MM-14753: Verifies that user can join teams and channels in spite of group constraints. (#10529)

* MM-147753: Verifies that users are allowed to be members of a team or a channel, based on group constraints, prior to allowing the API to add them.

* MM-14753: Allow methods to return meaningful results for deleted teams or channels.

* MM-14753: Renames methods to differentiate from permissions and other team and channel restrictions.

* MM-14753: Only check if users are team/channel members if team/channel is group constrained.

* MM-14753: Updates test function names.

* MM-14753: Changes a few method signatures.

* MM-14753: Small refactor and adds missing returns.

* MM-14753: Changes method names from Get* to Filter* name prefixes.

* MM-14753: Renames error variables.

* MM-14753: Updates method names for consistency with join table names.

* MM-14753: Adds case for non AppError return.

* Update i18n/en.json

api4/channel.go

@@ -1174,6 +1174,22 @@ func addChannelMember(c *Context, w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
+	if channel.GroupConstrained != nil && *channel.GroupConstrained {
+		nonMembers, err := c.App.FilterNonGroupChannelMembers([]string{member.UserId}, channel)
+		if err != nil {
+			if v, ok := err.(*model.AppError); ok {
+				c.Err = v
+			} else {
+				c.Err = model.NewAppError("addChannelMember", "api.channel.add_members.error", nil, err.Error(), http.StatusBadRequest)
+			}
+			return
+		}
+		if len(nonMembers) > 0 {
+			c.Err = model.NewAppError("addChannelMember", "api.channel.add_members.user_denied", map[string]interface{}{"UserIDs": nonMembers}, "", http.StatusBadRequest)
+			return
+		}
+	}
+
 	cm, err := c.App.AddChannelMember(member.UserId, channel, c.App.Session.UserId, postRootId, c.App.Session.Id)
 	if err != nil {
 		c.Err = err