fix(update): treat an incomplete Docker Hub response as unreadable

followed_tags caught OSError, ValueError, KeyError and RuntimeError, but
http.client.IncompleteRead, BadStatusLine and LineTooLong derive from
http.client.HTTPException only. One truncated digest response therefore
escaped to collect_updates (docker.py:344, no try) and aborted the whole
Docker update run instead of printing the follow warning for that image.
Every other fetch site of the updater already catches HTTPException
(docker.py, pip.py).

Reported by CodeRabbit on #1370.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/unit/python/utils/update/test_follow.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import contextlib
+import http.client
 import io
 import json
 import unittest
@@ -54,6 +55,20 @@ class TestFollowedTags(unittest.TestCase):
         self.assertEqual(found, [])
         self.assertIn("WARN fedora:44", stderr.getvalue())
 
+    def test_an_incomplete_response_proposes_nothing_and_says_so(self) -> None:
+        stderr = io.StringIO()
+        with (
+            mock.patch(
+                "utils.update.follow.get",
+                side_effect=http.client.IncompleteRead(b"{", 120),
+            ),
+            contextlib.redirect_stderr(stderr),
+        ):
+            found = followed_tags("fedora", list(FEDORA), {"follow": "latest"}, "44")
+        self.assertEqual(found, [])
+        self.assertIn("WARN fedora:44", stderr.getvalue())
+        self.assertIn("IncompleteRead", stderr.getvalue())
+
     def test_a_rate_limited_digest_is_retried(self) -> None:
         limited = urllib.error.HTTPError("url", 429, "Too Many Requests", {}, None)
         body = json.dumps({"digest": "sha256:45"}).encode()

utils/update/follow.py

@@ -9,6 +9,7 @@ rawhide) point elsewhere.
 
 from __future__ import annotations
 
+import http.client
 import json
 import sys
 import time
@@ -69,7 +70,13 @@ def followed_tags(
     try:
         target = tag_digest(image, str(follow))
         kept = [tag for tag in newer if tag_digest(image, tag) == target]
-    except (OSError, ValueError, KeyError, RuntimeError) as exc:
+    except (
+        OSError,
+        ValueError,
+        KeyError,
+        RuntimeError,
+        http.client.HTTPException,
+    ) as exc:
         print(
             f"[follow] WARN {image}:{current}: Docker Hub digests unreadable "
             f"({type(exc).__name__}: {exc}); proposing nothing",