Restart pg_usleep when interrupted

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Hi,

In the proposal by Bertrand [1] to implement vacuum cost delay tracking
in pg_stat_progress_vacuum, it was discovered that the vacuum cost delay
ends early on the leader process of a parallel vacuum due to parallel workers
reporting progress on index vacuuming, which was introduced in 17
with commit [2]. With this patch, everytime a parallel worker
completes a vacuum index, it will send a completion message to the leader.

The facility that allows a parallel worker to report progress to the leader was
introduced in commit [3].

In the case of the patch being proposed by Bertrand, the number of interrupts
will be much more frequent as parallel workers would send a message to the leader
to update the vacuum delay counters every vacuum_delay_point call.

Looking into this, the ideal solution appears to provide the ability for a pg_usleep
call to restart after being interrupted. Thanks to the usage of nanosleep as
of commit[4], this should be trivial to do as nanosleep
provides a remaining time, which can be used to restart the sleep. This
idea was also mentioned in [5].

I am attaching a POC patch that does the $SUBJECT. Rather than changing the
existing pg_usleep, the patch introduces a function, pg_usleep_handle_interrupt,
that takes in the sleep duration and a boolean flag to force handling of the
interrupt, or not.

This function can replace pg_usleep inside vacuum_delay_point, and could be
useful in other cases in which it’s important to handle interrupts.

For Windows, the “force” = “true” flag of the new function uses SleepEx which
from what I can tell based on the code comments is a non-interruptible sleep.

Thoughts?

[1] https://www.postgresql.org/message-id/ZnbIIUQpFJKAJx2W%40ip-10-97-1-34.eu-west-3.compute.internal
[2] https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=46ebdfe164c61fbac961d1eb7f40e9a684289ae6
[3] https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=f1889729dd3ab0352dc0ccc2ffcc1b1901f8e39f
[4] https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=a948e49e2ef11815be0b211723bfc5b53b7f75a8
[5] https://www.postgresql.org/message-id/24848.1473607575%40sss.pgh.pa.us

Regards,

Sami Imseih
Amazon Web Services (AWS)