On Mon, Jan 26, 2015 at 4:03 PM, Andres Freund <and...@2ndquadrant.com> wrote: >> I basically have two ideas to fix this. >> >> 1) Make do_pg_start_backup() acquire a SHARE lock on >> pg_database. That'll prevent it from starting while a movedb() is >> still in progress. Then additionally add pg_backup_in_progress() >> function to xlog.c that checks (XLogCtl->Insert.exclusiveBackup || >> XLogCtl->Insert.nonExclusiveBackups != 0). Use that in createdb() and >> movedb() to error out if a backup is in progress. > > Attached is a patch trying to this. Doesn't look too bad and lead me to > discover missing recovery conflicts during a AD ST. > > But: It doesn't actually work on standbys, because lock.c prevents any > stronger lock than RowExclusive from being acquired. And we need need a > lock that can conflict with WAL replay of DBASE_CREATE, to handle base > backups that are executed on the primary. Those obviously can't detect > whether any standby is currently doing a base backup... > > I currently don't have a good idea how to mangle lock.c to allow > this. I've played with doing it like in the second patch, but that > doesn't actually work because of some asserts around ProcSleep - leading > to locks on database objects not working in the startup process (despite > already being used). > > The easiest thing would be to just use a lwlock instead of a heavyweight > lock - but those aren't canceleable...
How about just wrapping an lwlock around a flag variable? movedb() increments the variable when starting and decrements it when done (must use PG_ENSURE_ERROR_CLEANUP). Starting a backup errors out (or waits in 1-second increments) if it's non-zero. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
