Implemented AGENT_ADDED and AGENT_REMOVED events.

Review: https://reviews.apache.org/r/52515/

src/common/protobuf_utils.cpp

@@ -529,6 +529,31 @@ mesos::master::Response::GetAgents::Agent createAgentResponse(
   return agent;
 }
 
+
+mesos::master::Event createAgentAdded(
+    const mesos::internal::master::Slave& slave)
+{
+  mesos::master::Event event;
+  event.set_type(mesos::master::Event::AGENT_ADDED);
+
+  event.mutable_agent_added()->mutable_agent()->CopyFrom(
+      createAgentResponse(slave));
+
+  return event;
+}
+
+
+mesos::master::Event createAgentRemoved(const SlaveID& slaveId)
+{
+  mesos::master::Event event;
+  event.set_type(mesos::master::Event::AGENT_REMOVED);
+
+  event.mutable_agent_removed()->mutable_agent_id()->CopyFrom(
+      slaveId);
+
+  return event;
+}
+
 } // namespace event {
 } // namespace master {
 

src/common/protobuf_utils.hpp

@@ -187,6 +187,15 @@ mesos::master::Event createTaskAdded(const Task& task);
 mesos::master::Response::GetAgents::Agent createAgentResponse(
     const mesos::internal::master::Slave& slave);
 
+
+// Helper for creating an `AGENT_ADDED` event from a `Slave`.
+mesos::master::Event createAgentAdded(
+    const mesos::internal::master::Slave& slave);
+
+
+// Helper for creating an `AGENT_REMOVED` event from a `SlaveID`.
+mesos::master::Event createAgentRemoved(const SlaveID& slaveId);
+
 } // namespace event {
 } // namespace master {
 

src/master/master.cpp

@@ -7341,6 +7341,10 @@ void Master::addSlave(
       unavailability,
       slave->totalResources,
       slave->usedResources);
+
+  if (!subscribers.subscribed.empty()) {
+    subscribers.send(protobuf::master::event::createAgentAdded(*slave));
+  }
 }
 
 
@@ -7507,6 +7511,10 @@ void Master::_removeSlave(
 
   sendSlaveLost(slave->info);
 
+  if (!subscribers.subscribed.empty()) {
+    subscribers.send(protobuf::master::event::createAgentRemoved(slave->id));
+  }
+
   delete slave;
 }