SOLR-18400: Plugins screen 500s when metrics are disabled

solr/core/src/java/org/apache/solr/response/PrometheusResponseWriter.java

@@ -63,6 +63,13 @@ public void write(
     // Otherwise handle MetricSnapshots
     var metrics = response.getValues().get("metrics");
     if (metrics == null) {
+      // e.g. metrics collection disabled: emit the reason as a comment; the EOF marker required
+      // by OpenMetrics is harmless in Prometheus format
+      var error = response.getValues().get("error");
+      if (error != null) {
+        out.write(("# " + error + "\n# EOF\n").getBytes(StandardCharsets.UTF_8));
+        return;
+      }
       throw new IOException("No metrics found in response");
     }
     MetricSnapshots snapshots = (MetricSnapshots) metrics;

solr/core/src/test/org/apache/solr/response/TestPrometheusResponseWriter.java

@@ -19,6 +19,7 @@
 import static org.apache.solr.client.solrj.response.InputStreamResponseParser.STREAM_KEY;
 import static org.apache.solr.core.CoreContainer.ALLOW_PATHS_SYSPROP;
 
+import java.io.ByteArrayOutputStream;
 import java.io.InputStream;
 import java.lang.invoke.MethodHandles;
 import java.nio.charset.StandardCharsets;
@@ -185,6 +186,16 @@ public void testDefaultPrometheusFormatNoWtParam() throws Exception {
     }
   }
 
+  @Test
+  public void testDisabledMetricsWritesErrorComment() throws Exception {
+    // SOLR-18400: disabled metrics must yield a graceful comment, not a 500
+    SolrQueryResponse rsp = new SolrQueryResponse();
+    rsp.add("error", "metrics collection is disabled");
+    ByteArrayOutputStream out = new ByteArrayOutputStream();
+    new PrometheusResponseWriter().write(out, null, rsp, null);
+    assertEquals("# metrics collection is disabled\n# EOF\n", out.toString(StandardCharsets.UTF_8));
+  }
+
   @Test
   public void testUnsupportedMetricsFormat() throws Exception {
     var req = new MetricsRequest();

solr/webapp/web/js/angular/controllers/plugins.js

@@ -35,6 +35,7 @@ solrAdminApp.controller('PluginsController',
             var type = $location.search().type;
 
             Metrics.raw(params, function (response) {
+                $scope.metricsDisabled = (response.data || '').indexOf('metrics collection is disabled') !== -1;
                 $scope.types = getPluginTypesFromMetrics(response.data, type);
                 $scope.type = getSelectedType($scope.types, type);
 

solr/webapp/web/partials/plugins.html

@@ -17,6 +17,9 @@
 <div id="plugins" class="clearfix">
 
   <div id="frame">
+      <div class="message-container" ng-show="metricsDisabled">
+        <div class="message">Metrics collection is disabled in this node's solr.xml (&lt;metrics enabled="false"&gt;), so no plugin information is available.</div>
+      </div>
       <ul>
       <li class="entry" ng-class="{changed: plugin.changed}" ng-repeat="plugin in type.plugins">
         <a ng-click="selectPlugin(plugin)">