complete-run.pl: automatically start Xdummy instances unless -d is specified

This makes running the testsuite incredibly easy:
    $ ./complete-run.pl
:)

testcases/lib/SocketActivation.pm

@@ -1,6 +1,8 @@
 package SocketActivation;
 # vim:ts=4:sw=4:expandtab
 
+use strict;
+use warnings;
 use IO::Socket::UNIX; # core
 use Cwd qw(abs_path); # core
 use POSIX; # core

testcases/lib/StartXDummy.pm

@@ -0,0 +1,80 @@
+package StartXDummy;
+# vim:ts=4:sw=4:expandtab
+
+use strict;
+use warnings;
+use POSIX ();
+use Exporter 'import';
+use Time::HiRes qw(sleep);
+use v5.10;
+
+our @EXPORT = qw(start_xdummy);
+
+# reads in a whole file
+sub slurp {
+    open(my $fh, '<', shift) or return '';
+    local $/;
+    <$fh>;
+}
+
+=head2 start_xdummy($parallel)
+
+Starts C<$parallel> (or number of cores * 2 if undef) Xdummy processes (see
+the file ./Xdummy) and returns two arrayrefs: a list of X11 display numbers to
+the Xdummy processes and a list of PIDs of the processes.
+
+=cut
+sub start_xdummy {
+    my ($parallel) = @_;
+
+    my @displays = ();
+    my @childpids = ();
+
+    # Yeah, I know it’s non-standard, but Perl’s POSIX module doesn’t have
+    # _SC_NPROCESSORS_CONF.
+    my $cpuinfo = slurp('/proc/cpuinfo');
+    my $num_cores = scalar grep { /model name/ } split("\n", $cpuinfo);
+    # If /proc/cpuinfo does not exist, we fall back to 2 cores.
+    $num_cores ||= 2;
+
+    $parallel ||= $num_cores * 2;
+
+    # First get the last used display number, then increment it by one.
+    # Effectively falls back to 1 if no X server is running.
+    my ($displaynum) = reverse ('0', sort </tmp/.X11-unix/X*>);
+    $displaynum =~ s/.*(\d)$/$1/;
+    $displaynum++;
+
+    say "Starting $parallel Xdummy instances, starting at :$displaynum...";
+
+    for my $idx (0 .. ($parallel-1)) {
+        my $pid = fork();
+        die "Could not fork: $!" unless defined($pid);
+        if ($pid == 0) {
+            # Child, close stdout/stderr, then start Xdummy.
+            POSIX::close(0);
+            POSIX::close(2);
+            exec './Xdummy', ":$displaynum", '-config', '/dev/null';
+            exit 1;
+        }
+        push(@childpids, $pid);
+        push(@displays, ":$displaynum");
+        $displaynum++;
+    }
+
+    # Wait until the X11 sockets actually appear. Pretty ugly solution, but as
+    # long as we can’t socket-activate X11…
+    my $sockets_ready;
+    do {
+        $sockets_ready = 1;
+        for (@displays) {
+            my $path = "/tmp/.X11-unix/X" . substr($_, 1);
+            $sockets_ready = 0 unless -S $path;
+        }
+        sleep 0.1;
+    } until $sockets_ready;
+
+    return \@displays, \@childpids;
+}
+
+1