[ruby-core:98630] [Ruby master Bug#15711] Remove use of _id2ref from DRb

Issue #15711 has been updated by Eregon (Benoit Daloze).


The commit 8980b53a48b1f55e09c5223008225e6bfa765405 added a new class WeakIdConv, but _id2ref is still used in `lib/drb/drb.rb`.

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Bug #15711: Remove use of _id2ref from DRb
https://bugs.ruby-lang.org/issues/15711#change-85954

* Author: headius (Charles Nutter)
* Status: Closed
* Priority: Normal
* Assignee: seki (Masatoshi Seki)
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
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This issue relates to https://bugs.ruby-lang.org/issues/15408

DRb uses `_idref` internally to implement a weak map, and this issue seeks to replace that code with an implementation that does not use `_id2ref`.

We will be deprecating `ObjectSpace._id2ref` in the near future since it fails to work like people expect (when implemented as a pointer address) or adds memory and invocation overhead to `object_id`.

An initial patch for this is provided by JRuby, which implements `object_id` using a monotonically-increasing value, and only allows `_id2ref` use with a command line flag.

https://github.com/ruby/ruby/compare/trunk...jruby:jruby-ruby_2_6_0#diff-e979bf2f831d9826629559b8628809e9

This implementation uses the stdlib `weakref` to implement a simple weak map, and it would be suitable as an implementation for now. However there's some inefficiency here because it has to periodically "clean" the hash of vacated references by scanning all entries.

There are two more efficient implementations that require additional work:

Alternate 1: Use `ObjectSpace::WeakMap`, which is an opaque VM-supported implementation of a weak Hash. Unfortunately I don't think `WeakMap` has ever been blessed as a public API, and since we're rapidly moving standard libraries to gems, it would not be appropriate to use an internal API. So, we can either make WeakMap an official part of the public standard API, or do alternate 2.

Alternate 2: Add weak reference queues to the weakref API, so users can implement their own efficient weak maps. Some of this has been discussed (at great length) in https://bugs.ruby-lang.org/issues/4168, and the JRuby team has supported the [weaklink](https://github.com/headius/weakling) gem for many years (which provides a WeakRef+RefQueue implementation for JRuby).

The original patch works well for small numbers of remoted objects.



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