3d80a1e0e3
After the WAL format changes, the calculation of the size of a checkpoint record became incorrect. Instead of trying to fix the math, check that the previous record, i.e. the xl_prev value that we'd write for the next record, matches the last checkpoint's redo pointer. That way it's not dependent on the size of the checkpoint record at all. The old logic was actually slightly wrong all along: if the previous checkpoint record crossed a page boundary, the page headers threw off the record size calculation, and the checkpoint was not skipped. The new checkpoint would not cross a page boundary, so this only resulted in at most one extra checkpoint after the system became idle. The new logic fixes that. (It's not worth fixing in backbranches). However, it makes some sense to try to keep the latest checkpoint contained fully in a page, or at least in a single WAL segment, just on general robustness grounds. If something goes awfully wrong, it's more likely that you can recover the latest WAL segment, than the last two WAL segments. So I added an extra check that the checkpoint is not skipped if the previous checkpoint crossed a WAL segment. Reported by Jeff Janes.
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