Add Stalwart CalDAV compatibility hints and fix expansion assertions

Stalwart has several non-RFC-conformant behaviours uncovered during
integration testing:

* VALUE=DATE (all-day) recurring events are not returned by time-range
  searches even though datetime recurring events are.  Marked as
  search.recurrences.includes-implicit.event: fragile (broken for
  VALUE=DATE events).

* Recurring VTODOs are returned in search results but without the RRULE,
  so client-side expansion cannot find specific occurrences.  Marked as
  search.recurrences.includes-implicit.todo: fragile.

* Server-side CALDAV:expand is broken for events with exceptions (returns
  3 items instead of 2, because exceptions are stored as separate objects).
  Marked as search.recurrences.expanded.exception: unsupported.

* Master+exception VEVENTs are stored as separate CalendarObjectResources
  rather than as a single multi-VEVENT object (RFC violation).  New feature
  flag save-load.event.recurrences.exception tracks this; when unsupported,
  CalDAVSearcher now falls back to server-side CALDAV:expand (if available)
  so client-side expansion of the master alone does not yield duplicates.

* VTODO date searches with no DTSTART on the task are skipped; open-ended
  VTODO date searches return no results.  Covered by existing old-flags
  vtodo_datesearch_nodtstart_task_is_skipped and new no_search_openended.

Test assertions in testRecurringDateWithExceptionSearch and
testTodoDatesearch are now guarded by the appropriate feature flags so
the tests pass (or skip gracefully) on Stalwart.

Also fix a KeyError crash in setUp when a rate-limit features dict lacks
'interval' or 'count' keys (e.g. Stalwart's rate-limit config uses
default_sleep/max_sleep, not interval/count).

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

Author: tobixen

caldav/compatibility_hints.py

@@ -130,6 +130,16 @@ class FeatureSet:
         "save-load.event": {"description": "it's possible to save and load events to the calendar"},
         "save-load.event.recurrences": {"description": "it's possible to save and load recurring events to the calendar - events with an RRULE property set, including recurrence sets"},
         "save-load.event.recurrences.count": {"description": "The server will receive and store a recurring event with a count set in the RRULE", "default": {"support": "full"}},
+        ## This was Claude's suggestion and it works as of today, the
+        ## "unsupported" description matches the behaviour of the Stalwart server.
+        ## Stalwart apparently (in a breach with the RFC) stores the exception
+        ## information as a separate CalendarObjectResource.
+        ## Currently the search logic will do server-side expansion
+        ## if this flag is set to "unsupported", which is the correct behaviour for Stalwart.
+        ## The problem is that logically, this feature would also be "unsupported" if the exception
+        ## information was simply discarded, and the current search behaviour would in
+        ## such a case be incorrect if the exception is simply discarded.
+        "save-load.event.recurrences.exception": {"description": "When a VCALENDAR containing a master VEVENT (with RRULE) and exception VEVENT(s) (with RECURRENCE-ID) is stored, the server keeps them together as a single calendar object resource. When unsupported, the server splits exception VEVENTs into separate calendar objects, making client-side expansion unreliable (the master expands without knowing about its exceptions)."},
         "save-load.todo": {"description": "it's possible to save and load tasks to the calendar"},
         "save-load.todo.recurrences": {"description": "it's possible to save and load recurring tasks to the calendar"},
         "save-load.todo.recurrences.count": {"description": "The server will receive and store a recurring task with a count set in the RRULE", "default": {"support": "full"}},
@@ -1014,6 +1024,7 @@ def dotted_feature_set_list(self, compact=False):
     'auto-connect.url': {'basepath': '/ucaldav/'},
     'save-load.journal': {'support': 'ungraceful'},
     'save-load.todo.recurrences.thisandfuture': {'support': 'ungraceful'},
+    'save-load.event.recurrences.exception': False,
     ## search.time-range.alarm: not checked by the server tester
     'search.time-range.alarm': {'support': 'unsupported'},
     ## Huh?  Non-deterministic behaviour of the checking script?
@@ -1365,8 +1376,27 @@ def dotted_feature_set_list(self, compact=False):
     },
     'create-calendar.auto': True,
     'principal-search': {'support': 'ungraceful'},
-    'search.recurrences.expanded.exception': False,
     'search.time-range.alarm': False,
+    ## Stalwart supports implicit recurrence for datetime events but not for
+    ## all-day (VALUE=DATE) recurring events in time-range searches.
+    'search.recurrences.includes-implicit.event': {'support': 'fragile', 'behaviour': 'broken for all-day (VALUE=DATE) events'},
+    ## Stalwart returns the recurring todo in search results but doesn't return the
+    ## RRULE intact, so client-side expansion can't expand it to specific occurrences.
+    'search.recurrences.includes-implicit.todo': {'support': 'fragile'},
+    ## Stalwart doesn't handle exceptions properly in server-side CALDAV:expand:
+    ## returns 3 items instead of 2 for a recurring event with one exception
+    ## (the exception is stored as a separate object and returned twice).
+    'search.recurrences.expanded.exception': False,
+    ## Stalwart doesn't store master+exception VEVENTs correctly as a single resource
+    ## (returns 3 VEVENTs instead of 2 when the master+exception event is expanded).
+    ## Since server-side expansion is also broken, both paths give wrong results.
+    'save-load.event.recurrences.exception': {'support': 'unsupported'},
+    'old_flags': [
+        ## Stalwart does not return VTODO items without DTSTART in date searches
+        'vtodo_datesearch_nodtstart_task_is_skipped',
+        ## Stalwart does not return results for open-ended date searches on VTODOs
+        'no_search_openended',
+    ],
 }
 
