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help / color / mirror / Atom feedQuestionable ping logic in LogicalRepApplyLoop
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Commits from this thread:
2020-09-05 9b47ee6e7c 7156a0eac3 9b8a8516ed c8746f999e 9b81a30f92 Fix over-eager ping'ing in logical replication receiver.
* Questionable ping logic in LogicalRepApplyLoop
@ 2020-09-04 19:41 Tom Lane <tgl@sss.pgh.pa.us>
0 siblings, 1 reply; 3+ messages in thread
From: Tom Lane @ 2020-09-04 19:41 UTC (permalink / raw)
To: pgsql-hackers@lists.postgresql.org
While playing around with clang's scan-build I noticed this warning:
worker.c:2281:7: warning: Value stored to 'ping_sent' is never read
ping_sent = true;
^ ~~~~
At first I thought it was a harmless unnecessary update, but looking
closer I wonder whether it isn't telling us there is a logic bug here.
Specifically, I wonder why the "ping_sent" variable is local to the
loop starting at line 2084, rather than having the same lifespan as
"last_recv_timestamp". Do we really want to forget that we sent a
ping anytime we have to wait for more data?
In fact, looking closer, it appears to me that
(1) The "ping_sent = false" at line 2124 is also dead code, because
ping_sent could never be anything but false at this point;
(2) The "if (!ping_sent)" at line 2274 is also dead code, because
ping_sent is still never anything but false at that point.
In short, we could remove the ping_sent variable entirely without
changing this code's behavior. I'm not 100% sure what semantics
it's trying to achieve, but I don't think it's achieving them.
regards, tom lane
^ permalink raw reply [nested|flat] 3+ messages in thread
* Re: Questionable ping logic in LogicalRepApplyLoop
@ 2020-09-04 20:41 Alvaro Herrera <alvherre@2ndquadrant.com>
parent: Tom Lane <tgl@sss.pgh.pa.us>
0 siblings, 1 reply; 3+ messages in thread
From: Alvaro Herrera @ 2020-09-04 20:41 UTC (permalink / raw)
To: Tom Lane <tgl@sss.pgh.pa.us>; +Cc: pgsql-hackers@lists.postgresql.org, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>; Petr Jelinek <petr@2ndquadrant.com>
On 2020-Sep-04, Tom Lane wrote:
> While playing around with clang's scan-build I noticed this warning:
>
> worker.c:2281:7: warning: Value stored to 'ping_sent' is never read
> ping_sent = true;
> ^ ~~~~
>
> At first I thought it was a harmless unnecessary update, but looking
> closer I wonder whether it isn't telling us there is a logic bug here.
> Specifically, I wonder why the "ping_sent" variable is local to the
> loop starting at line 2084, rather than having the same lifespan as
> "last_recv_timestamp". Do we really want to forget that we sent a
> ping anytime we have to wait for more data?
Ah ... maybe this bug is the reason why the bug fixed by 470687b4a5bb
did not affect logical replication.
> In short, we could remove the ping_sent variable entirely without
> changing this code's behavior. I'm not 100% sure what semantics
> it's trying to achieve, but I don't think it's achieving them.
I imagine that moving the variable one block-scope outwards (together
with last_recv_timestamp) is what was intended.
... oh look! commit 3f60f690fac1 moved last_recv_timestamp without
realizing that ping_sent had to get the same treatment.
--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
^ permalink raw reply [nested|flat] 3+ messages in thread
* Re: Questionable ping logic in LogicalRepApplyLoop
@ 2020-09-04 21:41 Tom Lane <tgl@sss.pgh.pa.us>
parent: Alvaro Herrera <alvherre@2ndquadrant.com>
0 siblings, 0 replies; 3+ messages in thread
From: Tom Lane @ 2020-09-04 21:41 UTC (permalink / raw)
To: Alvaro Herrera <alvherre@2ndquadrant.com>; +Cc: pgsql-hackers@lists.postgresql.org, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>; Petr Jelinek <petr@2ndquadrant.com>
Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> ... oh look! commit 3f60f690fac1 moved last_recv_timestamp without
> realizing that ping_sent had to get the same treatment.
Hah, I wondered if something like that had happened, but I didn't
get around to excavating in the git history yet. Thanks for doing so.
Will push a fix later.
regards, tom lane
^ permalink raw reply [nested|flat] 3+ messages in thread
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2020-09-04 19:41 Questionable ping logic in LogicalRepApplyLoop Tom Lane <tgl@sss.pgh.pa.us>
2020-09-04 20:41 ` Alvaro Herrera <alvherre@2ndquadrant.com>
2020-09-04 21:41 ` Tom Lane <tgl@sss.pgh.pa.us>
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