pg_basebackup WAL streamer shutdown is bogus - leading to slow tests

Hi,

I was wondering in [1] what we could do about the slowest tests on
windows.

On 2021-12-31 11:25:28 -0800, Andres Freund wrote:
> Picking a random successful cfbot run [1] I see the following tap tests taking
> more than 20 seconds:
>
> 67188 ms pg_basebackup t/010_pg_basebackup.pl
> 59710 ms recovery t/001_stream_rep.pl

Comparing these times to measurements taken on my normal linux workstation,
something seemed just *very* off, even with a slow CI instance and windows in
the mix.

A bunch of printf debugging later, I realized the problem is that several of
the pg_basebackups in tests take a *long* time. E.g. for t/001_stream_rep.pl
the backups from the standby each take just over 10s. That's awfully
specific...

# Taking pg_basebackup my_backup from node "standby_1"
# Running: pg_basebackup -D C:/dev/postgres/./tmp_check/t_001_stream_rep_standby_1_data/backup/my_backup -h C:/Users/myadmin/AppData/Local/Temp/yba26PBYX1 -p 59181 --checkpoint fast --no-sync --label my_backup -v
# ran in 10.145s
# Backup finished

This reproduceably happens and it's *not* related to the socket shutdown()
changes we've been debugging lately - even after a revert the problem
persists.

Because our logging for basebackups is quite weak, both for server and client
side, I needed to add a fair bit more debugging to figure it out:

pg_basebackup: wait to finish at 0.492
pg_basebackup: waiting for background process to finish streaming ...
pg_basebackup: stream poll timeout 10.112

The problem is that there's just no implemented way to timely shutdown the WAL
streaming thread in pg_basebackup. The code in pg_basebackup.c says:

if (verbose)
pg_log_info("waiting for background process to finish streaming ...");
...
/*
* On Windows, since we are in the same process, we can just store the
* value directly in the variable, and then set the flag that says
* it's there.
*/
...
xlogendptr = ((uint64) hi) << 32 | lo;
InterlockedIncrement(&has_xlogendptr);

But just setting a variable doesn't do much if the thread is in
HandleCopyStream()->CopyStreamPoll()->select()

The only reason we ever succeed shutting down, without more WAL coming in, is
that pg_basebackup defaults to sending a status message every 10 seconds. At
which point the thread sees has_xlogendptr = true, and shuts down.

A test specific workaround would be to just add --status-interval=1 to
Cluster.pm::backup(). But that seems very unsatisfying.

I don't immediately see a solution for this, other than to add
StreamCtl->stop_event (mirroring ->stop_socket) and then convert
CopyStreamPoll() to use WaitForMultipleObjects(). Microsoft's select()
doesn't support pipes and there's no socketpair().

Any more straightforward ideas?

From a cursory look at history, it used to be that pg_basebackup had this
behaviour on all platforms, but it got fixed for other platforms in
7834d20b57a by Tom (assuming the problem wasn't present there).

Greetings,

Andres Freund

[1] https://postgr.es/m/20211231192528.wirwj4qaaw3ted5g%40alap3.anarazel.de