Re: [Patch] ALTER SYSTEM READ ONLY

> On Sep 9, 2021, at 11:21 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> They have to check whether WAL has become prohibited
> and error out if so, and they need to do so before entering the
> critical section - because if the problem were detected for the first
> time inside the critical section it would escalate to a PANIC, which
> we do not want.

But that is the part that is still not clear. Should the comment say that a concurrent change to prohibit wal after the current process checks but before the current process exists the critical section will result in a panic? What is unclear about the comment is that it implies that a check before the critical section is sufficient, but ordinarily one would expect a lock to be held and the check-and-lock dance to carefully avoid any race condition. If somehow this is safe, the logic for why it is safe should be spelled out. If not, a mia culpa saying, "hey, were not terribly safe about this" should be explicit in the comment.

—
Mark Dilger
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