, VALUE): Assertion `ObjectCache_Get(key) == val' failed.
Aborted (core dumped)

Also could you rebase the PR from main? There have been some fixes to the tests in the meantime.

[haberman]

It would be helpful to add comments above the struct members that must only be changed through RB_OBJ_WRITE(), to caution future readers/writers of this invariant.

[casperisfine]

@haberman done. I rebased and added a comment above each reference to WB_PROTECTED.

[haberman]

Also could you rebase once more (sorry for the trouble, you caught us in the middle of a migration to GitHub Actions).

[casperisfine]

@haberman done

[casperisfine]

Sorry to be a bother, but is there any other blockers I can help with?

[haberman]

This was merged in d82d8a4

[casperisfine]

🙌

Thank you!