[Telemetry] Fix flaky browser test due to duplicated system stubbing

All the browser tests were flaky because we fail to restore the |sys| module in
desktop_browser_finder. The reason were because the module
desktop_browser_finder were overriden twice, hence in the second time
its original module |desktop_browser_finder.sys| is already stubbed. The
system_stub framework then tries to restore desktop_browser_finder.sys with this
"original" sys module, hence failing to restore desktop_browser_finder to real
original state.

This CL remove the duplicate stubbing & add assertion in system_stub to avoid overriding a module twice.

BUG=catapult:#3074

Review-Url: https://codereview.chromium.org/2574453002

telemetry/telemetry/internal/backends/chrome/desktop_browser_finder_unittest.py

@@ -32,15 +32,12 @@ def setUp(self):
     self._catapult_path_stubs = system_stub.Override(
         desktop_browser_finder.path_module.catapult_util, ['os', 'sys'])
     self._util_stubs = system_stub.Override(util, ['os', 'sys'])
-    self._browser_finder_stubs = system_stub.Override(desktop_browser_finder,
-                                                      ['os', 'sys'])
 
   def tearDown(self):
     self._finder_stubs.Restore()
     self._path_stubs.Restore()
     self._catapult_path_stubs.Restore()
     self._util_stubs.Restore()
-    self._browser_finder_stubs.Restore()
 
   @property
   def _files(self):
@@ -68,7 +65,6 @@ def setUp(self):
     self._finder_stubs.sys.platform = 'win32'
     self._path_stubs.sys.platform = 'win32'
     self._util_stubs.sys.platform = 'win32'
-    self._browser_finder_stubs.sys.platform = 'win32'
 
   def testFindProgramFiles(self):
     if not self.CanFindAvailableBrowsers():
@@ -104,7 +100,6 @@ def setUp(self):
     self._finder_stubs.sys.platform = 'win32'
     self._path_stubs.sys.platform = 'win32'
     self._util_stubs.sys.platform = 'win32'
-    self._browser_finder_stubs.sys.platform = 'win32'
 
   def testFindBuild(self):
     if not self.CanFindAvailableBrowsers():
@@ -134,7 +129,6 @@ def setUp(self):
     self._finder_stubs.sys.platform = 'darwin'
     self._path_stubs.sys.platform = 'darwin'
     self._util_stubs.sys.platform = 'darwin'
-    self._browser_finder_stubs.sys.platform = 'darwin'
     self._files.append('/Applications/Google Chrome Canary.app/'
                        'Contents/MacOS/Google Chrome Canary')
     self._files.append('/Applications/Google Chrome.app/' +
@@ -261,7 +255,6 @@ def setUp(self):
     self._finder_stubs.sys.platform = 'win32'
     self._path_stubs.sys.platform = 'win32'
     self._util_stubs.sys.platform = 'win32'
-    self._browser_finder_stubs.sys.platform = 'win32'
     self._path_stubs.os.local_app_data = 'c:\\Users\\Someone\\AppData\\Local'
     self._files.append('c:\\tmp\\chrome.exe')
     self._files.append('..\\..\\..\\build\\Release\\chrome.exe')

telemetry/telemetry/testing/system_stub.py

@@ -16,6 +16,9 @@
 
 
 class Override(object):
+
+  _overidden_modules = set()
+
   def __init__(self, base_module, module_list):
     self.cloud_storage = None
     self.open = None
@@ -41,6 +44,9 @@ def __init__(self, base_module, module_list):
     self.subprocess = None
     self.sys = None
 
+    assert base_module not in self._overidden_modules, (
+        '%s is already overridden' % base_module.__name__)
+    self._overidden_modules.add(base_module)
     self._base_module = base_module
     self._overrides = {}
 
@@ -65,6 +71,8 @@ def Restore(self):
       else:
         setattr(self._base_module, module_name, original_module)
     self._overrides = {}
+    self._overidden_modules.remove(self._base_module)
+    self._base_module = None
 
 
 class AdbDevice(object):