status: add status manager isReadyOverride func

isReadyOverride is a callback that, when set and only if `running` is
not yet true, will be used to determine if a system is ready for a call.
We will pass the request URI to this method along with the
`manualStatus`. The first returned boolean is true if the system should
be seen as ready and the second is true if the override does handle the
given request. If it does not, then we will fall back to our normal
is-ready check.

status/manager.go

@@ -13,6 +13,16 @@ import (
 // values of a SubServerStatus's fields.
 type SubServerOption func(status *SubServerStatus)
 
+// WithIsReadyOverride is a functional option that can be used to set a callback
+// function that is used to check if a system is ready _iff_ the system running
+// status is not yet true. The call-back will be passed the request URI along
+// with any manual status that has been set for the subsystem.
+func WithIsReadyOverride(fn func(string, string) (bool, bool)) SubServerOption {
+	return func(status *SubServerStatus) {
+		status.isReadyOverride = fn
+	}
+}
+
 // SubServerStatus represents the status of a sub-server.
 type SubServerStatus struct {
 	// Disabled is true if the sub-server is available in the LiT bundle but
@@ -32,6 +42,15 @@ type SubServerStatus struct {
 
 	// Err will be a non-empty string if the sub-server failed to start.
 	Err string
+
+	// isReadyOverride is a call back that, when set and only if `running`
+	// is not yet true, will be used to determine if a system is ready for
+	// a call. We will pass the request URI to this method along with the
+	// `manualStatus`. The first returned boolean is true if the system
+	// should be seen as ready and the second is true if the override does
+	// handle the given request. If it does not, then we will fall back to
+	// our normal is-ready check.
+	isReadyOverride func(string, string) (bool, bool)
 }
 
 // newSubServerStatus constructs a new SubServerStatus.
@@ -60,6 +79,41 @@ func NewStatusManager() *Manager {
 	}
 }
 
+// IsSystemReady shows if the given sub-server ready to handle the a request for
+// the passed request URI. The first returned boolean is true if the system
+// is ready to handle the request. The second returned boolean is true if the
+// system has been disabled.
+func (s *Manager) IsSystemReady(name, req string) (bool, bool, error) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	server, ok := s.subServers[name]
+	if !ok {
+		return false, false, errors.New("a sub-server with " +
+			"name %s has not yet been registered")
+	}
+
+	if server.Disabled {
+		return false, true, nil
+	}
+
+	// If there is no override for this server or if the server is already
+	// running then we just return the 'running' status.
+	if server.isReadyOverride == nil || server.Running {
+		return server.Running, false, nil
+	}
+
+	// Otherwise, we check the override to see if this request is handled
+	// by the override and if it is, then if the override permits this call.
+	isReady, handled := server.isReadyOverride(req, server.customStatus)
+	if handled {
+		return isReady, false, nil
+	}
+
+	// Otherwise, we just return the running status.
+	return server.Running, false, nil
+}
+
 // SubServerStatus queries the current status of a given sub-server.
 //
 // NOTE: this is part of the litrpc.StatusServer interface.