Re: Don't use the deprecated and insecure PQcancel in our frontend tools anymore

Hi,

While testing the patch, it seems like there's a change of behavior
with psql and pager.

On master, with PAGER=less set, querying 'table pgbench_accounts;' and
doing a Ctrl-C would display a ":Cancel request sent" at the end of
the page.

With your patch, either nothing is reported, or the "Sending cancel
request" shows in the middle of the screen.

HEAD writes "Cancel request sent" after the cancel request was
successfully sent:
- if (PQcancel(cancel: cancelConn, errbuf, errbufsize: sizeof(errbuf)))
- {
- write_stderr(cancel_sent_msg);

With your patch, the message is now sent before the cancel request:
+ write_stderr(cancel_sent_msg);
+ if (!PQcancelBlocking(cancelConn: cc))
+ {

I think there's a race between less as it handles the SIGINT and
repaint the screen, and the 'write_stderr(cancel_sent_msg)'. If
write_stderr is processed between or during repaint, the message will
be visible, but possibly in the middle of the screen. If it's
processed before (probably what happens most of the time), then
repainting will erase the message.

On HEAD, this is indirectly avoided thanks to the cancel request
delay. By the time the cancel request is sent, less has finished
repainting.

Maybe it would be better to keep the old behavior and only write the
cancel_sent_msg after the cancel request was sent?

Regards,
Anthonin Bonnefoy