Fix Calendar: No expanded occurrences are returned for a daily recurrences crossing DST

tests/src/webdav/cal_query.rs

@@ -13,6 +13,7 @@ use groupware::{
     calendar::{CalendarEventData, alarm::ExpandAlarm, expand::CalendarEventExpansion},
 };
 use hyper::StatusCode;
+use std::str::FromStr;
 use store::write::serialize::rkyv_unarchive;
 use types::TimeRange;
 
@@ -342,6 +343,44 @@ fn roundtrip_expansion(ics: &str, ignore_errors: bool) {
     assert_eq!(events, events_archive);
 }
 
+#[test]
+fn calendar_expand_dst_fallback() {
+    let akl = Tz::from_str("Pacific/Auckland").unwrap();
+    let ical = ICalendar::parse(ICAL_DST_FALLBACK_ICS).unwrap();
+    let event_data = CalendarEventData::new(ical, akl, 1000, &mut None);
+    let expanded_bytes = rkyv::to_bytes::<rkyv::rancor::Error>(&event_data).unwrap();
+    let archive = rkyv_unarchive::<CalendarEventData>(&expanded_bytes).unwrap();
+
+    let window = archive
+        .expand(
+            akl,
+            TimeRange {
+                start: 1782777600,
+                end: 1786579200,
+            },
+        )
+        .unwrap();
+    assert!(
+        !window.is_empty(),
+        "recurrence expansion aborted across the Pacific/Auckland DST fall-back overlap"
+    );
+
+    let across_overlap = archive
+        .expand(
+            akl,
+            TimeRange {
+                start: 1775260800,
+                end: 1775433600,
+            },
+        )
+        .unwrap();
+    assert_eq!(
+        across_overlap.len(),
+        1,
+        "the ambiguous fall-back occurrence must resolve to its earliest instant"
+    );
+}
+
 fn rfc_file_name(num: usize) -> String {
     format!(
         "{}/john%40example.com/default/abcd{num}.ics",
@@ -823,6 +862,20 @@ END:VFREEBUSY
 END:VCALENDAR
 "#;
 
+const ICAL_DST_FALLBACK_ICS: &str = r#"BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//Example Corp.//CalDAV Client//EN
+BEGIN:VEVENT
+DTSTAMP:20260108T000000Z
+DTSTART;TZID=Pacific/Auckland:20260108T022000
+DURATION:PT2H10M
+RRULE:FREQ=DAILY;INTERVAL=3
+SUMMARY:Auckland DST fall-back recurrence
+UID:auckland-dst-fallback@example.com
+END:VEVENT
+END:VCALENDAR
+"#;
+
 const ICAL_RFC_ABCD1_ICS: &str = r#"BEGIN:VCALENDAR
 VERSION:2.0
 PRODID:-//Example Corp.//CalDAV Client//EN