improve error handling for GDrive downloads (#5704)

* improve error handling for GDrive downloads

* perform HTML check regardless of MD5 check

Author: pmeier

torchvision/datasets/utils.py

@@ -1,4 +1,5 @@
 import bz2
+import contextlib
 import gzip
 import hashlib
 import itertools
@@ -262,6 +263,26 @@ def download_file_from_google_drive(file_id: str, root: str, filename: Optional[
 
         _save_response_content(content, fpath)
 
+    # In case we deal with an unhandled GDrive API response, the file should be smaller than 10kB and contain only text
+    if os.stat(fpath).st_size < 10 * 1024:
+        with contextlib.suppress(UnicodeDecodeError), open(fpath) as fh:
+            text = fh.read()
+            # Regular expression to detect HTML. Copied from https://stackoverflow.com/a/70585604
+            if re.search(r"</?\s*[a-z-][^>]*\s*>|(&(?:[\w\d]+|#\d+|#x[a-f\d]+);)", text):
+                warnings.warn(
+                    f"We detected some HTML elements in the downloaded file. "
+                    f"This most likely means that the download triggered an unhandled API response by GDrive. "
+                    f"Please report this to torchvision at https://github.com/pytorch/vision/issues including "
+                    f"the response:\n\n{text}"
+                )
+
+    if md5 and not check_md5(fpath, md5):
+        raise RuntimeError(
+            f"The MD5 checksum of the download file {fpath} does not match the one on record."
+            f"Please delete the file and try again. "
+            f"If the issue persists, please report this to torchvision at https://github.com/pytorch/vision/issues."
+        )
+
 
 def _extract_tar(from_path: str, to_path: str, compression: Optional[str]) -> None:
     with tarfile.open(from_path, f"r:{compression[1:]}" if compression else "r") as tar: