Provide a tab completion handler for JLine. Adds BUKKIT-4168

By: Phillip Schichtel <quick_wango@code-infection.de>

paper-server/src/main/java/org/bukkit/craftbukkit/command/ConsoleCommandCompleter.java

@@ -0,0 +1,47 @@
+package org.bukkit.craftbukkit.command;
+
+import java.util.List;
+import java.util.concurrent.ExecutionException;
+import java.util.logging.Level;
+
+import org.bukkit.craftbukkit.CraftServer;
+import org.bukkit.craftbukkit.util.Waitable;
+
+import jline.console.completer.Completer;
+
+public class ConsoleCommandCompleter implements Completer {
+    private final CraftServer server;
+
+    public ConsoleCommandCompleter(CraftServer server) {
+        this.server = server;
+    }
+
+    public int complete(final String buffer, final int cursor, final List<CharSequence> candidates) {
+        Waitable<List<String>> waitable = new Waitable<List<String>>() {
+            @Override
+            protected List<String> evaluate() {
+                return server.getCommandMap().tabComplete(server.getConsoleSender(), buffer);
+            }
+        };
+        this.server.getServer().processQueue.add(waitable);
+        try {
+            List<String> offers = waitable.get();
+            if (offers == null) {
+                return cursor;
+            }
+            candidates.addAll(offers);
+
+            final int lastSpace = buffer.lastIndexOf(' ');
+            if (lastSpace == -1) {
+                return cursor - buffer.length();
+            } else {
+                return cursor - (buffer.length() - lastSpace - 1);
+            }
+        } catch (ExecutionException e) {
+            this.server.getLogger().log(Level.WARNING, "Unhandled exception when tab completing", e);
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+        }
+        return cursor;
+    }
+}