HADOOP-15903. Allow HttpServer2 to discover resources in /static when symlinks are used. Contributed by Inigo Goiri.

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/http/HttpServer2.java

@@ -84,6 +84,7 @@ import org.eclipse.jetty.server.Server;
 import org.eclipse.jetty.server.ServerConnector;
 import org.eclipse.jetty.server.SessionManager;
 import org.eclipse.jetty.server.SslConnectionFactory;
+import org.eclipse.jetty.server.handler.AllowSymLinkAliasChecker;
 import org.eclipse.jetty.server.handler.ContextHandlerCollection;
 import org.eclipse.jetty.server.handler.HandlerCollection;
 import org.eclipse.jetty.server.handler.RequestLogHandler;
@@ -725,6 +726,7 @@ public final class HttpServer2 implements FilterContainer {
         asm.getSessionCookieConfig().setSecure(true);
       }
       logContext.setSessionHandler(handler);
+      logContext.addAliasCheck(new AllowSymLinkAliasChecker());
       setContextAttributes(logContext, conf);
       addNoCacheFilter(logContext);
       defaultContexts.put(logContext, true);
@@ -747,6 +749,7 @@ public final class HttpServer2 implements FilterContainer {
       asm.getSessionCookieConfig().setSecure(true);
     }
     staticContext.setSessionHandler(handler);
+    staticContext.addAliasCheck(new AllowSymLinkAliasChecker());
     setContextAttributes(staticContext, conf);
     defaultContexts.put(staticContext, true);
   }