 ## Lots of transient problems with purelymail

caldav/search.py

@@ -273,6 +273,17 @@ def _search_impl(
         if not self.expand and not server_expand:
             split_expanded = False
 
+        ## If the server stores exception VEVENTs as separate calendar objects, client-side
+        ## expansion is unreliable (the master expands without knowing its exceptions, yielding
+        ## duplicate occurrences).  Fall back to server-side expansion when it handles exceptions.
+        if (
+            self.expand
+            and not server_expand
+            and not calendar.client.features.is_supported("save-load.event.recurrences.exception")
+            and calendar.client.features.is_supported("search.recurrences.expanded.exception")
+        ):
+            server_expand = True
+
         if self.expand or server_expand:
             if not self.start or not self.end:
                 raise error.ReportError("can't expand without a date range")

tests/test_caldav.py

@@ -762,7 +762,7 @@ def setup_method(self):
 
         foo = self.is_supported("rate-limit", dict)
         if foo.get("enable"):
-            rate_delay = foo["interval"] / foo["count"]
+            rate_delay = foo.get("interval", 0) / foo.get("count", 1)
             self.caldav.request = _delay_decorator(self.caldav.request, t=rate_delay)
         foo = self.is_supported("search-cache", dict)
         if foo.get("behaviour") == "delay":
@@ -2642,9 +2642,9 @@ def testTodoDatesearch(self):
         todos2 = c.search(start=datetime(2025, 4, 14), todo=True, include_completed=True)
         todos3 = c.search(start=datetime(2025, 4, 14), todo=True)
 
-        assert isinstance(todos1[0], Todo)
-        assert isinstance(todos2[0], Todo)
         if not self.check_compatibility_flag("no_search_openended"):
+            assert isinstance(todos1[0], Todo)
+            assert isinstance(todos2[0], Todo)
             assert isinstance(todos3[0], Todo)
 
         ## * t6 should be returned, as it's a yearly task spanning over 2025
@@ -3283,12 +3283,19 @@ def testRecurringDateWithExceptionSearch(self):
             server_expand=True,
         )
 
-        assert len(r) == 2
-        if self.is_supported("search.recurrences.expanded.event"):
-            assert len(rs) == 2
+        ## Only assert exact count and RRULE-free output when exception handling
+        ## is reliable (either client-side or server-side expansion works correctly).
+        if self.is_supported("save-load.event.recurrences.exception") or self.is_supported(
+            "search.recurrences.expanded.exception"
+        ):
+            assert len(r) == 2
+            assert "RRULE" not in r[0].data
+            assert "RRULE" not in r[1].data
 
-        assert "RRULE" not in r[0].data
-        assert "RRULE" not in r[1].data
+        if self.is_supported("search.recurrences.expanded.event") and self.is_supported(
+            "search.recurrences.expanded.exception"
+        ):
+            assert len(rs) == 2
 
         asserts_on_results = [r]
         if self.is_supported("search.recurrences.expanded.exception"):