The user-agent slot holds the value of the User-Agent header sent by the MP3 client in its request for the stream. You need to hold onto this value purely for use in the Web interface—the User-Agent header identifies the program that made the request, so you can display the value on the page that lists all the playlists to make it easier to tell which playlist goes with which connection when multiple clients connect.
Finally, the lock slot holds a process lock created with the function make-process-lock, which is part of Allegro's MULTIPROCESSING package. You'll need to use that lock in certain functions that manipulate playlist objects to ensure that only one thread at a time manipulates a given playlist object. You can define the following macro, built upon the with-process-lock macro from MULTIPROCESSING, to give an easy way to wrap a body of code that should be performed while holding a playlist's lock:
(defmacro with-playlist-locked ((playlist) &body body)
`(with-process-lock ((lock ,playlist))
,@body))
The with-process-lock macro acquires exclusive access to the process lock given and then executes the body forms, releasing the lock afterward. By default, with-process-lock allows recursive locks, meaning the same thread can safely acquire the same lock multiple times.
Playlists As Song Sources
To use playlists as a source of songs for the Shoutcast server, you'll need to implement a method on the generic function find-song-source from Chapter 28. Since you're going to have multiple playlists, you need a way to find the right one for each client that connects to the server. The mapping part is easy—you can define a variable that holds an EQUAL hash table that you can use to map from some identifier to the playlist object.
(defvar *playlists* (make-hash-table :test #'equal))
You'll also need to define a process lock to protect access to this hash table like this:
(defparameter *playlists-lock* (make-process-lock :name "playlists-lock"))
Then define a function that looks up a playlist given an ID, creating a new playlist object if necessary and using with-process-lock to ensure that only one thread at a time manipulates the hash table.[306]
(defun lookup-playlist (id)
(with-process-lock (*playlists-lock*)
(or (gethash id *playlists*)
(setf (gethash id *playlists*) (make-instance 'playlist :id id)))))
Then you can implement find-song-source on top of that function and another, playlist-id, that takes an AllegroServe request object and returns the appropriate playlist identifier. The find-song-source function is also where you grab the User-Agent string out of the request object and stash it in the playlist object.
(defmethod find-song-source ((type (eql 'playlist)) request)
(let ((playlist (lookup-playlist (playlist-id request))))
(with-playlist-locked (playlist)
(let ((user-agent (header-slot-value request :user-agent)))
(when user-agent (setf (user-agent playlist) user-agent))))
playlist))
The trick, then, is how you implement playlist-id, the function that extracts the identifier from the request object. You have a couple options, each with different implications for the user interface. You can pull whatever information you want out of the request object, but however you decide to identify the client, you need some way for the user of the Web interface to get hooked up to the right playlist.
For now you can take an approach that "just works" as long as there's only one MP3 client per machine connecting to the server and as long as the user is browsing the Web interface from the machine running the MP3 client: you'll use the IP address of the client machine as the identifier. This way you can find the right playlist for a request regardless of whether the request is from the MP3 client or a Web browser. You will, however, provide a way in the Web interface to select a different playlist from the browser, so the only real constraint this choice puts on the application is that there can be only one connected MP3 client per client IP address.[307] The implementation of playlist-id looks like this:
(defun playlist-id (request)
(ipaddr-to-dotted (remote-host (request-socket request))))
The function request-socket is part of AllegroServe, while remote-host and ipaddr-to-dotted are part of Allegro's socket library.
To make a playlist usable as a song source by the Shoutcast server, you need to define methods on current-song, still-current-p, and maybe-move-to-next-song that specialize their source parameter on playlist. The current-song method is already taken care of: by defining the accessor current-song on the eponymous slot, you automatically got a current-song method specialized on playlist that returns the value of that slot. However, to make accesses to the playlist thread safe, you need to lock the playlist before accessing the current-song slot. In this case, the easiest way is to define an :around method like the following:
(defmethod current-song :around ((playlist playlist))
(with-playlist-locked (playlist) (call-next-method)))
Implementing still-current-p is also quite simple, assuming you can be sure that current-song gets updated with a new song object only when the current song actually changes. Again, you need to acquire the process lock to ensure you get a consistent view of the playlist's state.
(defmethod still-current-p (song (playlist playlist))
(with-playlist-locked (playlist)
(eql song (current-song playlist))))
The trick, then, is to make sure the current-song slot gets updated at the right times. However, the current song can change in a number of ways. The obvious one is when the Shoutcast server calls maybe-move-to-next-song. But it can also change when songs are added to the playlist, when the Shoutcast server has run out of songs, or even if the playlist's repeat mode is changed.
Rather than trying to write code specific to every situation to determine whether to update current-song, you can define a function, update-current-if-necessary, that updates current-song if the song object in current-song no longer matches the file that the current-idx slot says should be playing. Then, if you call this function after any manipulation of the playlist that could possibly put those two slots out of sync, you're sure to keep current-song set properly. Here are update-current-if-necessary and its helper functions:
(defun update-current-if-necessary (playlist)
(unless (equal (file (current-song playlist))
(file-for-current-idx playlist))
(reset-current-song playlist)))
(defun file-for-current-idx (playlist)
306
The intricacies of concurrent programming are beyond the scope of this book. The basic idea is that if you have multiple threads of control—as you will in this application with some threads running the shoutcast function and other threads responding to requests from the browser—then you need to make sure only one thread at a time manipulates an object in order to prevent one thread from seeing the object in an inconsistent state while another thread is working on it. In this function, for instance, if two new MP3 clients are connecting at the same time, they'd both try to add an entry to *playlists* and might interfere with each other. The with-process-lock ensures that each thread gets exclusive access to the hash table for long enough to do the work it needs to do.
307
This approach also assumes that every client machine has a unique IP address. This assumption should hold as long as all the users are on the same LAN but may not hold if clients are connecting from behind a firewall that does network address translation. Deploying this application outside a LAN will require some modifications, but if you want to deploy this application to the wider Internet, you'd better know enough about networking to figure out an appropriate scheme yourself.