GUAC-1101: Implement reserved concurrency policy.

extensions/guacamole-auth-jdbc/modules/guacamole-auth-jdbc-base/src/main/java/org/glyptodon/guacamole/auth/jdbc/socket/ReservedGuacamoleSocketService.java

@@ -24,9 +24,12 @@ package org.glyptodon.guacamole.auth.jdbc.socket;
 
 import com.google.inject.Singleton;
 import java.util.List;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
 import org.glyptodon.guacamole.auth.jdbc.user.AuthenticatedUser;
 import org.glyptodon.guacamole.auth.jdbc.connection.ModeledConnection;
 import org.glyptodon.guacamole.GuacamoleException;
+import org.glyptodon.guacamole.GuacamoleResourceConflictException;
 
 
 /**
@@ -43,17 +46,138 @@ import org.glyptodon.guacamole.GuacamoleException;
 public class ReservedGuacamoleSocketService
     extends AbstractGuacamoleSocketService {
 
+    /**
+     * An arbitrary number of reservations associated with a specific user.
+     * Initially, each Reservation instance represents exactly one reservation,
+     * but future calls to acquire() may increase this value. Once the
+     * reservation count is reduced to zero by calls to release(), a
+     * Reservation instance is empty and cannot be reused. It must be discarded
+     * and replaced with a fresh Reservation.
+     * 
+     * This is necessary as each Reservation will be stored within a Map, and
+     * the effect of acquire() must be deterministic. If Reservations could be
+     * reused, the internal count could potentially increase after being
+     * removed from the map, resulting in a successful acquire() that really
+     * should have failed.
+     */
+    private static class Reservation {
+
+        /**
+         * The username of the user associated with this reservation.
+         */
+        private final String username;
+
+        /**
+         * The number of reservations effectively present under the associated
+         * username.
+         */
+        private int count = 1;
+
+        /**
+         * Creates a new reservation which tracks the overall number of
+         * reservations for a given user.
+         * @param username 
+         */
+        public Reservation(String username) {
+            this.username = username;
+        }
+
+        /**
+         * Attempts to acquire a new reservation under the given username. If
+         * this reservation is for a different user, or the reservation has
+         * expired, this will fail.
+         *
+         * @param username
+         *     The username of the user to acquire the reservation for.
+         *
+         * @return
+         *     true if the reservation was successful, false otherwise.
+         */
+        public boolean acquire(String username) {
+
+            // Acquire always fails if for the wrong user
+            if (!this.username.equals(username))
+                return false;
+
+            // Determine success/failure based on count
+            synchronized (this) {
+
+                // If already expired, no further reservations are allowed
+                if (count == 0)
+                    return false;
+
+                // Otherwise, add another reservation, report success
+                count++;
+                return true;
+                
+            }
+            
+        }
+
+        /**
+         * Releases a previous reservation. The result of calling this function
+         * without a previous matching call to acquire is undefined.
+         *
+         * @return
+         *     true if the last reservation has been released and this
+         *     reservation is now empty, false otherwise.
+         */
+        public boolean release() {
+            synchronized (this) {
+
+                // Reduce reservation count
+                count--;
+
+                // Empty if no reservations remain
+                return count == 0;
+                
+            }
+        }
+        
+    }
+
+    /**
+     * Map of connection identifier to associated reservations.
+     */
+    private final ConcurrentMap<String, Reservation> reservations =
+            new ConcurrentHashMap<String, Reservation>();
+    
     @Override
     protected ModeledConnection acquire(AuthenticatedUser user,
             List<ModeledConnection> connections) throws GuacamoleException {
-        // STUB
-        throw new UnsupportedOperationException("STUB");
+
+        String username = user.getUser().getIdentifier();
+        
+        // Return the first successfully-reserved connection
+        for (ModeledConnection connection : connections) {
+
+            String identifier = connection.getIdentifier();
+
+            // Attempt to reserve connection, return if successful
+            Reservation reservation = reservations.putIfAbsent(identifier, new Reservation(username));
+            if (reservation == null || reservation.acquire(username))
+                return connection;
+
+        }
+
+        // Already in use
+        throw new GuacamoleResourceConflictException("Cannot connect. This connection is in use.");
+
     }
 
     @Override
     protected void release(AuthenticatedUser user, ModeledConnection connection) {
-        // STUB
-        throw new UnsupportedOperationException("STUB");
+
+        String identifier = connection.getIdentifier();
+        
+        // Retrieve active reservation (which must exist)
+        Reservation reservation = reservations.get(identifier);
+        assert(reservation != null);
+
+        // Release reservation, remove from map if empty
+        if (reservation.release())
+            reservations.remove(identifier);
+        
     }
 
 }