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## Problem There is "neon.pageserver_connstring" GUC with PGC_SIGHUP option, allowing to change it using pg_reload_conf(). It is used by control plane to update pageserver connection string if page server is crashed, relocated or new shards are added. It is copied to shared memory because config can not be loaded during query execution and we need to reestablish connection to page server. ## Summary of changes Copying connection string to shared memory is done by postmaster. And other backends should check update counter to determine of connection URL is changed and connection needs to be reestablished. We can not use standard Postgres LW-locks, because postmaster has proc entry and so can not wait on this primitive. This is why lockless access algorithm is implemented using two atomic counters to enforce consistent reading of connection string value from shared memory. ## Checklist before requesting a review - [ ] I have performed a self-review of my code. - [ ] If it is a core feature, I have added thorough tests. - [ ] Do we need to implement analytics? if so did you add the relevant metrics to the dashboard? - [ ] If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section. ## Checklist before merging - [ ] Do not forget to reformat commit message to not include the above checklist --------- Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech> Co-authored-by: Heikki Linnakangas <heikki@neon.tech>