client/web: add login client mode to web.Server

Adds new LoginOnly server option and swaps out API handler depending
on whether running in login mode or full web client mode.

Also includes some minor refactoring to the synology/qnap authorization
logic to allow for easier sharing between serveLoginAPI and serveAPI.

Updates tailscale/corp#14335

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

client/web/synology.go

@@ -16,11 +16,13 @@ import (
 )
 
 // authorizeSynology authenticates the logged-in Synology user and verifies
-// that they are authorized to use the web client.  It returns true if the
-// request was handled and no further processing is required.
-func authorizeSynology(w http.ResponseWriter, r *http.Request) (handled bool) {
+// that they are authorized to use the web client.
+// It reports true if the request is authorized to continue, and false otherwise.
+// authorizeSynology manages writing out any relevant authorization errors to the
+// ResponseWriter itself.
+func authorizeSynology(w http.ResponseWriter, r *http.Request) (ok bool) {
 	if synoTokenRedirect(w, r) {
-		return true
+		return false
 	}
 
 	// authenticate the Synology user
@@ -28,7 +30,7 @@ func authorizeSynology(w http.ResponseWriter, r *http.Request) (handled bool) {
 	out, err := cmd.CombinedOutput()
 	if err != nil {
 		http.Error(w, fmt.Sprintf("auth: %v: %s", err, out), http.StatusUnauthorized)
-		return true
+		return false
 	}
 	user := strings.TrimSpace(string(out))
 
@@ -36,14 +38,14 @@ func authorizeSynology(w http.ResponseWriter, r *http.Request) (handled bool) {
 	isAdmin, err := groupmember.IsMemberOfGroup("administrators", user)
 	if err != nil {
 		http.Error(w, err.Error(), http.StatusForbidden)
-		return true
+		return false
 	}
 	if !isAdmin {
 		http.Error(w, "not a member of administrators group", http.StatusForbidden)
-		return true
+		return false
 	}
 
-	return false
+	return true
 }
 
 func synoTokenRedirect(w http.ResponseWriter, r *http.Request) bool {