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https://issues.apache.org/jira/browse/LUCENE-5583?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13964245#comment-13964245
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Adrien Grand commented on LUCENE-5583:
--------------------------------------
I didn't think much about the thread-local, I just thought it would be nice to
minimize the number of buffer instances. I will make it work like copyBytes for
consistency.
> Should BufferedChecksumIndexInput have its own buffer?
> ------------------------------------------------------
>
> Key: LUCENE-5583
> URL: https://issues.apache.org/jira/browse/LUCENE-5583
> Project: Lucene - Core
> Issue Type: Bug
> Affects Versions: 4.8
> Reporter: Adrien Grand
> Priority: Blocker
> Fix For: 4.8, 5.0
>
> Attachments: LUCENE-5583.patch
>
>
> I was playing with on-the-fly checksum verification and this made me stumble
> upon an issue with {{BufferedChecksumIndexInput}}.
> I have some code that skips over a {{DataInput}} by reading bytes into
> /dev/null, eg.
> {code}
> private static final byte[] SKIP_BUFFER = new byte[1024];
> private static void skipBytes(DataInput in, long numBytes) throws
> IOException {
> assert numBytes >= 0;
> for (long skipped = 0; skipped < numBytes; ) {
> final int toRead = (int) Math.min(numBytes - skipped,
> SKIP_BUFFER.length);
> in.readBytes(SKIP_BUFFER, 0, toRead);
> skipped += toRead;
> }
> }
> {code}
> It is fine to read into this static buffer, even from multiple threads, since
> the content that is read doesn't matter here. However, it breaks with
> {{BufferedChecksumIndexInput}} because of the way that it updates the
> checksum:
> {code}
> @Override
> public void readBytes(byte[] b, int offset, int len)
> throws IOException {
> main.readBytes(b, offset, len);
> digest.update(b, offset, len);
> }
> {code}
> If you are unlucky enough so that a concurrent call to {{skipBytes}} started
> modifying the content of {{b}} before the call to {{digest.update(b, offset,
> len)}} finished, then your checksum will be wrong.
> I think we should make {{BufferedChecksumIndexInput}} read into a private
> buffer first instead of relying on the user-provided buffer.
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