On Sat, 20 Nov 2021 10:08:41 GMT, Jaikiran Pai <j...@openjdk.org> wrote:
> Can I please get a review for this change which proposes to fix the issue > reported in https://bugs.openjdk.java.net/browse/JDK-8003417? > > The issue notes that this is applicable for `WeakHashMap` which have `null` > keys. However, the issue is even applicable for `WeakHashMap` instances which > don't have `null` keys, as reproduced and shown by the newly added jtreg test > case in this PR. > > The root cause of the issue is that once the iterator is used to iterate till > the end and the `remove()` is called, then the > `WeakHashMap$HashIterator#remove()` implementation used to pass `null` as the > key to remove from the map, instead of the key of the last returned entry. > The commit in this PR fixes that part. > > A new jtreg test has been added which reproduces the issue as well as > verifies the fix. > `tier1` testing and this new test have passed after this change. However, I > guess this will require a JCK run to be run too, perhaps? If so, I will need > help from someone who has access to them to have this run against those > please. Changes requested by bchristi (Reviewer). src/java.base/share/classes/java/util/WeakHashMap.java line 826: > 824: throw new ConcurrentModificationException(); > 825: > 826: WeakHashMap.this.remove(lastReturned.get()); Unlike `currentKey`, `HashIterator` does not necessarily maintain a strong reference to the referent of `lastReturned`. If the `lastReturned` WeakReference were cleared between `lastReturned` being set, and the `remove()` call, the null key could again be erroneously removed. It would be cool to add a test case for this (maybe using large objects as keys would tempt the GC to clear the weakrefs via `jdk.test.lib.util.ForceGC`). Since we store the `NULL_KEY` sentinel value for the null key, I believe `Entry.get()` will only return null in the case that the weakref has been cleared. So could we instead fix this with: if (lastReturned.get() != null) { WeakHashMap.this.remove(lastReturned.get()); } ? Or, if we're worried about the ref being cleared between the null check and remove(), maybe: Object lastReturnedKey = lastReturned.get(); if (lastReturnedKey != null) { WeakHashMap.this.remove(lastReturned.get()); } ------------- PR: https://git.openjdk.java.net/jdk/pull/6488
