thumbnail: Support checking for images from streaming sources.

We may not always have trivial access to all of the bytes of the
uploaded file -- for instance, if the file was uploaded previously, or
by some other process.  Downloading the entire image in order to check
its headers is an inefficient use of time and bandwidth.

Adjust `maybe_thumbnail` and dependencies to potentially take a
`pyvips.Source` which supports streaming data from S3 or disk.  This
allows making the ImageAttachment row, if deemed appropriate, based on
only a few KB of data, and not the entire image.

zerver/lib/upload/local.py

@@ -7,13 +7,14 @@ from collections.abc import Callable, Iterator
 from datetime import datetime
 from typing import IO, Any, BinaryIO, Literal
 
+import pyvips
 from django.conf import settings
 from typing_extensions import override
 
 from zerver.lib.mime_types import guess_type
 from zerver.lib.thumbnail import resize_avatar, resize_logo
 from zerver.lib.timestamp import timestamp_to_datetime
-from zerver.lib.upload.base import ZulipUploadBackend
+from zerver.lib.upload.base import StreamingSourceWithSize, ZulipUploadBackend
 from zerver.lib.utils import assert_is_not_none
 from zerver.models import Realm, RealmEmoji, UserProfile
 
@@ -100,6 +101,13 @@ class LocalUploadBackend(ZulipUploadBackend):
     def save_attachment_contents(self, path_id: str, filehandle: BinaryIO) -> None:
         filehandle.write(read_local_file("files", path_id))
 
+    @override
+    def attachment_vips_source(self, path_id: str) -> StreamingSourceWithSize:
+        file_path = os.path.join(assert_is_not_none(settings.LOCAL_UPLOADS_DIR), "files", path_id)
+        assert_is_local_storage_path("files", file_path)
+        source = pyvips.Source.new_from_file(file_path)
+        return StreamingSourceWithSize(size=os.path.getsize(file_path), source=source)
+
     @override
     def delete_message_attachment(self, path_id: str) -> bool:
         return delete_local_file("files", path_id)