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From "Josh Elser (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ACCUMULO-3555) TabletServerBatchReaderIterator doesn't maintain reference to TabletServerBatchReader
Date Tue, 03 Feb 2015 15:49:35 GMT

    [ https://issues.apache.org/jira/browse/ACCUMULO-3555?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14303470#comment-14303470
] 

Josh Elser commented on ACCUMULO-3555:
--------------------------------------

(Speaking for [~bills] some more) One good argument for making such a change would be the
following code.

{code}
ArrayList<String> list = new ArrayList<>();
list.addAll(Arrays.asList("1", "2", "3"));
Iterator<String> iterator = list.iterator();
doThings(iterator);
{code}

This code is perfectly safe and normal Java code to write. 

{code}
BatchScanner bs = conn.createBatchScanner(tableName, Authorizations.EMPTY, 4);
bs.setRanges(Collections.singleton(new Range()));
Iterator<Entry<Key,Value>> iterator = bs.iterator();
doThings(iterator);
{code}

While the above is unsafe. Yes, we know that the BatchScanner may not get closed; however,
it has noticeably different semantics than the above version using an ArrayList.

One potential change would be to introduce the reference in the TSBRI to TSBR and then add
a hook to automatically close() the TSBR when the TSBRI exhausts itself. This would allow
users to treat the Iterator<Entry<Key,Value>> more like a "normal" Iterator from
a List. It still may let users avoid thinking about edge cases in their client code.

> TabletServerBatchReaderIterator doesn't maintain reference to TabletServerBatchReader
> -------------------------------------------------------------------------------------
>
>                 Key: ACCUMULO-3555
>                 URL: https://issues.apache.org/jira/browse/ACCUMULO-3555
>             Project: Accumulo
>          Issue Type: Bug
>          Components: client
>    Affects Versions: 1.5.0, 1.5.1, 1.5.2, 1.6.0, 1.6.1
>            Reporter: Josh Elser
>            Assignee: Josh Elser
>            Priority: Blocker
>
> Had a user in IRC run into this again today upgrading a 1.4 instance to 1.6.0.
> ACCUMULO-587 introduced a {{finalize}} implementation into {{TabletServerBatchReader}}
in an attempt to close the {{BatchScanner}} when the user might have forgotten to do so themselves.
The problem, however, is that the {{TabletServerBatchReaderIterator}} doesn't maintain a reference
to the {{TabletServerBatchReader}} (notice how it only uses it to create a new instnace of
{{ScannerOptions}} using the copy constructor).
> In other words, when the {{TabletServerBatchReaderIterator}} is constructed, it has no
references in the object graph to the {{TabletServerBatchReader}} it was created from. This
means that if clients don't hold onto the BatchScanner instance, it's possible that it gets
closed by the JVM calling {{finalize()}}.



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