fa7ff642c2
Avoid accessing the deletion target page's special area during nbtree page deletion at a point where there is no buffer lock held. This issue was detected by a patch that extends Valgrind's memcheck tool to mark nbtree pages that are unsafe to access (due to not having a buffer lock or buffer pin) as NOACCESS. We do hold a buffer pin at this point, and only access the special area, so the old approach was safe. Even still, it seems like a good idea to tighten up the rules in this area. There is no reason to not simply insist on always holding a buffer lock (not just a pin) when accessing nbtree pages. Update some related comments in passing. Discussion: https://postgr.es/m/CAH2-WzkLgyN3zBvRZ1pkNJThC=xi_0gpWRUb_45eexLH1+k2_Q@mail.gmail.com
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