pytest-server-fixtures: Support large pids in xvfb

Since a large number of 64 bit distributions are now using larger PIDs
than 65535 by default, and the xvfb fixtures makes the assumption that
PIDs are going to be less than that, check what the maximum is, falling
back to 65535 if we can't.

Author: s-t-e-v-e-n-k

pytest-server-fixtures/pytest_server_fixtures/xvfb.py

@@ -43,7 +43,10 @@ class XvfbServer(object):
 
     def __init__(self):
         tmpdir = mkdtemp(prefix='XvfbServer.', dir=Workspace.get_base_tempdir())
-        for servernum in range(os.getpid(), 65536):
+        pid_max = 65536
+        with open('/proc/sys/kernel/pid_max') as pid_max_file:
+            pid_max = int(pid_max_file.read())
+        for servernum in range(os.getpid(), pid_max):
             if os.path.exists('/tmp/.X{0}-lock'.format(servernum)):
                 continue
             self.display = ':' + str(servernum)