Re: Read Uncommitted

Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> So this is the same discussion as elsewhere about potentially aborted
> transactions...
> AFAIK, the worst that happens in that case is that the reading transaction
> will end with an ERROR, similar to a serializable error.

No, the worst case is transactions trying to read invalid data, resulting
in either crashes or exploitable security breaches (in the usual vein of
what can go wrong if you can get the C code to follow an invalid pointer).

This seems possible, for example, if you can get a transaction to read
uncommitted data that was written according to some other rowtype than
what the reading transaction thinks the table rowtype is. Casting my eyes
through AlterTableGetLockLevel(), it looks like all the easy ways to break
it like that are safe (for now) because they require AccessExclusiveLock.
But I am quite afraid that we'd introduce security holes by future
reductions of required lock levels --- or else that this feature would be
the sole reason why we couldn't reduce the lock level for some DDL
operation. I'm doubtful that its use-case is worth that.

> And that won't happen in the use cases I've explicitly described this as
> being useful for, which is where the writing transactions have completed
> executing.

My concerns, at least, are not about whether this has any interesting
use-cases. They're about whether the feature can be abused to cause
security problems. I think the odds are fair that that'd be true
even today, and higher that it'd become true sometime in the future.

regards, tom lane