[ruby-core:98776] [Ruby master Bug#16959] Weakmap has specs and third-party usage despite being a private API

Issue #16959 has been updated by headius (Charles Nutter).


> So I don't think it's right to blame the specs here. The specs reflect reality.

I'm not sure I'm seeing what you're seeing, since the specs appear to have been created and merged in mid-April and the Rails PR was created in May.

https://github.com/ruby/spec/commits/dd8437628a6f2de5b74b338d4960682bb1590a60/core/objectspace/weakmap

I can't tell if the merging of those specs in any way led to the Rails usage, so I'll retract that theory. There certainly are many other uses of WeakMap that predate the specs, including the Mustermann usage that (presumably) led to the specs being created.

The specs should reflect official, public Ruby APIs... not private APIs that were added unilaterally by one implementation without any discussion. We should always be vigilant about hidden features of Ruby becoming de facto standards, given the mess that RubyVM has become for alternative Rubies.

JRuby isn't entirely blameless here, since we implemented WeakMap shortly after it was introduced almost a decade ago (mostly in hopes of sharing the pure-Ruby weakref.rb, which didn't end up panning out). WeakMap itself was actually introduced because I and others proved that the previous implementation of weakref.rb was fundamentally broken. We did not push for specs then largely because discussions were still ongoing about how to support weak references in Ruby.

We should really make Weakref a core API and build WeakMap on top of it (in pure Ruby)... not the other way around.

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Bug #16959: Weakmap has specs and third-party usage despite being a private API
https://bugs.ruby-lang.org/issues/16959#change-86133

* Author: headius (Charles Nutter)
* Status: Open
* Priority: Normal
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
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Weakmap is still described as an internal API, and the documentation points users at WeakRef as the official public API:

https://github.com/ruby/ruby/blob/1fb16dbb6e28b9f32f92554d29e646e088b21a98/gc.c#L11928-L11936

However there are now specs for its current set of features, even though those features have never been discussed or approved as a public API:

https://github.com/ruby/spec/tree/dd8437628a6f2de5b74b338d4960682bb1590a60/core/objectspace/weakmap

And we are starting to see it being used by the community:

* https://github.com/jruby/jruby/issues/6267
* https://github.com/rsim/oracle-enhanced/issues/2027 
* https://github.com/rails/rails/pull/39121

One of two things needs to happen:

* Weakmap is made a public API after some discussion. It would be an official public feature only in 2.8/3.0 or higher.
* The specs are be removed and Weakmap remains a private API not to be used by the community. I suspect the addition of the specs led to folks starting to use this private API.

Personally, I'm in much more in favor of making WeakRef support all the features necessary to implement Weakmap in pure Ruby, rather than the other way around:

https://bugs.ruby-lang.org/issues/6309

But whatever happens it needs to happen soon, since this use case is now a merged feature in Rails master.



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