jiajunwang opened a new pull request #1107:
URL: https://github.com/apache/helix/pull/1107
### Issues
- [X] My PR addresses the following Helix issues and references them in the
PR description:
#767
### Description
- [X] Here are some details about my PR, including screenshots of any UI
changes:
Concurrent modification causes two issues.
1. Regular GC task fails due to concurrent list modification and the stale
versions are not removed at all.
2. If, by coincident, there is a newer version in the list other then the
current version, then because of the modification of the list inside the loop,
the final element (the newer version) won't be filtered but being left in the
to-be-removed list. Then the GC task removes the most recent version. For
example,
a) Input, current version "2"
b) Children = [1, 2, 3]
c) The task avoids checking "2", so the list for loop is: [1, 3]
d) When checking "1", it is removed from the list. So the list becomes
[3]. Then the loop ends, because the first item has already been looped from
the for iteration perspective.
e) The version to be removed is "3"!
This PR fixes the issue by avoiding concurrent modifications. Also, it
simplifies the logic so as to reduce the pending GC tasks.
The test is also updated accordingly.
### Tests
- [X] The following tests are written for this issue:
TestZkBucketDataAccessor
- [ ] The following is the result of the "mvn test" command on the
appropriate module:
Running
### Commits
- [X] My commits all reference appropriate Apache Helix GitHub issues in
their subject lines. In addition, my commits follow the guidelines from "[How
to write a good git commit message](http://chris.beams.io/posts/git-commit/)":
1. Subject is separated from body by a blank line
1. Subject is limited to 50 characters (not including Jira issue reference)
1. Subject does not end with a period
1. Subject uses the imperative mood ("add", not "adding")
1. Body wraps at 72 characters
1. Body explains "what" and "why", not "how"
### Documentation (Optional)
- [ ] In case of new functionality, my PR adds documentation in the
following wiki page:
(Link the GitHub wiki you added)
### Code Quality
- [X] My diff has been formatted using helix-style.xml
(helix-style-intellij.xml if IntelliJ IDE is used)
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