[telemetry] Move stale .pyc check from Python import to gclient hook.

This check turns out to be fairly expensive on Windows (~150ms), and having
it at import time means we're taking the hit every time we launch
memory_cache_http_server. We only need to do this if the Python files have
changed, so move it to runhooks.

The reason this check exists in the first place is that we've had problems in
the past where e.g. someone renames foo.py to foo/__init__.py. foo.pyc remains
on the filesystem of every Chromium checkout and causes a name conflict in
Python imports - foo can refer to the compiled module or the package.

BUG=388256
TEST=git rm a file in src/tools that has a .pyc; git commit; gclient runhooks; ensure .pyc doesn't exist; undo commit

Review URL: https://codereview.chromium.org/489693002

Cr-Commit-Position: refs/heads/master@{#291825}

DEPS

@@ -778,4 +778,15 @@ hooks = [
         '--running-as-hook',
     ],
   },
+  {
+    # Ensure that we don't accidentally reference any .pyc files whose
+    # corresponding .py files have already been deleted.
+    'name': 'remove_stale_pyc_files',
+    'pattern': 'src/tools/.*\\.py',
+    'action': [
+        'python',
+        'src/tools/remove_stale_pyc_files.py',
+        'src/tools',
+    ],
+  },
 ]

tools/remove_stale_pyc_files.py

+#!/usr/bin/env python
+# Copyright 2014 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import os
+import sys
+
+
+def RemoveAllStalePycFiles(base_dir):
+  """Scan directories for old .pyc files without a .py file and delete them."""
+  for dirname, _, filenames in os.walk(base_dir):
+    if '.svn' in dirname or '.git' in dirname:
+      continue
+    for filename in filenames:
+      root, ext = os.path.splitext(filename)
+      if ext != '.pyc':
+        continue
+
+      pyc_path = os.path.join(dirname, filename)
+      py_path = os.path.join(dirname, root + '.py')
+
+      try:
+        if not os.path.exists(py_path):
+          os.remove(pyc_path)
+      except OSError:
+        # Wrap OS calls in try/except in case another process touched this file.
+        pass
+
+    try:
+      os.removedirs(dirname)
+    except OSError:
+      # Wrap OS calls in try/except in case another process touched this dir.
+      pass
+
+
+if __name__ == '__main__':
+  for path in sys.argv[1:]:
+    RemoveAllStalePycFiles(path)

tools/telemetry/telemetry/util/global_hooks.py

@@ -4,49 +4,18 @@
 
 """Hooks that apply globally to all scripts that import or use Telemetry."""
 
-import os
 import signal
 import sys
 
-from telemetry.core import util
 from telemetry.util import exception_formatter
 
 
 def InstallHooks():
-  RemoveAllStalePycFiles(util.GetTelemetryDir())
-  RemoveAllStalePycFiles(util.GetBaseDir())
   InstallUnhandledExceptionFormatter()
   InstallStackDumpOnSigusr1()
   InstallTerminationHook()
 
 
-def RemoveAllStalePycFiles(base_dir):
-  """Scan directories for old .pyc files without a .py file and delete them."""
-  for dirname, _, filenames in os.walk(base_dir):
-    if '.svn' in dirname or '.git' in dirname:
-      continue
-    for filename in filenames:
-      root, ext = os.path.splitext(filename)
-      if ext != '.pyc':
-        continue
-
-      pyc_path = os.path.join(dirname, filename)
-      py_path = os.path.join(dirname, root + '.py')
-
-      try:
-        if not os.path.exists(py_path):
-          os.remove(pyc_path)
-      except OSError:
-        # Wrap OS calls in try/except in case another process touched this file.
-        pass
-
-    try:
-      os.removedirs(dirname)
-    except OSError:
-      # Wrap OS calls in try/except in case another process touched this dir.
-      pass
-
-
 def InstallUnhandledExceptionFormatter():
   """Print prettier exceptions that also contain the stack frame's locals."""
   sys.excepthook = exception_formatter.PrintFormattedException