refactor: accept generic <error>/<responsedescription> without server fingerprints

_parse_response hardcoded Stalwart's "No resources found" responsedescription
text and purelymail's {https://purelymail.com}does-not-exist error child to
recognise 404s in multistatus replies. Both <error> and <responsedescription>
are optional, server-defined children of <response> per RFC 4918, so the
content assertions were never warranted and adding the next server's 404 shape
meant editing the generic parser.

Accept either element generically. A genuinely novel tag still hits
error.weirdness(). The check_404 debug guard is now None-safe since a server
may legally send these elements without a response-level status.

Addresses §6.1 of docs/design/FULL_CODE_REVIEW_2026-06.md.

prompt: for point 6.1 in the code review document, perhaps the solution is to
fix so that any <error> or <responsedescription> is accepted, without any
hardcoded server fingerprints (though this will fail once a third server throws
in a <errortext> or something like that)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

AI Prompts:
claude-sonnet-4-6: PR #686.  Please look through it and describe what it does.
claude-sonnet-4-6: for point 6.1  in the code review document (docs/design/FULL_CODE_REVIEW_2026-06.md), perhaps the solutoin is to fix so that any <error> or <repsonsedescription> is accepted, without any hardcoded server fingerprints.  (though this will fail once a third server throws in a <errortext> or something like that)

Author: tobixen

caldav/response.py

@@ -488,28 +488,24 @@ def _parse_response(self, response: _Element) -> tuple[str, list[_Element], Any
                 href = _normalize_href(elem.text or "")
             elif elem.tag == dav.PropStat.tag:
                 propstats.append(elem)
-            elif elem.tag == "{DAV:}responsedescription":
-                ## This happens with Stalwart on a 404.
-                ## This code is mostly moot, but in debug
-                ## mode I want to be sure we do not toss away any data
-                error.assert_(elem.text == "No resources found")
-                check_404 = True
-            elif elem.tag == "{DAV:}error":
-                ## This happens with purelymail on a 404.
-                ## This code is mostly moot, but in debug
-                ## mode I want to be sure we do not toss away any data
-                children = elem.getchildren()
-                error.assert_(len(children) == 1)
-                error.assert_(children[0].tag == "{https://purelymail.com}does-not-exist")
+            elif elem.tag in ("{DAV:}responsedescription", "{DAV:}error"):
+                ## Both are optional children of <response> per RFC 4918
+                ## and carry server-defined content.  We've seen them on
+                ## 404s (Stalwart sends <responsedescription>No resources
+                ## found</responsedescription>, purelymail sends
+                ## <error><…:does-not-exist/></error>), so accept any such
+                ## element generically rather than fingerprinting servers.
                 check_404 = True
             else:
-                ## i.e. purelymail may contain one more tag, <error>...</error>
-                ## This is probably not a breach of the standard.  It may
-                ## probably be ignored.  But it's something we may want to
-                ## know.
+                ## A tag we don't recognise at all (e.g. a server inventing
+                ## an <errortext> element).  Not necessarily a standards
+                ## breach and probably ignorable, but worth surfacing.
                 error.weirdness("unexpected element found in response", elem)
         error.assert_(href)
-        if check_404:
+        if check_404 and status:
+            ## We've only ever observed <error>/<responsedescription> on
+            ## 404s; flag it in debug mode if a server pairs them with some
+            ## other status so we notice and revisit this handling.
             error.assert_("404" in status)
         return (cast(str, href), propstats, status)
 

docs/design/FULL_CODE_REVIEW_2026-06.md

@@ -454,11 +454,20 @@ producing drift bugs.
 ## 6. Altitude / design notes
 
 1. **`response.py:464` — server fingerprints hardcoded in the generic
-   parser.** purelymail's `{https://purelymail.com}does-not-exist` tag,
-   Stalwart's "No resources found" string, and SOGo status notes live in the
-   core multistatus path instead of going through the compatibility-hints
-   feature matrix. Adding the next server's 404 shape means editing generic
+   parser.** ✅ FIXED. purelymail's `{https://purelymail.com}does-not-exist`
+   tag, Stalwart's "No resources found" string, and SOGo status notes lived in
+   the core multistatus path instead of going through the compatibility-hints
+   feature matrix. Adding the next server's 404 shape meant editing generic
    parsing — the exact inversion the hints mechanism exists to avoid.
+
+   **Resolution:** `<error>` and `<responsedescription>` are optional children
+   of `<response>` per RFC 4918 with server-defined content, so no per-server
+   config (nor content fingerprint) is warranted at all — `_parse_response`
+   now accepts either element generically. A genuinely novel tag still hits
+   `error.weirdness()`. The `check_404` debug guard was made `None`-safe (a
+   server may legally send these without a response-level status). Tests:
+   `test_parse_sync_collection_generic_responsedescription` /
+   `test_parse_sync_collection_generic_error` in `tests/test_protocol.py`.
 2. **`vcal.fix()` is a regex-rewriting layer applied to every inbound
    object.** §2.1–§2.4 show the current fixups are individually broken in
    four different ways; the module's own TODOs flag the approach. Worth

tests/test_protocol.py

@@ -208,6 +208,53 @@ def test_parse_sync_collection_response(self):
         assert result.deleted[0] == "/cal/deleted.ics"
         assert result.sync_token == "new-token"
 
+    def test_parse_sync_collection_generic_responsedescription(self):
+        """A 404 <response> may carry an arbitrary <responsedescription>.
+
+        Per RFC 4918 <responsedescription> is an optional child of
+        <response>; its text is server-defined.  We must not hardcode
+        any particular server's wording (e.g. Stalwart's "No resources
+        found").
+        """
+        xml = b"""<?xml version="1.0"?>
+        <D:multistatus xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav">
+            <D:response>
+                <D:href>/cal/gone.ics</D:href>
+                <D:status>HTTP/1.1 404 Not Found</D:status>
+                <D:responsedescription>The thing you asked for is not here anymore</D:responsedescription>
+            </D:response>
+            <D:sync-token>tok</D:sync-token>
+        </D:multistatus>"""
+
+        result = DAVResponse.from_bytes(xml).parse_sync_collection()
+
+        assert result.deleted == ["/cal/gone.ics"]
+        assert result.changed == []
+
+    def test_parse_sync_collection_generic_error(self):
+        """A 404 <response> may carry an arbitrary <error> element.
+
+        Per RFC 4918 <error> is an optional child of <response> and its
+        children are server-defined.  We must not hardcode any
+        particular server's error condition (e.g. purelymail's
+        {https://purelymail.com}does-not-exist).
+        """
+        xml = b"""<?xml version="1.0"?>
+        <D:multistatus xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav"
+                       xmlns:X="https://example.com/ns">
+            <D:response>
+                <D:href>/cal/gone.ics</D:href>
+                <D:status>HTTP/1.1 404 Not Found</D:status>
+                <D:error><X:resource-must-be-null/></D:error>
+            </D:response>
+            <D:sync-token>tok</D:sync-token>
+        </D:multistatus>"""
+
+        result = DAVResponse.from_bytes(xml).parse_sync_collection()
+
+        assert result.deleted == ["/cal/gone.ics"]
+        assert result.changed == []
+
     def test_parse_complex_properties(self):
         """Parse complex properties like supported-calendar-component-set."""
         xml = b"""<?xml version="1.0"?>