... BTW, one other minor coding suggestion for
GetSafeSnapshotBlockingPids(): it might be better to avoid doing so much
palloc work while holding the SerializableXactHashLock. Even if it's
only held shared, I imagine that it's a contention bottleneck. You
could avoid that by returning an array rather than a list; the array
could be preallocated of size MaxBackends before ever taking the lock.
That would be a little bit space-wasteful, but since it's only short-lived
storage it doesn't seem like much of a problem.
regards, tom lane
