Consistently use clock_timestamp() instead of now() when working with vt

[brianpursley]

Most of the time, pgmq uses clock_timestamp() when setting or comparing vt. This is the case for the following functions:

• read
• read_with_poll
• send
• send_batch

But these functions use now():

• pop
• set_vt

Is there a reason for the difference?

Depending on which is used, the behavior can be different when used inside a transaction, which might be unexpected. I ran into this while testing something and it surprised me until I looked at the code.

Here is an example showing what I'm talking about:
Inside a transaction, send a message to the queue. You can't pop the message, but you can read it.

postgres=# begin transaction;
BEGIN
postgres=*# select pgmq.send('my_queue', '{"foo": "bar1"}');
 send 
------
   38
(1 row)

postgres=*# select pgmq.pop('my_queue');
 pop 
-----
(0 rows)

postgres=*# select pgmq.read('my_queue', 30, 1);
                                             read                                             
----------------------------------------------------------------------------------------------
 (37,2,"2024-12-20 19:44:53.600822+00","2024-12-20 19:46:10.624711+00","{""foo"": ""bar1""}")
(1 row)

postgres=*# commit;
COMMIT

If there isn't a reason for them to be different, I think pop and set_vt should use clock_timestamp(), to be consistent with the other functions.

[ChuckHend]

I don't think there's a reason that they are different other than the project starter using now() and we changed some of the functions as we were updating the codebase. We should be able to update pop() and set_vt() too.

[v0idpwn]

PRs welcome :)