[gcloud] Retry on connection/read timeout

Revert this comment after a version of `google-cloud-storage` with
googleapis/python-storage#727 gets released and
ScanForm is updated to use it.

storages/backends/gcloud.py

@@ -16,11 +16,15 @@
 )
 
 try:
+    import requests.exceptions
+    from google.api_core import exceptions as api_exceptions, retry
+    from google.auth import exceptions as auth_exceptions
     from google.cloud.exceptions import NotFound
     from google.cloud.storage import Blob, Client
     from google.cloud.storage.blob import _quote
     from google.cloud.storage.retry import (
-        DEFAULT_RETRY, DEFAULT_RETRY_IF_GENERATION_SPECIFIED,
+        _ADDITIONAL_RETRYABLE_STATUS_CODES, _RETRYABLE_TYPES,
+        ConditionalRetryPolicy, is_generation_specified,
     )
 except ImportError:
     raise ImproperlyConfigured("Could not load Google Cloud Storage bindings.\n"
@@ -30,14 +34,40 @@
 CONTENT_ENCODING = 'content_encoding'
 CONTENT_TYPE = 'content_type'
 
+# TODO(m1): revert the commit that introduced this after a version of
+#  `google-cloud-storage` with https://github.com/googleapis/python-storage/pull/727
+#  gets released and ScanForm is updated to use it.
+CUSTOM_RETRYABLE_TYPES = _RETRYABLE_TYPES + (
+    requests.exceptions.Timeout,
+)
+
+
+def _should_retry(exc):
+    """Predicate for determining when to retry."""
+    if isinstance(exc, CUSTOM_RETRYABLE_TYPES):
+        return True
+    elif isinstance(exc, api_exceptions.GoogleAPICallError):
+        return exc.code in _ADDITIONAL_RETRYABLE_STATUS_CODES
+    elif isinstance(exc, auth_exceptions.TransportError):
+        return _should_retry(exc.args[0])
+    else:
+        return False
+
+
+RETRY = retry.Retry(predicate=_should_retry)
+
+RETRY_IF_GENERATION_SPECIFIED = ConditionalRetryPolicy(
+    RETRY, is_generation_specified, ["query_params"]
+)
+
 
 class GoogleCloudFile(CompressedFileMixin, File):
     def __init__(self, name, mode, storage):
         self.name = name
         self.mime_type = mimetypes.guess_type(name)[0]
         self._mode = mode
         self._storage = storage
-        self.blob = storage.bucket.get_blob(name, timeout=storage.timeout)
+        self.blob = storage.bucket.get_blob(name, timeout=storage.timeout, retry=RETRY)
         if not self.blob and 'w' in mode:
             self.blob = Blob(
                 self.name, storage.bucket,
@@ -58,7 +88,8 @@ def _get_file(self):
             )
             if 'r' in self._mode:
                 self._is_dirty = False
-                self.blob.download_to_file(self._file, timeout=self._storage.timeout)
+                self.blob.download_to_file(self._file, timeout=self._storage.timeout,
+                                           retry=RETRY)
                 self._file.seek(0)
             if self._storage.gzip and self.blob.content_encoding == 'gzip':
                 self._file = self._decompress_file(mode=self._mode, file=self._file)
@@ -140,9 +171,9 @@ def get_default_settings(self):
     @property
     def retry_policy__if_generation_specified_or_immutable(self):
         if self.all_files_immutable:
-            return DEFAULT_RETRY
+            return RETRY
 
-        return DEFAULT_RETRY_IF_GENERATION_SPECIFIED
+        return RETRY_IF_GENERATION_SPECIFIED
 
     @property
     def client(self):
@@ -242,13 +273,13 @@ def delete(self, name):
     def exists(self, name):
         if not name:  # root element aka the bucket
             try:
-                self.client.get_bucket(self.bucket, timeout=self.timeout)
+                self.client.get_bucket(self.bucket, timeout=self.timeout, retry=RETRY)
                 return True
             except NotFound:
                 return False
 
         name = self._normalize_name(clean_name(name))
-        return bool(self.bucket.get_blob(name, timeout=self.timeout))
+        return bool(self.bucket.get_blob(name, timeout=self.timeout, retry=RETRY))
 
     def listdir(self, name):
         name = self._normalize_name(clean_name(name))
@@ -258,7 +289,7 @@ def listdir(self, name):
             name += '/'
 
         iterator = self.bucket.list_blobs(prefix=name, delimiter='/',
-                                          timeout=self.timeout)
+                                          timeout=self.timeout, retry=RETRY)
         blobs = list(iterator)
         prefixes = iterator.prefixes
 
@@ -276,7 +307,7 @@ def listdir(self, name):
 
     def _get_blob(self, name):
         # Wrap google.cloud.storage's blob to raise if the file doesn't exist
-        blob = self.bucket.get_blob(name, timeout=self.timeout)
+        blob = self.bucket.get_blob(name, timeout=self.timeout, retry=RETRY)
 
         if blob is None:
             raise NotFound('File does not exist: {}'.format(name))