Change the UserManager to use CompletableFutures to avoid on-thread IO

* EventContextKeys that were users are now UUIDs.

src/main/java/org/spongepowered/api/command/parameter/Parameter.java

@@ -655,8 +655,8 @@ static Parameter.Value.Builder<URL> url() {
      *
      * @return A {@link Parameter.Value.Builder}
      */
-    static Parameter.Value.Builder<User> user() {
-        return Parameter.builder(User.class, ResourceKeyedValueParameters.USER);
+    static Parameter.Value.Builder<UUID> user() {
+        return Parameter.builder(UUID.class, ResourceKeyedValueParameters.USER);
     }
 
     /**

src/main/java/org/spongepowered/api/command/parameter/managed/ValueParameter.java

@@ -111,7 +111,6 @@ default Optional<? extends T> parseValue(
          * you wish to do so, implement {@link ValueParameter} instead.
          *
          * {@inheritDoc}
-         * @return
          */
         @Override
         default List<CommandCompletion> complete(final CommandContext context, final String currentInput) {

src/main/java/org/spongepowered/api/command/parameter/managed/standard/ResourceKeyedValueParameters.java

@@ -43,6 +43,7 @@
 import org.spongepowered.api.registry.RegistryScope;
 import org.spongepowered.api.registry.RegistryScopes;
 import org.spongepowered.api.registry.RegistryTypes;
+import org.spongepowered.api.user.UserManager;
 import org.spongepowered.api.util.Color;
 import org.spongepowered.api.world.Locatable;
 import org.spongepowered.api.world.server.ServerLocation;
@@ -402,14 +403,27 @@ public final class ResourceKeyedValueParameters {
     public static final DefaultedRegistryReference<ResourceKeyedValueParameter<URL>> URL = ResourceKeyedValueParameters.key(ResourceKey.sponge("url"));
 
     /**
-     * Expect an argument to represent a player who has been online at some
-     * point, as a {@link User}.
+     * Expect an argument to represent the UUID of a player who has been online
+     * at some point - that is, a {@link UUID} where either:
+     *
+     * <ul>
+     *     <li>The player is online; or</li>
+     *     <li>The user with the returned {@link UUID} has user data available
+     *     in the {@link UserManager} (that is, for this UUID,
+     *     {@link UserManager#exists(java.util.UUID)} will return {@code true}).
+     *     </li>
+     * </ul>
+     *
+     * <p>As {@link User} objects are potentially slow to load, they are not
+     * created or returned here. In general, you will want to run
+     * {@link UserManager#load(java.util.UUID)} on the returned UUID to actually
+     * obtain the user.</p>
      *
      * <p>This parameter accepts selectors (to obtain players).</p>
      *
-     * <p>Returns a {@link User}.</p>
+     * <p>Returns a {@link UUID}.</p>
      */
-    public static final DefaultedRegistryReference<ResourceKeyedValueParameter<User>> USER = ResourceKeyedValueParameters.key(ResourceKey.sponge("user"));
+    public static final DefaultedRegistryReference<ResourceKeyedValueParameter<UUID>> USER = ResourceKeyedValueParameters.key(ResourceKey.sponge("user"));
 
     /**
      * Expect an argument to represent a {@link UUID}

src/main/java/org/spongepowered/api/entity/living/player/User.java

@@ -33,7 +33,9 @@
 import org.spongepowered.api.item.inventory.ArmorEquipable;
 import org.spongepowered.api.item.inventory.Carrier;
 import org.spongepowered.api.item.inventory.Inventory;
+import org.spongepowered.api.item.inventory.entity.PlayerInventory;
 import org.spongepowered.api.item.inventory.entity.UserInventory;
+import org.spongepowered.api.item.inventory.type.CarriedInventory;
 import org.spongepowered.api.profile.GameProfile;
 import org.spongepowered.api.service.permission.Subject;
 import org.spongepowered.api.util.annotation.DoNotStore;
@@ -129,12 +131,18 @@ public interface User extends DataHolder.Mutable, ArmorEquipable, Tamer, Subject
      */
     Vector3d rotation();
 
+    /**
+     * {@inheritDoc}
+     *
+     * Note that this may be either a {@link PlayerInventory} or
+     * {@link UserInventory}, depending on whether the user is online or not.
+     */
     @Override
-    UserInventory inventory();
+    CarriedInventory<? extends Carrier> inventory();
 
