Fix active_shards_percent_as_number NaN when state not recovered

server/src/main/java/org/elasticsearch/cluster/health/ClusterStateHealth.java

@@ -121,9 +121,9 @@ public ClusterStateHealth(
 
         // shortcut on green
         if (computeStatus.equals(ClusterHealthStatus.GREEN)) {
-            this.activeShardsPercent = 100;
+            this.activeShardsPercent = 100.0;
         } else {
-            this.activeShardsPercent = (((double) this.activeShards) / totalShardCount) * 100;
+            this.activeShardsPercent = totalShardCount == 0 ? 0.0 : (((double) this.activeShards) / totalShardCount) * 100;
         }
     }
 

server/src/test/java/org/elasticsearch/cluster/health/ClusterStateHealthTests.java

@@ -19,6 +19,9 @@
 import org.elasticsearch.cluster.ClusterName;
 import org.elasticsearch.cluster.ClusterState;
 import org.elasticsearch.cluster.TestShardRoutingRoleStrategies;
+import org.elasticsearch.cluster.block.ClusterBlock;
+import org.elasticsearch.cluster.block.ClusterBlockLevel;
+import org.elasticsearch.cluster.block.ClusterBlocks;
 import org.elasticsearch.cluster.metadata.IndexMetadata;
 import org.elasticsearch.cluster.metadata.IndexNameExpressionResolver;
 import org.elasticsearch.cluster.metadata.Metadata;
@@ -41,6 +44,7 @@
 import org.elasticsearch.common.util.set.Sets;
 import org.elasticsearch.index.IndexVersion;
 import org.elasticsearch.indices.TestIndexNameExpressionResolver;
+import org.elasticsearch.rest.RestStatus;
 import org.elasticsearch.tasks.CancellableTask;
 import org.elasticsearch.tasks.TaskId;
 import org.elasticsearch.test.ESTestCase;
@@ -71,6 +75,7 @@
 import static org.elasticsearch.test.ClusterServiceUtils.setState;
 import static org.hamcrest.CoreMatchers.allOf;
 import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.CoreMatchers.not;
 import static org.hamcrest.Matchers.greaterThanOrEqualTo;
 import static org.hamcrest.Matchers.is;
 import static org.hamcrest.Matchers.lessThanOrEqualTo;
@@ -633,6 +638,48 @@ private boolean primaryInactiveDueToRecovery(final String indexName, final Clust
         return true;
     }
 
+    /**
+     * Tests the case where totalShardCount == 0 during cluster health calculation.
+     * This happens when indices exist in metadata but their routing tables are missing,
+     * causing totalShardCount to remain 0 and triggering the division by zero fix.
+     */
+    public void testActiveShardsPercentDuringClusterRestart() {
+        final String indexName = "test-idx";
+        ProjectId projectId = randomUniqueProjectId();
+
+        final IndexMetadata indexMetadata = IndexMetadata.builder(indexName)
+            .settings(settings(IndexVersion.current()).put(IndexMetadata.SETTING_INDEX_UUID, UUIDs.randomBase64UUID()))
+            .numberOfShards(3)
+            .numberOfReplicas(1)
+            .build();
+
+        // Create cluster state with index metadata but WITHOUT routing table entry
+        // This simulates cluster restart where metadata is loaded but routing table is not yet built
+        final var mdBuilder = Metadata.builder().put(ProjectMetadata.builder(projectId).put(indexMetadata, true).build());
+        final var rtBuilder = GlobalRoutingTable.builder().put(projectId, RoutingTable.EMPTY_ROUTING_TABLE);
+
+        ClusterState clusterState = ClusterState.builder(new ClusterName("test_cluster"))
+            .metadata(mdBuilder.build())
+            .routingTable(rtBuilder.build())
+            // Add global block to force RED status while keeping totalShardCount == 0
+            .blocks(
+                ClusterBlocks.builder()
+                    .addGlobalBlock(new ClusterBlock(1, "test", true, true, true, RestStatus.SERVICE_UNAVAILABLE, ClusterBlockLevel.ALL))
+            )
+            .build();
+
+        String[] concreteIndices = new String[] { indexName };
+        ClusterStateHealth clusterStateHealth = new ClusterStateHealth(clusterState, concreteIndices, projectId);
+
+        assertThat(clusterStateHealth.getStatus(), not(equalTo(ClusterHealthStatus.GREEN)));
+
+        assertThat(
+            "activeShardsPercent should be 0.0 when totalShardCount is 0",
+            clusterStateHealth.getActiveShardsPercent(),
+            equalTo(0.0)
+        );
+    }
+
     private void assertClusterHealth(ClusterStateHealth clusterStateHealth, RoutingTableGenerator.ShardCounter counter) {
         assertThat(clusterStateHealth.getStatus(), equalTo(counter.status()));
         assertThat(clusterStateHealth.getActiveShards(), equalTo(counter.active));