[jbosscache-issues] [JBoss JIRA] Created: (JBCACHE-1556) A race to "children loaded" flag in CacheLoadedInterceptor.loadChildren()

Fri Nov 13 09:43:25 EST 2009

A race to "children loaded" flag in CacheLoadedInterceptor.loadChildren()
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                 Key: JBCACHE-1556
                 URL: https://jira.jboss.org/jira/browse/JBCACHE-1556
             Project: JBoss Cache
          Issue Type: Bug
      Security Level: Public (Everyone can see)
          Components: Cache loaders, Transactions
    Affects Versions: 3.2.1.GA
            Reporter: Krzysztof Sobolewski
            Assignee: Manik Surtani
         Attachments: ChildrenLoadingRaceTest-fix.diff, ChildrenLoadingRaceTest.java

We found a race in CacheLoaderInterceptor.loadChildren() that goes like this:

1) There is a  node /a with evicted children /a/a and /a/b; the node itself is *not* evicted
2) Transaction T1 wants to get the children list of /a
3) Transaction T2 wants to get the children list of /a
4) T1 gets to loadChildren() for /a, notices that the children are not loaded and proceeds to load them
4a) T1 loads the children and sets "children loaded" flag to true, but holds the names themselves in its context [this depends on isolation level, I guess]
5) T2 gets to the same point and sees that "children loaded" flag is set, so it skips loading
6a) T1 correctly returns "a" and "b"
6b) T2 returns whatever was in memory at that point [in the attached test it's "a", but I would expect empty set]

I fixed it by acquiring a lock on the parent node, forcing both transactions to serialize the access (but not preventing from loading the children twice).
The patch depends on the one proposed for JBCACHE-1553 (it's in the same place and they interact in non-trivial way)
Also note that the "lock parent for child insert remove" config option does not affect the result of the test.

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