feat: add configurable crc32c checksumming for downloads

google/resumable_media/_download.py

@@ -124,8 +124,23 @@ class Download(DownloadBase):
             ``start`` to the end of the media.
         headers (Optional[Mapping[str, str]]): Extra headers that should
             be sent with the request, e.g. headers for encrypted data.
+        checksum Optional([str]): The type of checksum to compute to verify
+            the integrity of the object. The response headers must contain
+            a checksum of the requested type. If the headers lack an
+            appropriate checksum (for instance in the case of transcoded or
+            ranged downloads where the remote service does not know the
+            correct checksum) an INFO-level log will be emitted. Supported
+            values are "md5", "crc32c" and None.
     """
 
+    def __init__(
+        self, media_url, stream=None, start=None, end=None, headers=None, checksum="md5"
+    ):
+        super(Download, self).__init__(
+            media_url, stream=stream, start=start, end=end, headers=headers
+        )
+        self.checksum = checksum
+
     def _prepare_request(self):
         """Prepare the contents of an HTTP request.
 

google/resumable_media/requests/_helpers.py

@@ -19,6 +19,7 @@
 
 
 import functools
+import warnings
 
 from google.resumable_media import _helpers
 from google.resumable_media import common
@@ -33,6 +34,12 @@
 # The number of seconds to wait between bytes sent from the server.
 _DEFAULT_READ_TIMEOUT = 60
 
+_SLOW_CRC32C_WARNING = (
+    "Currently using crcmod in pure python form. This is a slow "
+    "implementation. Python 3 has a faster implementation, `google-crc32c`, "
+    "which will be used if it is installed."
+)
+
 
 class RequestsMixin(object):
     """Mix-in class implementing ``requests``-specific behavior.
@@ -134,3 +141,36 @@ def http_request(
         transport.request, method, url, data=data, headers=headers, **transport_kwargs
     )
     return _helpers.wait_and_retry(func, RequestsMixin._get_status_code, retry_strategy)
+
+
+def _get_crc32c_object():
+    """ Get crc32c object
+    Attempt to use the Google-CRC32c package. If it isn't available, try
+    to use CRCMod. CRCMod might be using a 'slow' varietal. If so, warn...
+    """
+    try:
+        import crc32c
+
+        crc_obj = crc32c.Checksum()
+    except ImportError:
+        try:
+            import crcmod
+
+            crc_obj = crcmod.predefined.Crc("crc-32c")
+            _is_fast_crcmod()
+
+        except ImportError:
+            raise ImportError("Failed to import either `google-crc32c` or `crcmod`")
+
+    return crc_obj
+
+
+def _is_fast_crcmod():
+    # Determine if this is using the slow form of crcmod.
+    nested_crcmod = __import__(
+        "crcmod.crcmod", globals(), locals(), ["_usingExtension"], 0,
+    )
+    fast_crc = getattr(nested_crcmod, "_usingExtension", False)
+    if not fast_crc:
+        warnings.warn(_SLOW_CRC32C_WARNING, RuntimeWarning, stacklevel=2)
+    return fast_crc