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From "Francesco Mari (JIRA)" <j...@apache.org>
Subject [jira] [Created] (OAK-7721) Records of specific size bring SegmentBufferWriter#flush to fail
Date Wed, 29 Aug 2018 16:19:00 GMT
Francesco Mari created OAK-7721:
-----------------------------------

             Summary: Records of specific size bring SegmentBufferWriter#flush to fail
                 Key: OAK-7721
                 URL: https://issues.apache.org/jira/browse/OAK-7721
             Project: Jackrabbit Oak
          Issue Type: Bug
            Reporter: Francesco Mari
            Assignee: Francesco Mari
             Fix For: 1.10


In a handful of cases, {{SegmentBufferWriter#flush}} has been observed to fail with an ISE
without any other part of the system behaving incorrectly or suspiciously. This problem was
reported before (OAK-6452) but we were unable to reproduce it.

I investigated further and was able to reproduce the problem reliably in a test case. The
trick is to craft a record of a very specific length that fools the {{SegmentBufferWriter}}
into believing that it has enough space to store the record, even if the record consumes all
the available space in the buffer, including the space reserved for the segment header. This
is the reason why no call to {{SegmentBufferWriter#prepare}} fails, but calls to {{SegmentBufferWriter#flush}}
do.

The attached test case executes the following steps.
* The test creates a new {{SegmentBufferWriter}}. The buffer is fresh, but it is immediately
populated with the segment info, which is 38 bytes long. The size of the segment info is aligned
to 40 bytes, so at the end of the initialization the segment contains 40 bytes of record data.
Please note that at this point in time the buffer is not marked as dirty.
* The test prepares space for a block record of 262101 bytes, which is aligned to 262104 bytes.
The code in in {{prepare()}} figures out that adding this record to the current segment would
exceed the maximum size of 262144 bytes, so it flushes the current segment.
* The flush operation is skipped because the buffer is not dirty yet. The segment is not flushed.
* The code in {{prepare()}} resumes execution. Trusting that it's working on a fresh segment,
big enough to contain the new record, {{prepare()}} updates the state of the {{SegmentBufferWriter}}.
In particular, {{length}} is set to exactly {{262144}} and {{position}} to {{0}}. Even if
the record consumes all the available space in the buffer, {{prepare()}} is happy to go on
because {{position}} remains nonnegative.
* The test flushes the {{SegmentBufferWriter}}. When the size of the resulting segment is
computed, the size of the segment header is added to the size of the records. The size of
the records is {{262144}} bytes, which is exactly the segment maximum size, and adding the
size of the segment header obviously exceeds the maximum size for a segment. At this point,
{{flush()}} fails with an ISE.



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