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Previously, the transaction-property GUCs such as transaction_isolation could be reset after starting a transaction, because we marked them as GUC_NO_RESET_ALL but still allowed a targeted RESET. That leads to assertion failures or worse, because those properties aren't supposed to change after we've acquired a transaction snapshot. There are some NO_RESET_ALL variables for which RESET is okay, so we can't just redefine the semantics of that flag. Instead introduce a separate GUC_NO_RESET flag. Mark "seed", as well as the transaction property GUCs, as GUC_NO_RESET. We have to disallow GUC_ACTION_SAVE as well as straight RESET, because otherwise a function having a "SET transaction_isolation" clause can still break things: the end-of-function restore action is equivalent to a RESET. No back-patch, as it's conceivable that someone is doing something this patch will forbid (like resetting one of these GUCs at transaction start, or "CREATE FUNCTION ... SET transaction_read_only = 1") and not running into problems with it today. Given how long we've had this issue and not noticed, the side effects in non-assert builds can't be too serious. Per bug #17385 from Andrew Bille. Masahiko Sawada Discussion: https://postgr.es/m/17385-9ee529fb091f0ce5@postgresql.org
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