Issue #22251 has been updated by Eregon (Benoit Daloze). Both Java's `WeakHashMap` and JRuby's variant clean references in size(), so there is no issue there: https://github.com/openjdk/jdk/blob/05eaf783b270ec5cd901704c5e16b6201de7a9e4... https://github.com/jruby/jruby/blob/4d041a906d427b7fee577b9a2938e2fcda6c5522... I'm well aware it's tricky to GC an object reliably, but that's not the point of this issue, it's about consistency in what gets reported. ---------------------------------------- Bug #22251: ObjectSpace::WeakMap#size can return a stale value https://bugs.ruby-lang.org/issues/22251#change-118599 * Author: Eregon (Benoit Daloze) * Status: Open * ruby -v: ruby 4.0.6 (2026-07-14 revision 03b6d3f889) +PRISM [arm64-darwin25] * Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN ---------------------------------------- ``` $ ruby -e 'm = ObjectSpace::WeakMap.new; Thread.new { m[Object.new] = Object.new }.join; GC.start; p m.size; p m.keys; p m.size' 1 [] 0 ``` The first line is 1 but I would expect 0. Interestingly calling `#keys` seems to force an update. If I GC twice then it seems correct: ``` $ ruby -e 'm = ObjectSpace::WeakMap.new; Thread.new { m[Object.new] = Object.new }.join; GC.start; GC.start; p m.size; p m.keys; p m.size' 0 [] 0 ``` Could we decrease size whenever we "remove" an entry? Apologies if that's already known, I searched for an existing issue for this but didn't find anything. -- https://bugs.ruby-lang.org/