Implement FallbackUsers in API.

README.API

@@ -55,6 +55,14 @@ on-disk format but without any user definitions or cryptographic keys.
 Allowed methods are HEAD, GET, PUT and DELETE.  The only accepted
 content-type is `application/json`.
 
+### Fallback users
+
+    /galene-api/0/.groups/groupname/.fallback-users
+
+Contains fallback user descriptions, in the same format as the
+`fallbackUsers` field of the on-disk format.  Allowed methods are PUT and
+DELETE.  The only accepted content-type is `application/json`.
+
 ### Authentication keys
 
     /galene-api/0/.groups/groupname/.keys

group/description.go

@@ -540,6 +540,18 @@ func GetDescriptionNames() ([]string, error) {
 	return names, err
 }
 
+func SetFallbackUsers(group string, users []UserDescription) error {
+	groups.mu.Lock()
+	defer groups.mu.Unlock()
+
+	desc, err := readDescription(group, false)
+	if err != nil {
+		return err
+	}
+	desc.FallbackUsers = users
+	return rewriteDescriptionFile(desc.FileName, desc)
+}
+
 func SetKeys(group string, keys []map[string]any) error {
 	groups.mu.Lock()
 	defer groups.mu.Unlock()

webserver/api.go

@@ -128,6 +128,9 @@ func apiGroupHandler(w http.ResponseWriter, r *http.Request, pth string) {
 	if kind == ".users" {
 		apiUserHandler(w, r, g, rest)
 		return
+	} else if kind == ".fallback-users" && rest == "" {
+		fallbackUsersHandler(w, r, g)
+		return
 	} else if kind == ".keys" && rest == "" {
 		keysHandler(w, r, g)
 		return
@@ -433,6 +436,46 @@ func passwordHandler(w http.ResponseWriter, r *http.Request, g, user string) {
 	return
 }
 
+func fallbackUsersHandler(w http.ResponseWriter, r *http.Request, g string) {
+	if !checkAdmin(w, r) {
+		return
+	}
+
+	if r.Method == "PUT" {
+		ctype := parseContentType(r.Header.Get("Content-Type"))
+		if !strings.EqualFold(ctype, "application/json") {
+			http.Error(w, "unsupported content type",
+				http.StatusUnsupportedMediaType)
+			return
+		}
+		d := json.NewDecoder(r.Body)
+		var users []group.UserDescription
+		err := d.Decode(&users)
+		if err != nil {
+			httpError(w, err)
+			return
+		}
+		err = group.SetFallbackUsers(g, users)
+		if err != nil {
+			httpError(w, err)
+			return
+		}
+		w.WriteHeader(http.StatusNoContent)
+		return
+	} else if r.Method == "DELETE" {
+		err := group.SetFallbackUsers(g, nil)
+		if err != nil {
+			httpError(w, err)
+			return
+		}
+		w.WriteHeader(http.StatusNoContent)
+		return
+	}
+
+	methodNotAllowed(w, "PUT", "DELETE")
+	return
+}
+
 type jwkset = struct {
 	Keys []map[string]any `json:"keys"`
 }

webserver/api_test.go

@@ -168,6 +168,13 @@ func TestApi(t *testing.T) {
 		t.Errorf("Get groups: %v %#v", err, s)
 	}
 
+	resp, err = do("PUT", "/galene-api/0/.groups/test/.fallback-users",
+		"application/json", "", "",
+		`[{"password": "topsecret"}]`)
+	if err != nil || resp.StatusCode != http.StatusNoContent {
+		t.Errorf("Set fallback users: %v %v", err, resp.StatusCode)
+	}
+
 	resp, err = do("PUT", "/galene-api/0/.groups/test/.keys",
 		"application/jwk-set+json", "", "",
 		`{"keys": [{
@@ -261,10 +268,20 @@ func TestApi(t *testing.T) {
 		t.Errorf("Users (after delete): %#v", desc.Users)
 	}
 
+	if len(desc.FallbackUsers) != 1 {
+		t.Errorf("Keys: %v", len(desc.AuthKeys))
+	}
+
 	if len(desc.AuthKeys) != 1 {
 		t.Errorf("Keys: %v", len(desc.AuthKeys))
 	}
 
+	resp, err = do("DELETE", "/galene-api/0/.groups/test/.fallback-users",
+		"", "", "", "")
+	if err != nil || resp.StatusCode != http.StatusNoContent {
+		t.Errorf("Delete fallback users: %v %v", err, resp.StatusCode)
+	}
+
 	resp, err = do("DELETE", "/galene-api/0/.groups/test/.keys",
 		"", "", "", "")
 	if err != nil || resp.StatusCode != http.StatusNoContent {