Support compaction filter in db_bench (#4106)

Summary:
Right now there is no support for enabling compaction filter in db_bench, we should add support for that to facilitate testing of compaction filter.
This PR adds a compaction filter called KeepFilter and make `Filter` always returns false, essentially a noop compaction filter. This will allow us to test compaction filter code path without having to support arbitrary compaction filters
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4106

Differential Revision: D8828517

Pulled By: miasantreble

fbshipit-source-id: 9ad76d04103eaa9d00da98334b4a39e542d26c41

tools/db_bench_tool.cc

@@ -525,6 +525,9 @@ DEFINE_bool(read_cache_direct_write, true,
 DEFINE_bool(read_cache_direct_read, true,
             "Whether to use Direct IO for reading from read cache");
 
+DEFINE_bool(use_keep_filter, false,
+            "Whether to use a noop compaction filter");
+
 static bool ValidateCacheNumshardbits(const char* flagname, int32_t value) {
   if (value >= 20) {
     fprintf(stderr, "Invalid value for --%s: %d, must be < 20\n",
@@ -2199,6 +2202,17 @@ class Benchmark {
     std::shared_ptr<TimestampEmulator> timestamp_emulator_;
   };
 
+  class KeepFilter : public CompactionFilter {
+   public:
+    virtual bool Filter(int /*level*/, const Slice& /*key*/,
+                        const Slice& /*value*/, std::string* /*new_value*/,
+                        bool* /*value_changed*/) const override {
+      return false;
+    }
+
+    virtual const char* Name() const override { return "KeepFilter"; }
+  };
+
   std::shared_ptr<Cache> NewCache(int64_t capacity) {
     if (capacity <= 0) {
       return nullptr;
@@ -3427,6 +3441,12 @@ void VerifyDBFromDB(std::string& truth_db_name) {
       }
       options.wal_dir = wal_dir;
     }
+
+    // KeepFilter is a noop filter, this can be used to test compaction filter
+    if (FLAGS_use_keep_filter) {
+      options.compaction_filter = new KeepFilter();
+      fprintf(stdout, "A noop compaction filter is used\n");
+    }
   }
 
   void Open(Options* opts) {