This is the preferred mysql specific fix, I am opening another pull request with the vendor neutral fix.

Abstract

A bug exists in JSessionStorageDatabase::write() (libraries/joomla/session/storage/database.php:94). The logic assumes that an update will have one affected row if a session exists. However, this is not true. The timestamp is only granular in seconds and may be the same with multiple ajax requests, refreshes on different tabs, etc. When the timestamp is the same the entry is exactly the same and the update will not affect any rows. With the current logic an insert will be attempted and this results in a 500.

The fix

This fix uses the mysql specific ON DUPLICATE KEY UPDATE. I consider this the preferred fix because it reduces the sql statements to one. I have however prepared a second vendor neutral fix that uses an additional select statement.

Example Error

500 - JDatabaseMySQLi::query: 1062 - Duplicate entry 'e33c3ab2b05a9dc97dc25187a823b8e6' for key 'PRIMARY' SQL=INSERT INTO `jos_session` (`session_id`, `data`, `time`) VALUES ('e33c3ab2b05a9dc97dc25187a823b8e6', '__default|a:8:{s:22:\"session.client.browser\";s:119:\"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_6_8) AppleWebKit/535.19 (KHTML, like Gecko) Chrome/18.0.1025.54 Safari/535.19\";s:15:\"session.counter\";i:43;s:8:\"registry\";O:9:\"JRegistry\":1:{s:7:\"\0*\0data\";O:8:\"stdClass\":0:{}}s:4:\"user\";O:5:\"JUser\":23:{s:9:\"\0*\0isRoot\";b:0;s:2:\"id\";i:0;s:4:\"name\";N;s:8:\"username\";N;s:5:\"email\";N;s:8:\"password\";N;s:14:\"password_clear\";s:0:\"\";s:8:\"usertype\";N;s:5:\"block\";N;s:9:\"sendEmail\";i:0;s:12:\"registerDate\";N;s:13:\"lastvisitDate\";N;s:10:\"activation\";N;s:6:\"params\";N;s:6:\"groups\";a:0:{}s:5:\"guest\";i:1;s:10:\"\0*\0_params\";O:9:\"JRegistry\":1:{s:7:\"\0*\0data\";O:8:\"stdClass\":0:{}}s:14:\"\0*\0_authGroups\";a:1:{i:0;i:1;}s:14:\"\0*\0_authLevels\";a:2:{i:0;i:1;i:1;i:1;}s:15:\"\0*\0_authActions\";N;s:12:\"\0*\0_errorMsg\";N;s:10:\"\0*\0_errors\";a:0:{}s:3:\"aid\";i:0;}s:13:\"session.token\";s:32:\"cc3f66462deb72a899115b2ea51f8608\";s:19:\"session.timer.start\";i:1331743210;s:18:\"session.timer.last\";i:1331744856;s:17:\"session.timer.now\";i:1331744857;}', '1331744857')