Fix db_bench default compression level (#4248)

Summary:
db_bench's previous default compression level (-1) was not the default compression level in all libraries. In particular, in ZSTD negative values are valid compression levels, while ZSTD's default compression level is three.

This PR changes db_bench's default to be RocksDB's library-independent default compression level (see #3895). I also changed a couple other flags to get their default values from an options object directly rather than hardcoding.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4248

Differential Revision: D9235140

Pulled By: ajkr

fbshipit-source-id: be4e0722d59fa1968832183db36d1d20fcf11e5b

tools/db_bench_tool.cc

@@ -769,30 +769,21 @@ DEFINE_string(compression_type, "snappy",
 static enum rocksdb::CompressionType FLAGS_compression_type_e =
     rocksdb::kSnappyCompression;
 
-DEFINE_int32(compression_level, -1,
-             "Compression level. For zlib this should be -1 for the "
-             "default level, or between 0 and 9.");
+DEFINE_int32(compression_level, rocksdb::CompressionOptions().level,
+             "Compression level. The meaning of this value is library-"
+             "dependent. If unset, we try to use the default for the library "
+             "specified in `--compression_type`");
 
-DEFINE_int32(compression_max_dict_bytes, 0,
+DEFINE_int32(compression_max_dict_bytes,
+             rocksdb::CompressionOptions().max_dict_bytes,
              "Maximum size of dictionary used to prime the compression "
              "library.");
 
-DEFINE_int32(compression_zstd_max_train_bytes, 0,
+DEFINE_int32(compression_zstd_max_train_bytes,
+             rocksdb::CompressionOptions().zstd_max_train_bytes,
              "Maximum size of training data passed to zstd's dictionary "
              "trainer.");
 
-static bool ValidateCompressionLevel(const char* flagname, int32_t value) {
-  if (value < -1 || value > 9) {
-    fprintf(stderr, "Invalid value for --%s: %d, must be between -1 and 9\n",
-            flagname, value);
-    return false;
-  }
-  return true;
-}
-
-static const bool FLAGS_compression_level_dummy __attribute__((__unused__)) =
-    RegisterFlagValidator(&FLAGS_compression_level, &ValidateCompressionLevel);
-
 DEFINE_int32(min_level_to_compress, -1, "If non-negative, compression starts"
              " from this level. Levels with number < min_level_to_compress are"
              " not compressed. Otherwise, apply compression_type to "