debian: Provide last_update to produced ListedOrigin models

Use the value of the "Last-Modified" header from the HTTP response
resulting of the debian sources index HTTP request.

It will prevent to create loading tasks for debian packages with no
changes since last listing.

Related to T2400

swh/lister/debian/lister.py

@@ -7,6 +7,7 @@
 import bz2
 from collections import defaultdict
 from dataclasses import dataclass, field
+from email.utils import parsedate_to_datetime
 import gzip
 from itertools import product
 import logging
@@ -134,6 +135,10 @@ class DebianLister(Lister[DebianListerState, DebianPageType]):
             response = requests.get(url, stream=True)
             logging.debug("Fetched URL: %s, status code: %s", url, response.status_code)
             if response.status_code == 200:
+                last_modified = response.headers.get("Last-Modified")
+                self.last_sources_update = (
+                    parsedate_to_datetime(last_modified) if last_modified else None
+                )
                 decompressor = decompressors.get(compression)
                 if decompressor:
                     data = decompressor(response.raw).readlines()
@@ -224,6 +229,7 @@ class DebianLister(Lister[DebianListerState, DebianPageType]):
                     url=origin_url,
                     visit_type="deb",
                     extra_loader_arguments={"packages": {}},
+                    last_update=self.last_sources_update,
                 )
                 # origin will be yielded at the end of that method
                 origins_to_send[origin_url] = self.listed_origins[origin_url]

swh/lister/debian/tests/test_lister.py

@@ -4,6 +4,7 @@
 # See top-level LICENSE file for more information
 
 from collections import defaultdict
+from email.utils import formatdate
 import os
 from pathlib import Path
 from typing import Dict, List, Set, Tuple
@@ -78,7 +79,11 @@ def _init_test(
             if compression:
                 requests_mock.get(idx_url, status_code=404)
             else:
-                requests_mock.get(idx_url, text=sources)
+                requests_mock.get(
+                    idx_url,
+                    text=sources,
+                    headers={"Last-Modified": formatdate(usegmt=True)},
+                )
 
         for idx_url, _ in lister.debian_index_urls(suite, _components[1]):
             requests_mock.get(idx_url, status_code=404)
@@ -122,6 +127,7 @@ def _check_listed_origins(
                     ]
 
                     assert filtered_origins
+                    assert filtered_origins[0].last_update is not None
                     packages = filtered_origins[0].extra_loader_arguments["packages"]
                     # check the version info are available
                     assert package_version_key in packages