fix(caldav): expand RRULE for recurring events

internal/icalfix/icalfix.go

@@ -27,8 +27,20 @@ import "regexp"
 var tzidParamRe = regexp.MustCompile(`TZID=("?)([^";:\r\n]+)("?)`)
 
 // tzidLineRe matches a standalone "TZID:<name>" property line, as found
-// inside a VTIMEZONE component.
-var tzidLineRe = regexp.MustCompile(`(?m)^TZID:([^\r\n]+)$`)
+// inside a VTIMEZONE component. The trailing "(\r?)" capture group is
+// required, not cosmetic: Go's RE2 engine only matches "$" in multi-line
+// mode immediately before a literal "\n", not before "\r\n" — so without
+// consuming (and re-emitting) an optional trailing "\r" explicitly, this
+// regex silently fails to match on any CRLF-terminated feed (which is
+// every Outlook/Exchange-published ICS feed in practice), leaving the
+// VTIMEZONE's own TZID line un-normalized even though the "TZID=..."
+// parameter form (tzidParamRe, used on DTSTART/DTEND/etc.) still matches
+// fine. The practical effect was a VTIMEZONE whose declared TZID
+// ("W. Europe Standard Time") never matched the now-normalized
+// "TZID=Europe/Berlin" parameters referencing it elsewhere in the same
+// object, so any code trying to find "the VTIMEZONE for Europe/Berlin"
+// (see internal/icssub.Series.referencedTimezones) would never find one.
+var tzidLineRe = regexp.MustCompile(`(?m)^TZID:([^\r\n]+)(\r?)$`)
 
 // NormalizeTimeZones rewrites any recognized Windows timezone identifier
 // in data to its IANA equivalent, leaving everything else (including
@@ -51,7 +63,12 @@ func NormalizeTimeZones(data []byte) []byte {
 		if !ok {
 			return m
 		}
-		return []byte("TZID:" + iana)
+		// sub[2] is the optional trailing "\r" the regex consumed as part
+		// of the match (see the doc comment above) — it must be re-emitted
+		// here, or a CRLF-terminated line loses its "\r" and every
+		// subsequent line in the file ends up misaligned relative to the
+		// original byte offsets a caller might have recorded.
+		return []byte("TZID:" + iana + string(sub[2]))
 	})
 	return data
 }