Add fail-safe to VirtualShardRoutingHelper and valid configuration tracking test

Signed-off-by: Atri Sharma <atri.jiit@gmail.com>

Author: atris

server/src/main/java/org/opensearch/cluster/metadata/VirtualShardRoutingHelper.java

@@ -52,7 +52,16 @@ public static int resolvePhysicalShardId(IndexMetadata indexMetadata, int vShard
             }
         }
 
-        int virtualShardsPerPhysical = indexMetadata.getNumberOfVirtualShards() / indexMetadata.getNumberOfShards();
+        int numVirtualShards = indexMetadata.getNumberOfVirtualShards();
+        int numPhysicalShards = indexMetadata.getNumberOfShards();
+
+        if (numVirtualShards < numPhysicalShards || numVirtualShards % numPhysicalShards != 0) {
+            throw new IllegalArgumentException(
+                "Virtual shards must be enabled and be a multiple of the number of physical shards to resolve routing."
+            );
+        }
+
+        int virtualShardsPerPhysical = numVirtualShards / numPhysicalShards;
         return vShardId / virtualShardsPerPhysical;
     }
 }

server/src/test/java/org/opensearch/cluster/metadata/VirtualShardRoutingHelperTests.java

@@ -89,4 +89,30 @@ public void testInvalidOverridesFallBackToRangeBased() {
         // vShard 19 -> 19 / 4 = 4
         assertEquals(4, VirtualShardRoutingHelper.resolvePhysicalShardId(metadata, 19));
     }
+
+    public void testResolvePhysicalShardIdInvalidConfigurations() {
+        int numPhysicalShards = 5;
+
+        // Disabled virtual shards
+        IndexMetadata metadataDisabled = org.mockito.Mockito.mock(IndexMetadata.class);
+        org.mockito.Mockito.when(metadataDisabled.getNumberOfVirtualShards()).thenReturn(-1);
+        org.mockito.Mockito.when(metadataDisabled.getNumberOfShards()).thenReturn(numPhysicalShards);
+
+        IllegalArgumentException e1 = expectThrows(
+            IllegalArgumentException.class,
+            () -> VirtualShardRoutingHelper.resolvePhysicalShardId(metadataDisabled, 0)
+        );
+        assertTrue(e1.getMessage().contains("must be enabled and be a multiple"));
+
+        // Invalid multiple
+        IndexMetadata metadataInvalid = org.mockito.Mockito.mock(IndexMetadata.class);
+        org.mockito.Mockito.when(metadataInvalid.getNumberOfVirtualShards()).thenReturn(13);
+        org.mockito.Mockito.when(metadataInvalid.getNumberOfShards()).thenReturn(numPhysicalShards);
+
+        IllegalArgumentException e2 = expectThrows(
+            IllegalArgumentException.class,
+            () -> VirtualShardRoutingHelper.resolvePhysicalShardId(metadataInvalid, 0)
+        );
+        assertTrue(e2.getMessage().contains("must be enabled and be a multiple"));
+    }
 }