MySQL Bugs: #51034: NdbApi : Read of blob data with lockmode LM

Bug #51034	NdbApi : Read of blob data with lockmode LM_SimpleRead does not upgrade
Submitted:	9 Feb 2010 17:07	Modified:	3 Mar 2010 5:51
Reporter:	Frazer Clement	Email Updates:
Status:	Closed	Impact on me:	None
Category:	MySQL Cluster: NDB API	Severity:	S3 (Non-critical)
Version:	mysql-5.1-telco-6.2+	OS:	Any
Assigned to:	Frazer Clement	CPU Architecture:	Any

Description:
Due to multi-row nature of the Blob implementation, an LM_Read lock must be taken on the Blob head row and held until after read operations on the Blob part table rows have completed.  If this is not the case then reads of Blob parts can return an inconsistent set of snapshots of Blob parts and / or result in transaction rollback.

Bug#19285 implements an automatic lock-upgrade from LM_CommittedRead to LM_Read.

However, there is currently no automatic upgrade from LM_SimpleRead to LM_Read.

This means that a Blob access using LM_SimpleRead may return part data from different concurrently committing update transactions.  If the Blob is shortened by a concurrent transaction then the Blob read may result in transaction abort. 

Note that MySQLD does not currently use LM_SimpleRead lockmode, so this bug only affects NdbApi programs.

How to repeat:
- Modify testBlobs to also test LM_SimpleRead lock upgrade.

Suggested fix:
Modify Blobs code to perform automatic lock upgrade for LM_SimpleRead as well as LM_CommittedRead.

Proposed patch and testcase improvements

Attachment: blob-simpleread-fix.patch (text/x-patch), 9.81 KiB.

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/101449

3082 Frazer Clement	2010-02-25
      Bug#51034 NdbApi : Read of blob data with lockmode LM_SimpleRead does not upgrade
      
      Modify NdbApi to upgrade SimpleRead when reading Blob(s)

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/101605

3083 Frazer Clement	2010-02-26
      Bug#51034 NdbApi : Read of blob data with lockmode LM_SimpleRead does not upgrade
      
      Modify NdbApi to upgrade SimpleRead when reading Blob(s)

Pushed to : 
  6.2.19
  6.3.32
  7.0.13
  7.1.2

Documented as follows in the NDB-6.2.19, 6.3.32, 7.0.13, and 7.1.2 changelogs:

      When reading blob data with lock mode LM_SimpleRead, the lock was not
      upgraded as expected.

Closed.

NB: Fix went into 6.3.33/7.0.14, and not 6.3.32/7.0.13.

Changelog entry tagged with correct versions.