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Subject: Re: [Patch] ALTER SYSTEM READ ONLY
From: Mark Dilger <mark.dilger@enterprisedb.com>
In-Reply-To: 
 <CA+TgmobFnYkZv_yc4r_SJQe1zkYeJXorYF8TPc91ecaSundyQw@mail.gmail.com>
Date: Thu, 9 Sep 2021 11:49:34 -0700
Cc: Amul Sul <sulamul@gmail.com>,
 Andres Freund <andres@anarazel.de>,
 Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>,
 Dilip Kumar <dilipbalaut@gmail.com>,
 Ibrar Ahmed <ibrar.ahmad@gmail.com>,
 PostgreSQL-development <pgsql-hackers@postgresql.org>,
 Prabhat Sahu <prabhat.sahu@enterprisedb.com>
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To: Robert Haas <robertmhaas@gmail.com>
Archived-At: 
 <https://www.postgresql.org/message-id/2452D6AC-FEF2-4F59-9329-EAF6267A0C7C%40enterprisedb.com>
Precedence: bulk


> On Sep 9, 2021, at 11:21 AM, Robert Haas <robertmhaas@gmail.com> =
wrote:
>=20
> 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.

=E2=80=94
Mark Dilger
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company