     /**
-     * Gets the {@link Inventory} available for this Player's shared {@link EnderChest}
-     * contents.
+     * Gets the {@link Inventory} available for this Player's shared
+     * {@link EnderChest} contents.
      *
      * @return The ender chest inventory
      */

src/main/java/org/spongepowered/api/event/EventContextKeys.java

@@ -55,6 +55,8 @@
 import org.spongepowered.math.vector.Vector3d;
 import org.spongepowered.plugin.PluginContainer;
 
+import java.util.UUID;
+
 /**
  * Standard keys for use within {@link EventContext}s.
  */
@@ -111,7 +113,7 @@ public final class EventContextKeys {
     /**
      * Represents the creator of an {@link Entity} or a {@link BlockState} at a {@link Location}
      */
-    public static final EventContextKey<User> CREATOR = EventContextKeys.key(ResourceKey.sponge("creator"), User.class);
+    public static final EventContextKey<UUID> CREATOR = EventContextKeys.key(ResourceKey.sponge("creator"), UUID.class);
 
     /**
      * Represents the {@link DamageType} to an entity.
@@ -199,9 +201,9 @@ public final class EventContextKeys {
     public static final EventContextKey<BlockSnapshot> NEIGHBOR_NOTIFY_SOURCE = EventContextKeys.key(ResourceKey.sponge("neighbor_notify_source"), BlockSnapshot.class);
 
     /**
-     * Represents the {@link User} that notified a block.
+     * Represents the {@link UUID} of a {@link User} that notified a block.
      */
-    public static final EventContextKey<User> NOTIFIER = EventContextKeys.key(ResourceKey.sponge("notifier"), User.class);
+    public static final EventContextKey<UUID> NOTIFIER = EventContextKeys.key(ResourceKey.sponge("notifier"), UUID.class);
 
     /**
      * Used when a {@link BlockTypes#PISTON_HEAD} extends.

src/main/java/org/spongepowered/api/user/UserManager.java

@@ -29,13 +29,16 @@
 import org.spongepowered.api.profile.GameProfile;
 import org.spongepowered.api.profile.GameProfileManager;
 
-import java.util.Collection;
 import java.util.Optional;
 import java.util.UUID;
+import java.util.concurrent.CompletableFuture;
 import java.util.stream.Stream;
 
 /**
  * Stores the persistent {@link User} data of a {@link Player}.
+ *
+ * <p>Any {@link User}s retrieved from this manager should not be stored, as
+ * they may become invalid at any time.</p>
  */
 public interface UserManager {
 
@@ -45,7 +48,7 @@ public interface UserManager {
      * @param uniqueId The UUID of the user
      * @return {@link User} or Optional.empty() if not found
      */
-    Optional<User> find(UUID uniqueId);
+    CompletableFuture<Optional<User>> load(UUID uniqueId);
 
     /**
      * Gets the data of a {@link User} by their last known user name
@@ -57,58 +60,76 @@ public interface UserManager {
      * @param lastKnownName The user name
      * @return {@link User} or Optional.empty() if not found
      */
-    Optional<User> find(String lastKnownName);
+    CompletableFuture<Optional<User>> load(String lastKnownName);
 
     /**
      * Gets the data of a {@link User} by their {@link GameProfile}.
      *
      * @param profile The profile
      * @return {@link User} or Optional.empty() if not found
      */
-    Optional<User> find(GameProfile profile);
+    CompletableFuture<Optional<User>> load(GameProfile profile);
 
     /**
-     * Gets or creates a persistent {@link User} associated with the given
-     * {@link GameProfile}.
-     *
-     * <p>To obtain a {@link GameProfile}, use the {@link GameProfileManager}.
-     * </p>
+     * Gets or creates a persistent {@link User} with the given UUID.
      *
-     * @param profile The profile
+     * @param uuid The {@link UUID} of the player to load or create.
      * @return The user object
      */
-    User findOrCreate(GameProfile profile);
+    CompletableFuture<User> loadOrCreate(UUID uuid);
 
