Abhishek Rai updated ZOOKEEPER-2574:
    Attachment: ZOOKEEPER-2574.5.patch

Thanks [~abrahamfine].

>> I switched logsToPurge from a List<File> to an ArrayList<File> so I can 
>> simply use remove(0) to remove the first element in the list on line 239
> I think I must be missing something as all of the lists are ArrayLists. For 
> example, this still passes:

Sorry I was confused about something, fixed the usage of logsToPurge as you 
suggested, thanks for persisting.

>> Is there a way to achieve both goals, logging and console output (preferably 
>> stdout) without any duplication.
> I'm not sure, perhaps system.err?

I tried System.err.println, but then this output comes at the end of the test 
log under "stderr" section.  It may have limited utility in debugging since 
it's not inline with other related logging.


> PurgeTxnLog can inadvertently delete required txn log files
> -----------------------------------------------------------
>                 Key: ZOOKEEPER-2574
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2574
>             Project: ZooKeeper
>          Issue Type: Bug
>          Components: server
>    Affects Versions: 3.4.7, 3.4.8, 3.5.0, 3.5.1, 3.5.2
>         Environment: Zookeeper 3.4.8, standalone, and 3-server quorum
>            Reporter: Abhishek Rai
>            Assignee: Abhishek Rai
>            Priority: Blocker
>             Fix For: 3.4.10, 3.5.3
>         Attachments: ZOOKEEPER-2574.2.patch, ZOOKEEPER-2574.3.patch, 
> ZOOKEEPER-2574.4.patch, ZOOKEEPER-2574.5.patch, ZOOKEEPER-2574.patch
> As part of the fix for ZOOKEEPER-1797, the call to 
> FileTxnSnapLog.getSnapshotLogs() was removed from PurgeTxnLog.java.  As a 
> result, some old-looking but required txn log files can be deleted, resulting 
> in data corruption or loss.
> For example, consider the following:
> 1. Configuration:
> autopurge.snapRetainCount=3
> 2. Following files exist:
> log.100 spans transactions from zxid=100 till zxid=140 (inclusive)
> snapshot.110 - snapshot as of zxid=110
> snapshot.120 - snapshot as of zxid=120
> snapshot.130 - snapshot as of zxid=130
> Above scenario is possible when snapshotting has happened multiple times but 
> without accompanying log rollover, which is possible if the server was 
> running as a learner.
> 3. PurgeTxnLog retains all snapshots but deletes log.100 because its zxid is 
> older than the zxid of the oldest snapshot (110).  This results in loss of 
> transactions in the range 131-140.
> Before the fix for ZOOKEEPER-1797, this was avoided by the call to 
> FileTxnSnapLog.getSnapshotLogs() which finds and retains the newest txn log 
> file with starting zxid < oldest retained snapshot's highest zxid.

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