We saw a case where the recovery stats reported `recovered_bytes` greater than `length_in_bytes` for one file:

```
GET /_recovery?detailed

200 OK
...
{
  "name": "_buk.fdt",
  "length_in_bytes": 26891307173,
  "reused": false,
  "recovered_in_bytes": 35324928266,
  "recovered_from_snapshot_in_bytes": 8433621093
}
...
```

NB `35324928266 == 26891307173 + 8433621093`

This causes Elasticsearch to compute a negative value for the reserved space on the target node of this recovery, which prevents stats APIs from working. This appears to be caused by a network-level failure which triggers a retry without first cancelling an ongoing download of a different file with the same name from the snapshot repository.

---

### Workaround

Restart the target node to clear these stats.
server/src/main/java/org/elasticsearch/index/shard/IndexShard.java
/*
                 * We have to check again since otherwise there is a race when a thread passes the first check next to another thread which
                 * performs the operation quickly enough to  finish before the current thread could flip the flag. In that situation, we
                 * have an extra operation.
                 *
                 * Additionally, a flush implicitly executes a translog generation roll so if we execute a flush then we do not need to
                 * check if we should roll the translog generation.
                 */
                if (shouldPeriodicallyFlush()) {
                    logger.debug("submitting async flush request");
                    final AbstractRunnable flush = new AbstractRunnable() {
                        @Override
                        public void onFailure(final Exception e) {
                            if (state != IndexShardState.CLOSED) {
                                logger.warn("failed to flush index", e);
                            }
                        }
                        @Override
                        protected void doRun() {
                            flush(new FlushRequest());
                            periodicFlushMetric.inc();
                        }
                        @Override
                        public void onAfter() {
                            flushOrRollRunning.compareAndSet(true, false);
                            afterWriteOperation();
                        }
                    };
                    threadPool.executor(ThreadPool.Names.FLUSH).execute(flush);
                } else if (shouldRollTranslogGeneration()) {