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)

pytest-server-fixtures/tests/integration/test_xvfb_server.py

@@ -5,7 +5,7 @@
 from itertools import chain, repeat
 
 try:
-    from unittest.mock import patch
+    from unittest.mock import mock_open, patch
 except ImportError:
     # python 2
     from mock import patch
@@ -69,3 +69,23 @@ def test_handles_unexpected_failure_to_start():
         with raises(RuntimeError) as ex:
             XvfbServer()
         assert 'Failed to start Xvfb' in str(ex)
+
+
+def test_handles_64_bit_pids():
+    with patch('os.getpid') as mock_getpid:
+         # This just has to be larger than 65535
+        mock_getpid.return_value = 65537
+        with XvfbServer() as server:
+            assert server.display
+
+
+def test_errors_64_bit_pids():
+    with patch('os.getpid') as mock_getpid:
+        target = 'pytest_server_fixtures.xvfb.open'
+        with patch(target, mock_open(read_data='4')) as m:
+            # This just has to be larger than the returned max PID
+            mock_getpid.return_value = 7
+            with raises(RuntimeError) as ex:
+                XvfbServer()
+    m.assert_called_once_with('/proc/sys/kernel/pid_max')
+    assert 'Unable to find a free server number to start Xvfb' in str(ex)