Datanode migration: fix invalid hostname (#19894)

* silently remove trailing slash from hostname (#19884)

* move check up further

graylog2-server/src/main/java/org/graylog/plugins/views/storage/migration/state/actions/MigrationActionsImpl.java

@@ -41,6 +41,7 @@
 import org.graylog2.plugin.certificates.RenewalPolicy;
 import org.graylog2.plugin.cluster.ClusterConfigService;
 import org.graylog2.rest.resources.datanodes.DatanodeRestApiProxy;
+import org.graylog2.shared.utilities.StringUtils;
 import org.graylog2.storage.providers.ElasticsearchVersionProvider;
 import org.graylog2.system.processing.control.ClusterProcessingControl;
 import org.graylog2.system.processing.control.ClusterProcessingControlFactory;
@@ -256,7 +257,11 @@ public boolean dataNodeStartupFinished() {
     @Override
     public void startRemoteReindex() {
         final String allowlist = getStateMachineContext().getActionArgumentOpt("allowlist", String.class).orElse(null);
-        final URI hostname = Objects.requireNonNull(URI.create(getStateMachineContext().getActionArgument("hostname", String.class)), "hostname has to be provided");
+        String host = StringUtils.requireNonBlank(getStateMachineContext().getActionArgument("hostname", String.class), "hostname has to be provided");
+        if (host.endsWith("/")) {
+            host = host.substring(0, host.length() - 1);
+        }
+        final URI hostname = URI.create(host);
         final String user = getStateMachineContext().getActionArgumentOpt("user", String.class).orElse(null);
         final String password = getStateMachineContext().getActionArgumentOpt("password", String.class).orElse(null);
         final List<String> indices = getStateMachineContext().getActionArgumentOpt("indices", List.class).orElse(Collections.emptyList()); // todo: generics!