caldav: add DiscoverContextURL

caldav/client.go

@@ -16,6 +16,12 @@ import (
 	"github.com/emersion/go-webdav/internal"
 )
 
+// DiscoverContextURL performs a DNS-based CardDAV service discovery as
+// described in RFC 6352 section 11. It returns the URL to the CardDAV server.
+func DiscoverContextURL(ctx context.Context, domain string) (string, error) {
+	return internal.DiscoverContextURL(ctx, "caldavs", domain)
+}
+
 // Client provides access to a remote CardDAV server.
 type Client struct {
 	*webdav.Client

carddav/client.go

@@ -5,7 +5,6 @@ import (
 	"context"
 	"fmt"
 	"mime"
-	"net"
 	"net/http"
 	"net/url"
 	"strconv"
@@ -20,37 +19,7 @@ import (
 // DiscoverContextURL performs a DNS-based CardDAV service discovery as
 // described in RFC 6352 section 11. It returns the URL to the CardDAV server.
 func DiscoverContextURL(ctx context.Context, domain string) (string, error) {
-	var resolver net.Resolver
-
-	// Only lookup carddavs (not carddav), plaintext connections are insecure
-	_, addrs, err := resolver.LookupSRV(ctx, "carddavs", "tcp", domain)
-	if dnsErr, ok := err.(*net.DNSError); ok {
-		if dnsErr.IsTemporary {
-			return "", err
-		}
-	} else if err != nil {
-		return "", err
-	}
-
-	if len(addrs) == 0 {
-		return "", fmt.Errorf("carddav: domain doesn't have an SRV record")
-	}
-	addr := addrs[0]
-
-	target := strings.TrimSuffix(addr.Target, ".")
-	if target == "" {
-		return "", fmt.Errorf("carddav: empty target in SRV record")
-	}
-
-	// TODO: perform a TXT lookup, check for a "path" key in the response
-	u := url.URL{Scheme: "https"}
-	if addr.Port == 443 {
-		u.Host = target
-	} else {
-		u.Host = fmt.Sprintf("%v:%v", target, addr.Port)
-	}
-	u.Path = "/.well-known/carddav"
-	return u.String(), nil
+	return internal.DiscoverContextURL(ctx, "carddavs", domain)
 }
 
 // Client provides access to a remote CardDAV server.

internal/client.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io"
 	"mime"
+	"net"
 	"net/http"
 	"net/url"
 	"path"
@@ -14,6 +15,42 @@ import (
 	"unicode"
 )
 
+// DiscoverContextURL performs a DNS-based CardDAV/CalDAV service discovery as
+// described in RFC 6352 section 11. It returns the URL to the CardDAV server.
+func DiscoverContextURL(ctx context.Context, service, domain string) (string, error) {
+	var resolver net.Resolver
+
+	// Only lookup TLS records, plaintext connections are insecure
+	_, addrs, err := resolver.LookupSRV(ctx, service+"s", "tcp", domain)
+	if dnsErr, ok := err.(*net.DNSError); ok {
+		if dnsErr.IsTemporary {
+			return "", err
+		}
+	} else if err != nil {
+		return "", err
+	}
+
+	if len(addrs) == 0 {
+		return "", fmt.Errorf("webdav: domain doesn't have an SRV record")
+	}
+	addr := addrs[0]
+
+	target := strings.TrimSuffix(addr.Target, ".")
+	if target == "" {
+		return "", fmt.Errorf("webdav: empty target in SRV record")
+	}
+
+	// TODO: perform a TXT lookup, check for a "path" key in the response
+	u := url.URL{Scheme: "https"}
+	if addr.Port == 443 {
+		u.Host = target
+	} else {
+		u.Host = fmt.Sprintf("%v:%v", target, addr.Port)
+	}
+	u.Path = "/.well-known/" + service
+	return u.String(), nil
+}
+
 // HTTPClient performs HTTP requests. It's implemented by *http.Client.
 type HTTPClient interface {
 	Do(req *http.Request) (*http.Response, error)