     /**
-     * Gets the collection of all {@link GameProfile}s with stored {@link User}
-     * data.
+     * Deletes the data associated with a {@link User}, if the player is
+     * offline.
      *
-     * <p>This method may be resource intensive, particularly for servers that
-     * have a large number of {@link User}s. If you require a subset of this
-     * {@link Collection}, use {@link #streamOfMatches(String)} or
-     * {@link #streamAll()} and use {@link Stream} operations for your queries
-     * instead.</p>
+     * @param uuid The uuid of the user to delete
+     * @return true if the deletion was successful
+     */
+    CompletableFuture<Boolean> delete(UUID uuid);
+
+    /**
+     * If the implementation supports caching user objects, this will hint
+     * to the implementation that the user with the given UUID should no
+     * longer be cached. Any {@link User} objects held that this point
+     * will become invalid (though developers should not be storing
+     * users).
      *
-     * <p>This {@link Stream} may contain profiles that only hold a result for
-     * {@link GameProfile#uniqueId()}, that is, do not return a user's name.
-     * Such profiles should thus be treated as incomplete and are no more than
-     * an indicator that a {@link User} associated with the given {@link UUID}
-     * exists.</p>
+     * <p>Be aware, any changes that have been made to the user may not
+     * be saved.</p>
      *
-     * <p>Similarly, for {@link GameProfile}s that are filled and thus contain
-     * name data, the profile information is based on the latest information
-     * the server holds and no attempt is made to update this information.</p>
+     * <p>Users that are online will not be affected by this call.</p>
      *
-     * <p>If you require up to date {@link GameProfile}s, use the appropriate
-     * methods on the {@link GameProfileManager} and/or its associated
-     * {@link GameProfileManager}.</p>
+     * @param uuid The UUID of the user to save.
+     * @return {@code true} if the user was removed from a cache.
+     */
+    boolean removeFromCache(UUID uuid);
+
+    /**
+     * If the implementation supports caching user objects, this will hint
+     * to the implementation that the user with the given UUID should be saved
+     * to the disk immediately.
      *
-     * <p>Use {@link #find(GameProfile)} to load the {@link User} data associated
-     * with the associated {@link GameProfile}.</p>
+     * <p>If an exception is encountered during save, the completed future
+     * will be exceptional and the boolean will be {@code null}. It is therefore
+     * recommended that you check for any exceptions this future holds.</p>
      *
-     * @return A {@link Stream} of {@link GameProfile}s
+     * @param uuid The user to attempt to save.
+     * @return A completed future that returns {@code true} if the implementation
+     *         saved the user.
+     */
+    CompletableFuture<Boolean> forceSave(UUID uuid);
+
+    /**
+     * Returns whether data to create a {@link User} exists for a given
+     * player with a specified {@link UUID}.
+     *
+     * <p>If this is {@code false}, then {@link #load(UUID)} will return
+     * an {@linkplain Optional#empty() empty optional}.</p>
+     *
+     * @param playerUuid The {@link UUID} of the player to check.
+     * @return If the player has existing user data that can be loaded.
      */
-    Collection<GameProfile> all();
+    boolean exists(UUID playerUuid);
 
     /**
      * Gets a {@link Stream} that returns a {@link GameProfile} for each stored
@@ -128,33 +149,13 @@ public interface UserManager {
      * methods on the {@link GameProfileManager} and/or its associated
      * {@link GameProfileManager}.</p>
      *
-     * <p>Use {@link #find(GameProfile)} to load the {@link User} data associated
+     * <p>Use {@link #load(GameProfile)} to load the {@link User} data associated
      * with the associated {@link GameProfile}.</p>
      *
      * @return A {@link Stream} of {@link GameProfile}s
      */
     Stream<GameProfile> streamAll();
 
-    /**
-     * Deletes the data associated with a {@link User}.
-     *
-     * <p>This may not work if the user is logged in.</p>
-     *
-     * @param profile The profile of the user to delete
-     * @return true if the deletion was successful
-     */
-    boolean delete(GameProfile profile);
-
-    /**
-     * Deletes the data associated with a {@link User}.
-     *
-     * <p>This may not work if the user is logged in.</p>
-     *
-     * @param user The user to delete
-     * @return true if the deletion was successful
-     */
-    boolean delete(User user);
-
     /**
      * Gets a {@link Stream} that returns a {@link GameProfile} for each stored
      * {@link User} whose last known user names start with the given string
@@ -168,7 +169,7 @@ public interface UserManager {
      * methods on the {@link GameProfileManager} and/or its associated
      * {@link GameProfileManager}.</p>
      *
-     * <p>Use {@link #find(GameProfile)} to load associated {@link User} data.
+     * <p>Use {@link #load(GameProfile)} to load associated {@link User} data.
      * </p>
      *
      * @param lastKnownName The name to check for