SubscriptionManagerService::shouldRedirectCurrentUserOnLogin() used to delegate the redirect decision to the default_connector plugin alone. It now polls every installed connector and redirects only if all of them agree (#3616773: shouldRedirectCurrentUserOnLogin() only consults the default connector): any connector answering FALSE vetoes the redirect, since a veto means the user is already engaged with that connector (for example a pending checkout session).
Failure semantics: a connector that throws during instantiation or while answering is logged at WARNING and treated as an abstention — the remaining connectors decide — so a broken plugin can never break login or permanently suppress the redirect. If no connector responds at all, no redirect happens.
What you need to do
Single-connector sites see no behavior change. On multi-connector sites, the redirect now requires unanimity where previously only the default connector's answer counted. Connector modules should implement shouldRedirectCurrentUserOnLogin() to return FALSE only when they have a concrete reason to suppress the redirect (the user is mid-flow with this service), TRUE otherwise — an unconditional FALSE now suppresses the post-login subscribe redirect for the whole site.
