Implement "expires" and "not-before" for groups.

README

@@ -157,6 +157,8 @@ nobody will be able to join the group.  The following fields are allowed:
    a time;
  - `max-history-age`: the time, in seconds, during which chat history is
    kept (default 14400, i.e. 4 hours);
+ - `not-before` and `expires`: the times (in ISO 8601 or RFC 3339 format)
+   between which joining the group is allowed;
  - `allow-recording`: if true, then recording is allowed in this group;
  - `unrestricted-tokens`: if true, then ordinary users (without the "op"
    privilege) are allowed to create tokens;

group/description.go

@@ -46,6 +46,12 @@ type Description struct {
 	// The time for which history entries are kept.
 	MaxHistoryAge int `json:"max-history-age,omitempty"`
 
+	// Time after which joining is no longer allowed
+	Expires *time.Time `json:"expires"`
+
+	// Time before which joining is not allowed
+	NotBefore *time.Time `json:"not-before,omitempty"`
+
 	// Whether users are allowed to log in with an empty username.
 	AllowAnonymous bool `json:"allow-anonymous,omitempty"`
 

group/group.go

@@ -607,6 +607,22 @@ func AddClient(group string, c Client, creds ClientCredentials) (*Group, error)
 				}
 				return nil, UserError(m)
 			}
+			if g.description.NotBefore != nil ||
+				g.description.Expires != nil {
+				now := time.Now()
+				if g.description.NotBefore != nil &&
+					g.description.NotBefore.After(now) {
+					return nil, UserError(
+						"this group is not open yet",
+					)
+				}
+				if g.description.Expires != nil &&
+					g.description.Expires.Before(now) {
+					return nil, UserError(
+						"this group is closed",
+					)
+				}
+			}
 			if g.description.Autokick {
 				ops := false
 				for _, c := range clients {