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Fetch the next-item pointer before the call not after, so that we aren't dereferencing a dangling pointer if the callback deregistered itself during the call. The risky coding pattern appears in CallXactCallbacks, CallSubXactCallbacks, and ResourceOwnerReleaseInternal. (There are some other places that might be at hazard if they offered deregistration functionality, but they don't.) I (tgl) considered back-patching this, but desisted because it wouldn't be very safe for extensions to rely on this working in pre-v16 branches. Hao Wu Discussion: https://postgr.es/m/CAH+9SWXTiERkmhRke+QCcc+jRH8d5fFHTxh8ZK0-Yn4BSpyaAg@mail.gmail.com