db_bench: sanity check CuckooTable with mmap_read option

Summary:
This is to avoid run time error. Fail the db_bench immediately if cuckoo table is used but mmap_read is not specified.
Closes https://github.com/facebook/rocksdb/pull/3420

Differential Revision: D6838284

Pulled By: siying

fbshipit-source-id: 20893fa28d40fadc31e4ff154bed02f5a1bad341

tools/db_bench_tool.cc

@@ -3069,6 +3069,12 @@ void VerifyDBFromDB(std::string& truth_db_name) {
         fprintf(stderr, "Invalid cuckoo_hash_ratio\n");
         exit(1);
       }
+
+      if (!FLAGS_mmap_read) {
+        fprintf(stderr, "cuckoo table format requires mmap read to operate\n");
+        exit(1);
+      }
+
       rocksdb::CuckooTableOptions table_options;
       table_options.hash_table_ratio = FLAGS_cuckoo_hash_ratio;
       table_options.identity_as_first_hash = FLAGS_identity_as_first_hash;