fixup! Encapsulate file mirroring in service class

Author: nadove-ucsc

src/azul/indexer/mirror_service.py

@@ -52,8 +52,8 @@ class FilePart(SerializableAttrs):
     """
     A part of a mirrored file
     """
-    #: The part number, starting at 0 for the first part. Note that the S3 API
-    #: numbers parts starting at 1.
+    #: The part number, starting at 0 for the first part, unlike S3 API part
+    #: numbers, which start at 1.
     #:
     index: int
 
@@ -64,7 +64,7 @@ class FilePart(SerializableAttrs):
     #:
     size: int
 
-    #: Various quotas related to parts and part sizes
+    #: Various S3 quotas related to parts and part sizes
     #: https://docs.aws.amazon.com/AmazonS3/latest/userguide/qfacts.html
     #:
     min_size: ClassVar[int] = 5 * 1024 ** 2
@@ -102,14 +102,13 @@ def next(self, file: File) -> Self | None:
         last part.
         """
         assert file.size is not None, R('File size unknown', file)
-        stop = self.offset + self.size
-        if stop == file.size:
+        next_offset = self.offset + self.size
+        if next_offset == file.size:
             return None
-        elif 0 < stop < file.size:
-            return attr.evolve(self,
-                               index=self.index + 1,
-                               offset=stop,
-                               size=min(self.size, file.size - stop))
+        elif 0 < next_offset < file.size:
+            next_index = self.index + 1
+            next_size = min(self.size, file.size - next_offset)
+            return attr.evolve(self, index=next_index, offset=next_offset, size=next_size)
         else:
             assert False, R('Part range exceeds file size', self, file)