Expose object modification evidence

tests/test_object_storage.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+from datetime import datetime, timezone
 from io import BytesIO
 from pathlib import Path
 import tempfile
@@ -26,10 +27,12 @@ class _S3Error(RuntimeError):
 class _FakeS3Client:
     def __init__(self) -> None:
         self.objects: dict[str, bytes] = {}
+        self.modified_at: dict[str, datetime] = {}
         self.head_error: Exception | None = None
 
     def put_object(self, *, Key: str, Body: bytes, **_kwargs) -> None:
         self.objects[Key] = Body
+        self.modified_at[Key] = datetime.now(timezone.utc)
 
     def get_object(self, *, Key: str, **_kwargs):
         try:
@@ -45,10 +48,40 @@ class _FakeS3Client:
             payload = self.objects[Key]
         except KeyError as exc:
             raise _S3Error("NotFound", 404) from exc
-        return {"ContentLength": len(payload)}
+        return {
+            "ContentLength": len(payload),
+            "LastModified": self.modified_at[Key],
+        }
 
     def delete_object(self, *, Key: str, **_kwargs) -> None:
         self.objects.pop(Key, None)
+        self.modified_at.pop(Key, None)
+
+    def list_objects_v2(
+        self,
+        *,
+        Prefix: str,
+        MaxKeys: int,
+        StartAfter: str | None = None,
+        **_kwargs,
+    ):
+        keys = [
+            key
+            for key in sorted(self.objects)
+            if key.startswith(Prefix) and (StartAfter is None or key > StartAfter)
+        ]
+        selected = keys[:MaxKeys]
+        return {
+            "Contents": [
+                {
+                    "Key": key,
+                    "Size": len(self.objects[key]),
+                    "LastModified": self.modified_at[key],
+                }
+                for key in selected
+            ],
+            "IsTruncated": len(keys) > len(selected),
+        }
 
 
 class ObjectStorageTests(unittest.TestCase):
@@ -67,6 +100,7 @@ class ObjectStorageTests(unittest.TestCase):
 
             self.assertEqual(b"a", backend.get_bytes("campaign/a.eml"))
             self.assertEqual(1, backend.stat("campaign/a.eml").size_bytes)
+            self.assertIsNotNone(backend.stat("campaign/a.eml").modified_at)
             self.assertEqual(
                 ("campaign/a.eml",), tuple(item.key for item in first.objects)
             )
@@ -134,6 +168,10 @@ class ObjectStorageTests(unittest.TestCase):
         backend.put_bytes("campaign/message.eml", b"message/rfc822")
         self.assertTrue(backend.exists("campaign/message.eml"))
         self.assertEqual(b"message/rfc822", backend.get_bytes("campaign/message.eml"))
+        self.assertIsNotNone(backend.stat("campaign/message.eml").modified_at)
+        self.assertIsNotNone(
+            backend.list_objects(prefix="campaign/").objects[0].modified_at
+        )
         self.assertFalse(backend.exists("campaign/missing.eml"))
 
         client.head_error = _S3Error("AccessDenied", 403)