backport: Extend background_psql() to be able to start asynchronously

This is a backport of ba08edb065. Originally it was only applied to master,
but I (Andres) am planning to fix a few bugs in BackgroundPsql, which would be
somewhat harder with the behavioural differences across branches. It's also
generally good for test infrastructure to behave similarly across branches, to
avoid pain during backpatching.

Discussion: https://postgr.es/m/ilcctzb5ju2gulvnadjmhgatnkxsdpac652byb2u3d3wqziyvx@fbuqcglker46

Michael's original commit message:

This commit extends the constructor routine of BackgroundPsql.pm with a
new "wait" parameter.  If set to 0, the routine returns without waiting
for psql to start, ready to consume input.

background_psql() in Cluster.pm gains the same "wait" parameter.  The
default behavior is still to wait for psql to start.  It becomes now
possible to not wait, giving to TAP scripts the possibility to perform
actions between a BackgroundPsql startup and its wait_connect() call.

Author: Jacob Champion
Discussion: https://postgr.es/m/CAOYmi+=60deN20WDyCoHCiecgivJxr=98s7s7-C8SkXwrCfHXg@mail.gmail.com

src/test/perl/PostgreSQL/Test/BackgroundPsql.pm

@@ -68,7 +68,7 @@ use Test::More;
 
 =over
 
-=item PostgreSQL::Test::BackgroundPsql->new(interactive, @psql_params, timeout)
+=item PostgreSQL::Test::BackgroundPsql->new(interactive, @psql_params, timeout, wait)
 
 Builds a new object of class C<PostgreSQL::Test::BackgroundPsql> for either
 an interactive or background session and starts it. If C<interactive> is
@@ -76,12 +76,15 @@ true then a PTY will be attached. C<psql_params> should contain the full
 command to run psql with all desired parameters and a complete connection
 string. For C<interactive> sessions, IO::Pty is required.
 
+This routine will not return until psql has started up and is ready to
+consume input. Set B<wait> to 0 to return immediately instead.
+
 =cut
 
 sub new
 {
 	my $class = shift;
-	my ($interactive, $psql_params, $timeout) = @_;
+	my ($interactive, $psql_params, $timeout, $wait) = @_;
 	my $psql = {
 		'stdin' => '',
 		'stdout' => '',
@@ -119,14 +122,25 @@ sub new
 
 	my $self = bless $psql, $class;
 
-	$self->_wait_connect();
+	$wait = 1 unless defined($wait);
+	if ($wait)
+	{
+		$self->wait_connect();
+	}
 
 	return $self;
 }
 
-# Internal routine for awaiting psql starting up and being ready to consume
-# input.
-sub _wait_connect
+=pod
+
+=item $session->wait_connect
+
+Returns once psql has started up and is ready to consume input. This is called
+automatically for clients unless requested otherwise in the constructor.
+
+=cut
+
+sub wait_connect
 {
 	my ($self) = @_;
 
@@ -187,7 +201,7 @@ sub reconnect_and_clear
 	$self->{stdin} = '';
 	$self->{stdout} = '';
 
-	$self->_wait_connect();
+	$self->wait_connect();
 }
 
 =pod

src/test/perl/PostgreSQL/Test/Cluster.pm

@@ -2002,6 +2002,12 @@ connection.
 
 If given, it must be an array reference containing additional parameters to B<psql>.
 
+=item wait => 1
+
+By default, this method will not return until connection has completed (or
+failed). Set B<wait> to 0 to return immediately instead. (Clients can call the
+session's C<wait_connect> method manually when needed.)
+
 =back
 
 =cut
@@ -2025,13 +2031,15 @@ sub background_psql
 		'-');
 
 	$params{on_error_stop} = 1 unless defined $params{on_error_stop};
+	$params{wait} = 1 unless defined $params{wait};
 	$timeout = $params{timeout} if defined $params{timeout};
 
 	push @psql_params, '-v', 'ON_ERROR_STOP=1' if $params{on_error_stop};
 	push @psql_params, @{ $params{extra_params} }
 	  if defined $params{extra_params};
 
-	return PostgreSQL::Test::BackgroundPsql->new(0, \@psql_params, $timeout);
+	return PostgreSQL::Test::BackgroundPsql->new(0, \@psql_params, $timeout,
+		$params{wait});
 }
 
 =pod