Add new module search algorithm for Python.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=184720075

Author: emrekultursay

src/googleclouddebugger/__init__.py

@@ -51,10 +51,13 @@ def _StartDebugger():
   cdbg_native.InitializeModule(_flags)
 
   _hub_client = gcp_hub_client.GcpHubClient()
-
   visibility_policy = _GetVisibilityPolicy()
+  use_new_module_search = _flags.get('use_new_module_search')
+
   _breakpoints_manager = breakpoints_manager.BreakpointsManager(
-      _hub_client, visibility_policy)
+      _hub_client,
+      visibility_policy,
+      use_new_module_search=use_new_module_search)
 
   # Set up loggers for logpoints.
   capture_collector.SetLogger(logging.getLogger())

src/googleclouddebugger/breakpoints_manager.py

@@ -36,11 +36,17 @@ class BreakpointsManager(object):
         breakpoint updates back to the backend.
     data_visibility_policy: An object used to determine the visibiliy
         of a captured variable.  May be None if no policy is available.
+    use_new_module_search: If true, the new module search algorithm will be
+        used.
   """
 
-  def __init__(self, hub_client, data_visibility_policy):
+  def __init__(self,
+               hub_client,
+               data_visibility_policy,
+               use_new_module_search=False):
     self._hub_client = hub_client
     self.data_visibility_policy = data_visibility_policy
+    self.use_new_module_search = use_new_module_search
 
     # Lock to synchronize access to data across multiple threads.
     self._lock = RLock()
@@ -75,7 +81,11 @@ def SetActiveBreakpoints(self, breakpoints_data):
       self._active.update([
           (x['id'],
            python_breakpoint.PythonBreakpoint(
-               x, self._hub_client, self, self.data_visibility_policy))
+               x,
+               self._hub_client,
+               self,
+               self.data_visibility_policy,
+               self.use_new_module_search))
           for x in breakpoints_data
           if x['id'] in ids - self._active.viewkeys() - self._completed])
 

src/googleclouddebugger/imphook.py

@@ -12,7 +12,18 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-"""Support for breakpoints on modules that haven't been loaded yet."""
+"""Support for breakpoints on modules that haven't been loaded yet.
+
+This is the old module import hook which:
+  1. Takes full path of the module with file extension as input.
+  2. At each (top-level-only) import statement:
+    a. Uses path to guess all possible names the module may be loaded as.
+    b. Checks sys.modules if there are any modules loaded with those names,
+       using exact path match between the __file__ attribute of the module
+       from sys.modules and the input path.
+
+For the new module import hook, see imphook2.py file.
+"""
 
 import os
 import sys  # Must be imported, otherwise import hooks don't work.
@@ -35,7 +46,7 @@ def AddImportCallback(abspath, callback):
   """Register import hook.
 
   This function overrides the default import process. Then whenever a module
-  corresponding to source_path is imported, the callback will be invoked.
+  corresponding to abspath is imported, the callback will be invoked.
 
   A module may be imported multiple times. Import event only means that the
   Python code contained an "import" statement. The actual loading and