Implementing a simple pub/sub design pattern in Go

This code defines a Agent struct that implements a simple pub/sub agent in Golang. The Agent struct has a mu field of type sync.Mutex, a subs field of type map[string][]chan string, a quit field of type chan struct{}, and a closed field of type bool. The mu field is used to protect access to the subs and closed fields using a mutex, which is a synchronization mechanism that allows only one goroutine to access these fields at a time. The subs field is a map that holds a list of channels for each topic, allowing subscribers to receive messages published to that topic. The quit field is a channel that is closed when the agent is closed, allowing goroutines that are blocked on the channel to unblock and exit. The closed field is a flag that indicates whether the agent has been closed.

The NewAgent function creates and returns a new Agent struct. This function initializes the subs field as a new empty map, and the quit field as a new channel. The Publish method allows a message to be published to a topic. This method acquires a lock on the mu mutex, and then sends the message to all channels in the subs map for the specified topic. The Subscribe method allows a goroutine to subscribe to a topic. This method acquires a lock on the mu mutex, and then creates a new channel and adds it to the list of channels for the specified topic in the subs map. This method returns the newly-created channel, allowing the subscriber to receive messages published to the topic.

The Close method closes the agent and all channels in the subs map. This method acquires a lock on the mu mutex, sets the closed flag to true, closes the quit channel, and then closes all channels in the subs map. This allows goroutines that are blocked on these channels to unblock and exit.

The main function creates a new agent, subscribes to the "foo" topic, publishes a message to the "foo" topic, receives and prints the message, and then closes the agent.