Fix active_shards_percent_as_number NaN when total shard count is zero

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

@@ -56,10 +56,29 @@ public ClusterIndexHealth(final IndexMetadata indexMetadata, final IndexRoutingT
         this.numberOfReplicas = indexMetadata.getNumberOfReplicas();
 
         shards = new HashMap<>();
-        for (int i = 0; i < indexRoutingTable.size(); i++) {
-            IndexShardRoutingTable shardRoutingTable = indexRoutingTable.shard(i);
-            int shardId = shardRoutingTable.shardId().id();
-            shards.put(shardId, new ClusterShardHealth(shardId, shardRoutingTable));
+        if (indexRoutingTable != null) {
+            for (int i = 0; i < indexRoutingTable.size(); i++) {
+                IndexShardRoutingTable shardRoutingTable = indexRoutingTable.shard(i);
+                int shardId = shardRoutingTable.shardId().id();
+                shards.put(shardId, new ClusterShardHealth(shardId, shardRoutingTable));
+            }
+        } else {
+            for (int shardId = 0; shardId < numberOfShards; shardId++) {
+                // Create a shard health representing completely unassigned shard
+                // All replicas for this shard are unassigned, including the primary
+                int replicasCount = numberOfReplicas + 1;
+                ClusterShardHealth clusterShardHealth = new ClusterShardHealth(
+                    shardId,
+                    ClusterHealthStatus.RED,
+                    0,
+                    0,
+                    0,
+                    replicasCount,
+                    1,
+                    false
+                );
+                shards.put(shardId, clusterShardHealth);
+            }
         }
 
         // update the index status

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

@@ -86,9 +86,6 @@ public ClusterStateHealth(
         for (String index : concreteIndices) {
             IndexRoutingTable indexRoutingTable = routingTable.index(index);
             IndexMetadata indexMetadata = project.index(index);
-            if (indexRoutingTable == null) {
-                continue;
-            }
 
             ClusterIndexHealth indexHealth = new ClusterIndexHealth(indexMetadata, indexRoutingTable);
             indices.put(indexHealth.getIndex(), indexHealth);
@@ -121,7 +118,7 @@ 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;
         }

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;
@@ -633,6 +637,64 @@ private boolean primaryInactiveDueToRecovery(final String indexName, final Clust
         return true;
     }
 
+    /**
+     * Tests the case where indices exist in metadata but their routing tables are missing.
+     * This happens during cluster restart where metadata is loaded but routing table is not yet built.
+     * All shards should be considered completely unassigned and the cluster should be RED.
+     */
+    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())
+            .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);
+
+        // The cluster should be RED because all shards are unassigned
+        assertThat(clusterStateHealth.getStatus(), equalTo(ClusterHealthStatus.RED));
+
+        // All shards are unassigned, so activeShardsPercent should be 0.0
+        assertThat(
+            "activeShardsPercent should be 0.0 when all shards are unassigned",
+            clusterStateHealth.getActiveShardsPercent(),
+            equalTo(0.0)
+        );
+
+        // Verify that totalShardCount is correctly calculated
+        int expectedTotalShards = indexMetadata.getTotalNumberOfShards();
+        assertThat("All shards should be counted as unassigned", clusterStateHealth.getUnassignedShards(), equalTo(expectedTotalShards));
+
+        // All primary shards should be unassigned
+        assertThat(
+            "All primary shards should be unassigned",
+            clusterStateHealth.getUnassignedPrimaryShards(),
+            equalTo(indexMetadata.getNumberOfShards())
+        );
+
+        // No active shards
+        assertThat(clusterStateHealth.getActiveShards(), equalTo(0));
+        assertThat(clusterStateHealth.getActivePrimaryShards(), equalTo(0));
+    }
+
     private void assertClusterHealth(ClusterStateHealth clusterStateHealth, RoutingTableGenerator.ShardCounter counter) {
         assertThat(clusterStateHealth.getStatus(), equalTo(counter.status()));
         assertThat(clusterStateHealth.getActiveShards(), equalTo(counter.active));