feat: add Ensurer interface for leadership guarantees

core/leadership/interface.go

@@ -104,7 +104,7 @@ type Token interface {
 	Check() error
 }
 
-// Checker exposes leadership testing capabilities.
+// Checker exposes leadership checking capabilities.
 type Checker interface {
 
 	// LeadershipCheck returns a Token representing the supplied unit's
@@ -117,6 +117,14 @@ type Checker interface {
 	LeadershipCheck(applicationId, unitId string) Token
 }
 
+// Ensurer describes the ability to guarantee leadership
+// for the duration of some operation.
+type Ensurer interface {
+	// WithLeader ensures that the input unit, holds leadership of the input
+	// application for the duration of execution of the input function.
+	WithLeader(ctx context.Context, appName, unitName string, fn func(context.Context) error) error
+}
+
 // Ticket is used to communicate leadership status to Tracker clients.
 type Ticket interface {