Add does_storage_object_exist to gs_utils

Needed for recreate_skps script

BUG=skia:3008

Review URL: https://codereview.chromium.org/661563002

.gitignore

diff --git a/.gitignore b/.gitignore
index 2f78cf5..f4bf1d0 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,2 @@
 *.pyc
-
+third_party/externals

py/utils/gs_utils.py

diff --git a/py/utils/gs_utils.py b/py/utils/gs_utils.py
index d5c5b95..fd9999c 100644
--- a/py/utils/gs_utils.py
+++ b/py/utils/gs_utils.py
@@ -52,7 +52,7 @@
 # each core sits idle waiting for network I/O to complete.
 DEFAULT_UPLOAD_THREADS = 10
 
-_GS_PREFIX = 'gs://'
+GS_PREFIX = 'gs://'
 
 
 class AnonymousGSConnection(GSConnection):
@@ -132,19 +132,34 @@
 
     Params:
       boto_file_path: full path (local-OS-style) on local disk where .boto
-          credentials file can be found.  If None, then the GSUtils object
-          created will be able to access only public files in Google Storage.
+          credentials file can be found.  If None, fall back on the
+          AWS_CREDENTIAL_FILE environment variable, then look in a set of
+          common paths for the .boto file.  If no .boto file is found, then the
+          GSUtils object created will be able to access only public files in
+          Google Storage.
 
     Raises an exception if no file is found at boto_file_path, or if the file
     found there is malformed.
     """
     self._gs_access_key_id = None
     self._gs_secret_access_key = None
+    if not boto_file_path:
+      if os.environ.get('AWS_CREDENTIAL_FILE'):
+        boto_file_path = os.path.expanduser(os.environ['AWS_CREDENTIAL_FILE'])
+    if not boto_file_path:
+      for path in (os.path.join(os.path.expanduser('~'), '.boto'),):
+        if os.path.isfile(path):
+          boto_file_path = path
+          break
+
     if boto_file_path:
       print ('Reading boto file from %s' % boto_file_path)
       boto_dict = _config_file_as_dict(filepath=boto_file_path)
       self._gs_access_key_id = boto_dict['gs_access_key_id']
       self._gs_secret_access_key = boto_dict['gs_secret_access_key']
+    else:
+      print >> sys.stderr, 'Warning: no .boto file found.'
+
     # Which field we get/set in ACL entries, depending on IdType.
     self._field_by_id_type = {
         self.IdType.GROUP_BY_DOMAIN: 'domain',
@@ -582,13 +597,24 @@
         dirs.append(item.name[prefix_length:-1])
     return (dirs, files)
 
+  def does_storage_object_exist(self, bucket, object_name):
+    """Determines whether an object exists in Google Storage.
+
+    Returns True if it exists else returns False.
+    """
+    b = self._connect_to_bucket(bucket=bucket)
+    if object_name in b:
+      return True
+    dirs, files = self.list_bucket_contents(bucket, object_name)
+    return bool(dirs or files)
+
   @staticmethod
   def is_gs_url(url):
     """Returns True if url is a legal Google Storage URL ("gs://bucket/file").
     """
     try:
-      if url.lower().startswith(_GS_PREFIX) and len(url) > len(_GS_PREFIX):
-        return url[len(_GS_PREFIX)].isalnum()
+      if url.lower().startswith(GS_PREFIX) and len(url) > len(GS_PREFIX):
+        return url[len(GS_PREFIX)].isalnum()
       else:
         return False
     except AttributeError:
@@ -602,7 +628,7 @@
     """
     if not GSUtils.is_gs_url(url):
       raise AttributeError('"%s" is not a legal Google Storage URL' % url)
-    prefix_removed = url[len(_GS_PREFIX):]
+    prefix_removed = url[len(GS_PREFIX):]
     pathsep_index = prefix_removed.find('/')
     if pathsep_index < 0:
       return (prefix_removed, '')

py/utils/gs_utils_manualtest.py

diff --git a/py/utils/gs_utils_manualtest.py b/py/utils/gs_utils_manualtest.py
index 3365055..2ba63e4 100755
--- a/py/utils/gs_utils_manualtest.py
+++ b/py/utils/gs_utils_manualtest.py
@@ -388,6 +388,17 @@
                      path=posixpath.join(remote_dir, subdir, filename))
 
 
+def _test_does_storage_object_exist():
+  gs = gs_utils.GSUtils()
+  cases = (('gs_utils_manualtest/4843645167453050644/subdir/file1', True),
+           ('gs_utils_manualtest', True),
+           ('bogus', False))
+  for obj, expect in cases:
+    msg = 'expected exists(%s) to be %s' % (
+        '/'.join(('gs:/', TEST_BUCKET, obj)), expect)
+    assert gs.does_storage_object_exist(TEST_BUCKET, obj) == expect, msg
+
+
 if __name__ == '__main__':
   _test_static_methods()
   _test_upload_if_multiple_files()
@@ -395,5 +406,6 @@
   _test_public_read()
   _test_authenticated_round_trip()
   _test_dir_upload_and_download()
+  _test_does_storage_object_exist()
   # TODO(epoger): Add _test_unauthenticated_access() to make sure we raise
   # an exception when we try to access without needed credentials.