recipe engine: add remote_run command

recipe_engine/fetch.py

Index: recipe_engine/fetch.py
diff --git a/recipe_engine/fetch.py b/recipe_engine/fetch.py
new file mode 100644
index 0000000000000000000000000000000000000000..63e404ef623fe09f4a53f0c7e609c4bc9ea9bd44
--- /dev/null
+++ b/recipe_engine/fetch.py
@@ -0,0 +1,68 @@
+# Copyright 2016 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 logging
+import os
+import sys
+
+from .third_party import subprocess42
+
+
+class FetchError(Exception):
+  pass
+
+
+class UncleanFilesystemError(FetchError):
+  pass
+
+
+class FetchNotAllowedError(FetchError):
+  pass
+
+
+def _run_git(checkout_dir, *args):
+  if sys.platform.startswith(('win', 'cygwin')):
+    cmd = ['git.bat']
+  else:
+    cmd = ['git']
+
+  if checkout_dir is not None:
+    cmd += ['-C', checkout_dir]
+  cmd += list(args)
+
+  logging.info('Running: %s', cmd)
+  return subprocess42.check_output(cmd)
+
+
+def ensure_git_checkout(repo, revision, checkout_dir, allow_fetch):
+  """Fetches given |repo| at |revision| to |checkout_dir| using git.
+
+  Network operations are performed only if |allow_fetch| is True.
+  """
+  logging.info('Freshening repository %s in %s', repo, checkout_dir)
+
+  if not os.path.isdir(checkout_dir):
+    if not allow_fetch:
+      raise FetchNotAllowedError(
+          'need to clone %s but fetch not allowed' % repo)
+    _run_git(None, 'clone', '-q', repo, checkout_dir)
+  elif not os.path.isdir(os.path.join(checkout_dir, '.git')):
+    raise UncleanFilesystemError(
+        '%s exists but is not a git repo' % checkout_dir)
+
+  actual_origin = _run_git(checkout_dir, 'config', 'remote.origin.url').strip()
+  if actual_origin != repo:
+    raise UncleanFilesystemError(
+        ('workdir %r exists but uses a different origin url %r '
+         'than requested %r') % (checkout_dir, actual_origin, repo))
+
+  try:
+    _run_git(checkout_dir, 'rev-parse', '-q', '--verify',
+             '%s^{commit}' % revision)
+  except subprocess42.CalledProcessError:
+    if not allow_fetch:
+      raise FetchNotAllowedError(
+          'need to fetch %s but fetch not allowed' % repo)
+    _run_git(checkout_dir, 'fetch')
+  _run_git(checkout_dir, 'reset', '-q', '--hard', revision)