pgsql: Make smgr access for a BufferManagerRelation safer in relcache i

Make smgr access for a BufferManagerRelation safer in relcache inval

Currently there's no bug, because we have no code path where we
invalidate relcache entries where it'd cause a problem. But it's more
robust to do it this way in case we introduce such a path later, as some
Postgres forks reportedly already have.

Author: Daniil Davydov <3danissimo(at)gmail(dot)com>
Reviewed-by: Stepan Neretin <slpmcf(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAJDiXgj3FNzAhV+jjPqxMs3jz=OgPohsoXFj_fh-L+nS+13CKQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b7cc6474e930d4429b15657d6910e1e32066de5e

Modified Files
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src/backend/storage/buffer/bufmgr.c | 59 ++++++++++++++++-------------------
src/backend/storage/buffer/localbuf.c | 10 +++---
src/include/storage/bufmgr.h | 15 ++++++---
3 files changed, 44 insertions(+), 40 deletions(-)