Import script: Make sure medium avatars exist during import.

During a slack import, we don't have medium-size avatars already
available in the export data set (and possibly also with a normal
import/export?).  The medium size avatar can be created by the
'ensure_medium_avatar_image' function, which checks if the medium
image exists, and if it doesn't, it creates the image.

This commit was substantially edited by tabbott to get rid of an
undefined variable bug, avoid initializing the upload backend classes
in a loop, and add some TODO notes on things that could be improved
later.

zerver/lib/export.py

@@ -16,6 +16,7 @@ import shutil
 import subprocess
 import tempfile
 from zerver.lib.avatar_hash import user_avatar_hash, user_avatar_path_from_ids
+from zerver.lib.upload import S3UploadBackend, LocalUploadBackend
 from zerver.lib.create_user import random_api_key
 from zerver.lib.bulk_create import bulk_create_users
 from zerver.models import UserProfile, Realm, Client, Huddle, Stream, \
@@ -1323,7 +1324,20 @@ def import_uploads_local(import_dir: Path, processing_avatars: bool=False) -> No
             subprocess.check_call(["mkdir", "-p", os.path.dirname(file_path)])
         shutil.copy(orig_file_path, file_path)
 
+    if processing_avatars:
+        # Ensure that we have medium-size avatar images for every
+        # avatar.  TODO: This implementation is hacky, both in that it
+        # does get_user_profile_by_id for each user, and in that it
+        # might be better to require the export to just have these.
+        upload_backend = LocalUploadBackend()
+        for record in records:
+            if record['s3_path'].endswith('.original'):
+                user_profile = get_user_profile_by_id(record['user_profile_id'])
+                # If medium sized avatar does not exist, this creates it using the original image
+                upload_backend.ensure_medium_avatar_image(user_profile=user_profile)
+
 def import_uploads_s3(bucket_name: str, import_dir: Path, processing_avatars: bool=False) -> None:
+    upload_backend = S3UploadBackend()
     conn = S3Connection(settings.S3_KEY, settings.S3_SECRET_KEY)
     bucket = conn.get_bucket(bucket_name, validate=True)
 
@@ -1358,6 +1372,14 @@ def import_uploads_s3(bucket_name: str, import_dir: Path, processing_avatars: bo
 
         key.set_contents_from_filename(os.path.join(import_dir, record['path']), headers=headers)
 
+        if processing_avatars:
+            # TODO: Ideally, we'd do this in a separate pass, after
+            # all the avatars have been uploaded, since we may end up
+            # unnecssarily resizing images just before the medium-size
+            # image in the export is uploaded.  See the local uplods
+            # code path for more notes.
+            upload_backend.ensure_medium_avatar_image(user_profile=user_profile)
+
 def import_uploads(import_dir: Path, processing_avatars: bool=False) -> None:
     if processing_avatars:
         logging.info("Importing avatars")