Backend/requests: Fix calendar event cancellation

app/backend/src/couchers/email/calendar_events.py

@@ -25,7 +25,10 @@ def create_host_request_ics(
     host_request: HostRequest, other_name: str, hosting: bool, loc_context: LocalizationContext
 ) -> str:
     event = create_host_request_event(host_request, other_name, hosting, loc_context)
-    return event_to_ics(event, loc_context)
+
+    # METHOD:PUBLISH means this is part of a stream of calendar event information.
+    # It allows for later cancellation, and doesn't expose accept/decline functionality.
+    return event_to_ics(event, "PUBLISH", loc_context)
 
 
 def create_host_request_event(
@@ -36,6 +39,9 @@ def create_host_request_event(
     event = Event()
     event.uid = get_host_request_event_uid(host_request.host_request_id)
 
+    # Explicitly allow later sequencing of a cancellation with SEQUENCE:1
+    event.extra.append(ContentLine(name="SEQUENCE", value="0"))
+
     if hosting:
         event.name = get_emails_i18next().localize(
             "calendar_events.host_requests.title_host", loc_context.locale, {"name": other_name}
@@ -76,17 +82,20 @@ def create_host_request_cancellation_ics(
     )
     event.status = "CANCELLED"
 
-    # Cancellation is sequenced after creation (which defaults to sequence number 0)
+    # Sequence cancellation is after creation (which uses SEQUENCE:0)
     event.extra.append(ContentLine(name="SEQUENCE", value="1"))
 
-    return event_to_ics(event, loc_context)
+    # METHOD:PUBLISH means this is part of a stream of calendar event information.
+    # Gmail™ will immediately remove the event from the user's calendar.
+    # METHOD:CANCEL might leave the event in cancelled state or not work.
+    return event_to_ics(event, "PUBLISH", loc_context)
 
 
-def event_to_ics(event: Event, loc_context: LocalizationContext) -> str:
+def event_to_ics(event: Event, method: str | None, loc_context: LocalizationContext) -> str:
     # PRODID is mandatory and generally follows "-//[Organization]//[Product Name]//[Language]"
     calendar = Calendar(creator=f"-//Couchers.org//Couchers//{loc_context.locale.upper()}")
-    if event.status == "CANCELLED":
-        calendar.method = "CANCEL"
+    if method:
+        calendar.method = method
     calendar.events.add(event)
     return cast(str, calendar.serialize())
 

app/backend/src/tests/test_calendar_events.py

@@ -45,6 +45,7 @@ def test_ics_content():
 VERSION:2.0
 PRODID:-//Couchers.org//Couchers//EN
 BEGIN:VEVENT
+SEQUENCE:0
 DTSTART;VALUE=DATE:20000101
 DTEND;VALUE=DATE:20000103
 DESCRIPTION:<stripped>/42
@@ -53,6 +54,7 @@ def test_ics_content():
 UID:host_request.42@<stripped>
 URL:<stripped>/42
 END:VEVENT
+METHOD:PUBLISH
 END:VCALENDAR
     """.strip()
     )