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If a tuple is locked, and this lock is later upgraded either to an update or to a stronger lock, and in the meantime some other process tries to lock, update or delete the same tuple, it (the tuple) could end up being updated twice, or having conflicting locks held. The reason for this is that the second updater checks for a change in Xmax value, or in the HEAP_XMAX_IS_MULTI infomask bit, after noticing the first lock; and if there's a change, it restarts and re-evaluates its ability to update the tuple. But it neglected to check for changes in lock strength or in lock-vs-update status when those two properties stayed the same. This would lead it to take the wrong decision and continue with its own update, when in reality it shouldn't do so but instead restart from the top. This could lead to either an assertion failure much later (when a multixact containing multiple updates is detected), or duplicate copies of tuples. To fix, make sure to compare the other relevant infomask bits alongside the Xmax value and HEAP_XMAX_IS_MULTI bit, and restart from the top if necessary. Also, in the belt-and-suspenders spirit, add a check to MultiXactCreateFromMembers that a multixact being created does not have two or more members that are claimed to be updates. This should protect against other bugs that might cause similar bogus situations. Backpatch to 9.3, where the possibility of multixacts containing updates was introduced. (In prior versions it was possible to have the tuple lock upgraded from shared to exclusive, and an update would not restart from the top; yet we're protected against a bug there because there's always a sleep to wait for the locking transaction to complete before continuing to do anything. Really, the fact that tuple locks always conflicted with concurrent updates is what protected against bugs here.) Per report from Andrew Dunstan and Josh Berkus in thread at http://www.postgresql.org/message-id/534C8B33.9050807@pgexperts.com Bug analysis by Andres Freund. |
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