[ruby-core:44857] [ruby-trunk - Feature #6309] Add a reference queue for weak references

Issue #6309 has been updated by headius (Charles Nutter).


I linked to a concrete use case in the original report...an implementation of a "weak ID map" entirely in Ruby without scanning for dead references:  https://github.com/headius/weakling/blob/master/lib/weakling/collections.rb

It is not possible to implement weak data structures efficiently without a reference queue, since you would be forced to periodically do an O(N) scan for dead references to clean them out.
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Feature #6309: Add a reference queue for weak references
https://bugs.ruby-lang.org/issues/6309#change-26432

Author: headius (Charles Nutter)
Status: Assigned
Priority: Normal
Assignee: matz (Yukihiro Matsumoto)
Category: 
Target version: 


Most interesting uses of WeakRef are much harder to do efficiently without a reference queue.

A reference queue, as implemented by the JVM, is basically a queue into which weak references are placed some time after the object they refer to has been collected. The queue can be polled cheaply to look for collected references.

A simple example of usage can be seen in the weakling gem, with an efficient implementation of an ID hash: https://github.com/headius/weakling/blob/master/lib/weakling/collections.rb

Notice the _cleanup method is called for every operation, to keep the hash clear of dead references. Failure to have a _cleanup method would mean the hash grows without bounds.

_cleanup cannot be implemented efficiently on MRI at present because there's no reference queue implementation. On MRI, _cleanup would have to perform a linear scan of all stored values periodically to search for dead references. For a heavily used hash with many live values, this becomes a very expensive operation.

It's probably possible to implement reference queues efficiently atop the new ObjectSpace::WeakMap internals, since it already keeps track of weak references and can run code when a weak reference no longer refers to a live object.


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