On Mon, Mar 16, 2020 at 12:43:41AM -0500, Justin Pryzby wrote: > I think I see. Julien's v3 patch did this: > https://www.postgresql.org/message-id/attachment/106429/pgsa_leader_pid-v3.diff > + if (proc->lockGroupLeader) > + values[29] = > Int32GetDatum(proc->lockGroupLeader->pid); > > ..which is racy because a proc with a leader might die and be replaced by > another proc without a leader between 1 and 2. > > But the code since v4 does: > > + leader = proc->lockGroupLeader; > + if (leader) > + values[29] = > Int32GetDatum(leader->pid); > > ..which is safe because PROCs are allocated in shared memory, so leader is for > sure a non-NULL pointer to a PROC. leader->pid may be read inconsistently, > which is what the comment says: "no extra lock is being held".
Yes, you have the correct answer here. As shaped, the code relies on the state of a PGPROC entry in shared memory. -- Michael
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