users: Fix uploading user avatars.

Due to recent refactoring in 9fb03cb2c7, a user could not
upload avatar if the server uses local upload backend and there
was already an avatar file for that user.

This commit fixes it to just check if there exists a file only
when importing and not when the user is actually trying to
change the avatar.

Fixes #30676.

zerver/lib/upload/__init__.py

@@ -260,6 +260,17 @@ def copy_avatar(source_profile: UserProfile, target_profile: UserProfile) -> Non
 def ensure_avatar_image(user_profile: UserProfile, medium: bool = False) -> None:
     file_path = user_avatar_path(user_profile)
 
+    final_file_path = upload_backend.get_avatar_path(file_path, medium)
+
+    if settings.LOCAL_AVATARS_DIR is not None:
+        output_path = os.path.join(
+            settings.LOCAL_AVATARS_DIR,
+            final_file_path,
+        )
+
+        if os.path.isfile(output_path):
+            return
+
     image_data, _ = upload_backend.get_avatar_contents(file_path)
 
     if medium:
@@ -267,7 +278,7 @@ def ensure_avatar_image(user_profile: UserProfile, medium: bool = False) -> None
     else:
         resized_avatar = resize_avatar(image_data)
     upload_backend.upload_single_avatar_image(
-        upload_backend.get_avatar_path(file_path, medium),
+        final_file_path,
         user_profile=user_profile,
         image_data=resized_avatar,
         content_type="image/png",

zerver/lib/upload/local.py

@@ -126,12 +126,7 @@ class LocalUploadBackend(ZulipUploadBackend):
         image_data: bytes,
         content_type: Optional[str],
     ) -> None:
-        output_path = os.path.join(
+        write_local_file("avatars", file_path, image_data)
-            assert_is_not_none(settings.LOCAL_AVATARS_DIR),
-            file_path,
-        )
-        if not os.path.isfile(output_path):
-            write_local_file("avatars", file_path, image_data)
 
     @override
     def delete_avatar_image(self, path_id: str) -> None: