On 04/16/2014 02:37 PM, Craig Ringer wrote: > Hi all > > I've been using the dynamic BGWorker support for some recent work, and I > think I've found an issue with how postmaster restarts are handled. > > TL;DR: I don't think there's a safe way to use a BGWorker (static or > dynamic) with bgw_restart_time != BGW_NEVER_RESTART and a bgw_main_arg > Datum that points into shared memory, and think we might need a API > change to fix that.
Andres sensibly points out that part of this is easily solved by passing the bgworker an index into an array in a named shmem block. A simple pass-by-value Datum that can be turned into a pointer to a shmem struct. This still doesn't solve the other half of the issue, which is how to handle dynamic bgworkers after a postmaster restart. They're effectively lost/leaked: there's no way to retain a bgworker handle across restart, and no way to list bgworkers, nor is there any idempotent way to say "Start a worker to do <x> only if it doesn't already exist" (unique names, magic cookie hashes, whatever). With the current API the only solution to the second half that I see is to have bgworkers run in non-restart mode and manage them yourself. Not ideal. Instead we need one of: - A flag like BGW_UNREGISTER_ON_RESTART; - To always unregister dynamic bgws on postmaster shm clear + restart; - A way to list bgws, inspect their BackgroundWorker structs and obtain their handles; or - A way to idempotently register a bgw only if it doesn't already exist -- Craig Ringer http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
