Report shared_cache shards as zero size on disk (#69820)

Searchable snapshot shards that are using the shared_cache are not actually consuming as much disk space as they
report today in the "store" section of the node/indices stats. This blocks plan changes on Cloud as they check if the new
nodes have enough disk space to host all the (at least primary) shards from the old nodes. With this change here,
searchable snapshot shards that are using the shared_cache will report 0 as store size.

Author: ywelsch

server/src/main/java/org/elasticsearch/index/shard/IndexShard.java

@@ -1085,16 +1085,14 @@ public GetStats getStats() {
     }
 
     public StoreStats storeStats() {
+        if (DiskThresholdDecider.SETTING_IGNORE_DISK_WATERMARKS.get(indexSettings.getSettings())) {
+            // if this shard has no disk footprint then its size is reported as 0
+            return new StoreStats(0, 0);
+        }
         try {
-            final long reservedBytes;
-            if (DiskThresholdDecider.SETTING_IGNORE_DISK_WATERMARKS.get(indexSettings.getSettings())) {
-                // if this shard has no disk footprint then it also needs no reserved space
-                reservedBytes = 0L;
-            } else {
-                final RecoveryState recoveryState = this.recoveryState;
-                final long bytesStillToRecover = recoveryState == null ? -1L : recoveryState.getIndex().bytesStillToRecover();
-                reservedBytes = bytesStillToRecover == -1 ? StoreStats.UNKNOWN_RESERVED_BYTES : bytesStillToRecover;
-            }
+            final RecoveryState recoveryState = this.recoveryState;
+            final long bytesStillToRecover = recoveryState == null ? -1L : recoveryState.getIndex().bytesStillToRecover();
+            final long reservedBytes = bytesStillToRecover == -1 ? StoreStats.UNKNOWN_RESERVED_BYTES : bytesStillToRecover;
             return store.stats(reservedBytes);
         } catch (IOException e) {
             failShard("Failing shard because of exception during storeStats", e);

x-pack/plugin/searchable-snapshots/src/internalClusterTest/java/org/elasticsearch/xpack/searchablesnapshots/SearchableSnapshotsIntegTests.java

@@ -519,6 +519,7 @@ public void testCreateAndRestorePartialSearchableSnapshot() throws Exception {
                         shardStats.getStats().getStore().getReservedSize().getBytes(),
                         equalTo(0L)
                     );
+                    assertThat(shardStats.getShardRouting().toString(), shardStats.getStats().getStore().getSize().getBytes(), equalTo(0L));
                 }
             }
         }, "test-stats-watcher");