Re: Allow non-superuser to cancel superuser tasks.

> I don't think we should rely on !OidIsValid(proc->roleId) for signaling
> autovacuum workers.  That might not always be true, and I don't see any
> need to rely on that otherwise.  IMHO we should just add a few lines before
> the existing code, which doesn't need to be changed at all:
> 
>	if (pg_stat_is_backend_autovac_worker(proc->backendId) &&
>	    !has_privs_of_role(GetUserId(), ROLE_PG_SIGNAL_AUTOVACUUM))
>	    return SIGNAL_BACKEND_NOAUTOVACUUM;

I tried to add them above the existing code. When I test it locally, a user without pg_signal_autovacuum will actually fail at this block because the user is not superuser and !OidIsValid(proc->roleId) is also true in the following:

	/*
	 * Only allow superusers to signal superuser-owned backends.  Any process
	 * not advertising a role might have the importance of a superuser-owned
	 * backend, so treat it that way.
	 */
	if ((!OidIsValid(proc->roleId) || superuser_arg(proc->roleId)) &&
		!superuser())
		return SIGNAL_BACKEND_NOSUPERUSER;

This is what Im planning to do - If the backend is autovacuum worker and the user is not superuser or has pg_signal_autovacuum role, we return the new value and provide the relevant error message	

              /*
	 * If the backend is autovacuum worker, allow user with privileges of the 
               * pg_signal_autovacuum role to signal the backend.
	 */
	if (pgstat_get_backend_type(proc->backendId) == B_AUTOVAC_WORKER)
	{
		if (!has_privs_of_role(GetUserId(), ROLE_PG_SIGNAL_AUTOVACUUM) || !superuser())
			return SIGNAL_BACKEND_NOAUTOVACUUM;
	}
	/*
	 * Only allow superusers to signal superuser-owned backends.  Any process
	 * not advertising a role might have the importance of a superuser-owned
	 * backend, so treat it that way.
	*/
	else if ((!OidIsValid(proc->roleId) || superuser_arg(proc->roleId)) &&
			 !superuser())
	{
		return SIGNAL_BACKEND_NOSUPERUSER;
	}
	/* Users can signal backends they have role membership in. */
	else if (!has_privs_of_role(GetUserId(), proc->roleId) &&
			 !has_privs_of_role(GetUserId(), ROLE_PG_SIGNAL_BACKEND))
	{
		return SIGNAL_BACKEND_NOPERMISSION;
	}


> We can add tests just like [0] with injection points.
> I mean replace that "sleep 1" with something like "$node->wait_for_event('autovacuum worker', 'autocauum-runing');".
> Currently we have no infrastructure to wait for autovacuum of particular table, but I think it's doable.
> Also I do not like that test is changing system-wide autovac settings, AFAIR these settings can be set for particular table.

Thanks for the suggestion. I will take a look at this. Let me also separate the test into a different patch file.

--
Anthony Leung
Amazon Web Services: https://aws.amazon.com