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[CCACHE-28] SoftCache NullPointerException in has? with many threads Created: 24/Oct/12  Updated: 09/Feb/13

Status: Open
Project: core.cache
Component/s: None
Affects Version/s: None
Fix Version/s: None

Type: Defect Priority: Major
Reporter: James Aley Assignee: Fogus
Resolution: Unresolved Votes: 0
Labels: None
Environment:

Linux, Oracle JRE 1.6


Attachments: Text File 0001-CCACHE-28-fixes-concurrency-bug-in-has.patch    
Patch: Code

 Description   

"cell" can be null on this call to SoftReference.get():
https://github.com/clojure/core.cache/blob/master/src/main/clojure/clojure/core/cache.clj#L501

If the cache did not contain "item" in the call to (get cache item) in the let binding, but another thread pre-empts and adds it before the subsequent call to contains?, we'll attempt to call .get() on nil.

Would it perhaps be sufficient to just check whether cell is nil instead of calling contains? in the check on the previous line?



 Comments   
Comment by Paul Stadig [ 09/Feb/13 6:13 AM ]

Good catch!

This is rather simple to reproduce:

(use 'clojure.core.cache)
(def cache (soft-reference-cache {}))
(def mutator (Thread. #(loop [] (assoc cache :foo :bar) (dissoc cache :foo) (recur))))
(.start mutator)
(loop [] (has? cache :foo) (recur))

The attached patch fixes the issue, by not referring back to the cache after attempting to pull out a SoftReference.

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