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28d3c2ddcf
Yet another bug in the ilk of commits a7ee7c851 and 741b88435. In 741b88435, we took care to clear the memorized location of the downlink when we split the parent page, because splitting the parent page can move the downlink. But we missed that even *updating* a tuple on the parent can move it, because updating a tuple on a gist page is implemented as a delete+insert, so the updated tuple gets moved to the end of the page. This commit fixes the bug in two different ways (belt and suspenders): 1. Clear the downlink when we update a tuple on the parent page, even if it's not split. This the same approach as in commits a7ee7c851 and 741b88435. I also noticed that gistFindCorrectParent did not clear the 'downlinkoffnum' when it stepped to the right sibling. Fix that too, as it seems like a clear bug even though I haven't been able to find a test case to hit that. 2. Change gistFindCorrectParent so that it treats 'downlinkoffnum' merely as a hint. It now always first checks if the downlink is still at that location, and if not, it scans the page like before. That's more robust if there are still more cases where we fail to clear 'downlinkoffnum' that we haven't yet uncovered. With this, it's no longer necessary to meticulously clear 'downlinkoffnum', so this makes the previous fixes unnecessary, but I didn't revert them because it still seems nice to clear it when we know that the downlink has moved. Also add the test case using the same test data that Alexander posted. I tried to reduce it to a smaller test, and I also tried to reproduce this with different test data, but I was not able to, so let's just include what we have. Backpatch to v12, like the previous fixes. Reported-by: Alexander Lakhin Discussion: https://www.postgresql.org/message-id/18129-caca016eaf0c3702@postgresql.org
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