Push Command.destroy down through DispatchCommand redirection

When we redirect through a DispatchCommand to a different name we
need to ensure the destroy event is pushed down too, in case the
daemon terminates before the command is run.

Change-Id: Idc4446f4a0426aadcbb03c48ff2321071cded25b
Signed-off-by: Shawn O. Pearce <sop@google.com>

gerrit-sshd/src/main/java/com/google/gerrit/sshd/DispatchCommand.java

@@ -37,6 +37,7 @@ final class DispatchCommand extends BaseCommand {
   private final Provider<CurrentUser> currentUser;
   private final String prefix;
   private final Map<String, Provider<Command>> commands;
+  private Command cmd;
 
   @Inject
   DispatchCommand(final Provider<CurrentUser> cu, @Assisted final String pfx,
@@ -74,6 +75,11 @@ final class DispatchCommand extends BaseCommand {
     final Provider<Command> p = commands.get(name);
     if (p != null) {
       final Command cmd = p.get();
+
+      synchronized (this) {
+        this.cmd = cmd;
+      }
+
       if (cmd.getClass().getAnnotation(AdminCommand.class) != null) {
         final CurrentUser u = currentUser.get();
         if (!u.isAdministrator()) {
@@ -102,6 +108,16 @@ final class DispatchCommand extends BaseCommand {
     }
   }
 
+  @Override
+  public void destroy() {
+    synchronized (this) {
+      if (cmd != null) {
+        cmd.destroy();
+        cmd = null;
+      }
+    }
+  }
+
   private void usage() throws IOException, UnsupportedEncodingException {
     final StringBuilder usage = new StringBuilder();
     if (prefix.indexOf(' ') < 0) {