Re: [Patch] ALTER SYSTEM READ ONLY

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From: Mark Dilger <mark.dilger@enterprisedb.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Amul Sul <sulamul@gmail.com>
Cc: Andres Freund <andres@anarazel.de>
Cc: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Cc: Dilip Kumar <dilipbalaut@gmail.com>
Cc: Ibrar Ahmed <ibrar.ahmad@gmail.com>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Cc: Prabhat Sahu <prabhat.sahu@enterprisedb.com>
Subject: Re: [Patch] ALTER SYSTEM READ ONLY
Date: Thu, 9 Sep 2021 11:49:34 -0700
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> On Sep 9, 2021, at 11:21 AM, Robert Haas <robertmhaas@gmail.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
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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