Add function to validate calendar for CalDAV

CalDAV imposes a set of constraints on iCal Calendar objects. They are
spelled out in RFC 4791, section 4.1 [1]. Add an exported function to
validate a calendar according to those constraints, and return data that
is necessary for further CalDAV processing and which can only be
extracted if the calendar meets these constraints.

[1]: https://datatracker.ietf.org/doc/html/rfc4791#section-4.1

caldav/caldav.go

@@ -4,6 +4,7 @@
 package caldav
 
 import (
+	"fmt"
 	"time"
 
 	"github.com/emersion/go-ical"
@@ -15,6 +16,51 @@ func NewCalendarHomeSet(path string) webdav.BackendSuppliedHomeSet {
 	return &calendarHomeSet{Href: internal.Href{Path: path}}
 }
 
+// ValidateCalendarObject checks the validity of a calendar object according to
+// the contraints layed out in RFC 4791 section 4.1 and returns the only event
+// type and UID occuring in this calendar, or an error if the calendar could
+// not be validated.
+func ValidateCalendarObject(cal *ical.Calendar) (eventType string, uid string, err error) {
+	// Calendar object resources contained in calendar collections
+	// MUST NOT specify the iCalendar METHOD property.
+	if prop := cal.Props.Get(ical.PropMethod); prop != nil {
+		return "", "", fmt.Errorf("calendar resource must not specify METHOD property")
+	}
+
+	for _, comp := range cal.Children {
+		// Calendar object resources contained in calendar collections
+		// MUST NOT contain more than one type of calendar component
+		// (e.g., VEVENT, VTODO, VJOURNAL, VFREEBUSY, etc.) with the
+		// exception of VTIMEZONE components, which MUST be specified
+		// for each unique TZID parameter value specified in the
+		// iCalendar object.
+		if comp.Name != ical.CompTimezone {
+			if eventType == "" {
+				eventType = comp.Name
+			}
+			if eventType != comp.Name {
+				return "", "", fmt.Errorf("conflicting event types in calendar: %s, %s", eventType, comp.Name)
+			}
+			// TODO check VTIMEZONE for each TZID?
+		}
+
+		// Calendar components in a calendar collection that have
+		// different UID property values MUST be stored in separate
+		// calendar object resources.
+		compUID, err := comp.Props.Text(ical.PropUID)
+		if err != nil {
+			return "", "", fmt.Errorf("error checking component UID: %v", err)
+		}
+		if uid == "" {
+			uid = compUID
+		}
+		if uid != compUID {
+			return "", "", fmt.Errorf("conflicting UID values in calendar: %s, %s", uid, compUID)
+		}
+	}
+	return eventType, uid, nil
+}
+
 type Calendar struct {
 	Path                  string
 	Name                  string