Support custom user principal and home set paths

Currently, the user principal path and the home set path are both
hardcoded to "/", for both CalDAV and CardDAV. This poses a challenge if
one wishes to run a CardDAV and CalDAV server in the same server.

This commit introduces the concept of a UserPrincipalBackend. This
backend must provide the path of the current user's principal URL from
the given request context.

The CalDAV and CardDAV backends are extended to also function as
UserPrincipalBackend. In addition, they are required to supply the path
of the respective home set (`calendar-home-set` and
`addressbook-home-set`). The CardDAV and CalDAV servers act accordingly.

The individual servers will continue to work as before (including the
option of keeping everything at "/"). If one wishes to run CardDAV and
CalDAV in parallel, the new `webdav.ServeUserPrincipal()` can be used as
a convenience function to serve a common user principal URL for both
servers. The input for this function can be easily computed by the
application by getting the home set paths from the backends and using
`caldav.NewCalendarHomeSet()` and `carddav.NewAddressbookHomeSet()` to
create the home sets.

Note that the storage backend will have to know about these paths as
well. For any non-trivial use case, a storage backend should probably
have access to the same UserPrincipalBackend. That is, however, an
implementation detail and doesn't have to be reflected in the
interfaces.

caldav/server.go

@@ -8,7 +8,7 @@ import (
 	"time"
 
 	"github.com/emersion/go-ical"
-
+	"github.com/emersion/go-webdav"
 	"github.com/emersion/go-webdav/internal"
 )
 
@@ -16,10 +16,13 @@ import (
 
 // Backend is a CalDAV server backend.
 type Backend interface {
+	CalendarHomeSetPath(ctx context.Context) (string, error)
 	Calendar(ctx context.Context) (*Calendar, error)
 	GetCalendarObject(ctx context.Context, path string, req *CalendarCompRequest) (*CalendarObject, error)
 	ListCalendarObjects(ctx context.Context, req *CalendarCompRequest) ([]CalendarObject, error)
 	QueryCalendarObjects(ctx context.Context, query *CalendarQuery) ([]CalendarObject, error)
+
+	webdav.UserPrincipalBackend
 }
 
 // Handler handles CalDAV HTTP requests. It can be used to create a CalDAV
@@ -35,12 +38,17 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if r.URL.Path == "/.well-known/caldav" {
-		http.Redirect(w, r, "/", http.StatusMovedPermanently)
+	principalPath, err := h.Backend.CurrentUserPrincipal(r.Context())
+	if err != nil {
+		http.Error(w, "caldav: failed to determine current user principal", http.StatusInternalServerError)
+		return
+	}
+
+	if r.URL.Path == "/.well-known/caldav" {
+		http.Redirect(w, r, principalPath, http.StatusMovedPermanently)
 		return
 	}
 
-	var err error
 	switch r.Method {
 	case "REPORT":
 		err = h.handleReport(w, r)
@@ -182,7 +190,17 @@ type backend struct {
 func (b *backend) Options(r *http.Request) (caps []string, allow []string, err error) {
 	caps = []string{"calendar-access"}
 
-	if r.URL.Path == "/" {
+	homeSetPath, err := b.Backend.CalendarHomeSetPath(r.Context())
+	if err != nil {
+		return nil, nil, err
+	}
+
+	principalPath, err := b.Backend.CurrentUserPrincipal(r.Context())
+	if err != nil {
+		return nil, nil, err
+	}
+
+	if r.URL.Path == "/" || r.URL.Path == principalPath || r.URL.Path == homeSetPath {
 		return caps, []string{http.MethodOptions, "PROPFIND", "REPORT"}, nil
 	}
 
@@ -209,14 +227,30 @@ func (b *backend) HeadGet(w http.ResponseWriter, r *http.Request) error {
 }
 
 func (b *backend) Propfind(r *http.Request, propfind *internal.Propfind, depth internal.Depth) (*internal.Multistatus, error) {
+	homeSetPath, err := b.Backend.CalendarHomeSetPath(r.Context())
+	if err != nil {
+		return nil, err
+	}
+	principalPath, err := b.Backend.CurrentUserPrincipal(r.Context())
+	if err != nil {
+		return nil, err
+	}
+
 	var resps []internal.Response
-	if r.URL.Path == "/" {
+
+	if r.URL.Path == principalPath {
+		resp, err := b.propfindUserPrincipal(r.Context(), propfind, homeSetPath)
+		if err != nil {
+			return nil, err
+		}
+		resps = append(resps, *resp)
+	} else if r.URL.Path == homeSetPath {
 		cal, err := b.Backend.Calendar(r.Context())
 		if err != nil {
 			return nil, err
 		}
 
-		resp, err := b.propfindCalendar(propfind, cal)
+		resp, err := b.propfindCalendar(r.Context(), propfind, cal)
 		if err != nil {
 			return nil, err
 		}
@@ -225,13 +259,38 @@ func (b *backend) Propfind(r *http.Request, propfind *internal.Propfind, depth i
 		if depth != internal.DepthZero {
 			// TODO
 		}
+	} else {
+		// TODO
 	}
 
 	return internal.NewMultistatus(resps...), nil
 }
 
-func (b *backend) propfindCalendar(propfind *internal.Propfind, cal *Calendar) (*internal.Response, error) {
+func (b *backend) propfindUserPrincipal(ctx context.Context, propfind *internal.Propfind, homeSetPath string) (*internal.Response, error) {
+	principalPath, err := b.Backend.CurrentUserPrincipal(ctx)
+	if err != nil {
+		return nil, err
+	}
 	props := map[xml.Name]internal.PropfindFunc{
+		internal.CurrentUserPrincipalName: func(*internal.RawXMLValue) (interface{}, error) {
+			return &internal.CurrentUserPrincipal{Href: internal.Href{Path: principalPath}}, nil
+		},
+		calendarHomeSetName: func(*internal.RawXMLValue) (interface{}, error) {
+			return &calendarHomeSet{Href: internal.Href{Path: homeSetPath}}, nil
+		},
+	}
+	return internal.NewPropfindResponse(principalPath, propfind, props)
+}
+
+func (b *backend) propfindCalendar(ctx context.Context, propfind *internal.Propfind, cal *Calendar) (*internal.Response, error) {
+	principalPath, err := b.Backend.CurrentUserPrincipal(ctx)
+	if err != nil {
+		return nil, err
+	}
+	props := map[xml.Name]internal.PropfindFunc{
+		internal.CurrentUserPrincipalName: func(*internal.RawXMLValue) (interface{}, error) {
+			return &internal.CurrentUserPrincipal{Href: internal.Href{Path: principalPath}}, nil
+		},
 		internal.ResourceTypeName: func(*internal.RawXMLValue) (interface{}, error) {
 			return internal.NewResourceType(internal.CollectionName, calendarName), nil
 		},
@@ -252,14 +311,6 @@ func (b *backend) propfindCalendar(propfind *internal.Propfind, cal *Calendar) (
 				},
 			}, nil
 		},
-		// TODO: this is a principal property
-		calendarHomeSetName: func(*internal.RawXMLValue) (interface{}, error) {
-			return &calendarHomeSet{Href: internal.Href{Path: "/"}}, nil
-		},
-		// TODO: this should be set on all resources
-		internal.CurrentUserPrincipalName: func(*internal.RawXMLValue) (interface{}, error) {
-			return &internal.CurrentUserPrincipal{Href: internal.Href{Path: "/"}}, nil
-		},
 	}
 
 	if cal.Description != "" {