Block cache tracer: StartTrace return busy if trace is already started. (#5519)

Summary:
This PR is needed for integration into MyRocks. A second call on StartTrace returns Busy so that MyRocks may return an error to the user.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5519

Test Plan: make clean && USE_CLANG=1 make check -j32

Differential Revision: D16055476

Pulled By: HaoyuHuang

fbshipit-source-id: a51772fb0965c873922757eb470a332b1e02a91d

trace_replay/block_cache_tracer.cc

@@ -233,7 +233,7 @@ Status BlockCacheTracer::StartTrace(
     std::unique_ptr<TraceWriter>&& trace_writer) {
   InstrumentedMutexLock lock_guard(&trace_writer_mutex_);
   if (writer_.load()) {
-    return Status::OK();
+    return Status::Busy();
   }
   trace_options_ = trace_options;
   writer_.store(

trace_replay/block_cache_tracer_test.cc

@@ -195,6 +195,17 @@ TEST_F(BlockCacheTracerTest, AtomicWrite) {
   }
 }
 
+TEST_F(BlockCacheTracerTest, ConsecutiveStartTrace) {
+  TraceOptions trace_opt;
+  std::unique_ptr<TraceWriter> trace_writer;
+  ASSERT_OK(
+      NewFileTraceWriter(env_, env_options_, trace_file_path_, &trace_writer));
+  BlockCacheTracer writer;
+  ASSERT_OK(writer.StartTrace(env_, trace_opt, std::move(trace_writer)));
+  ASSERT_NOK(writer.StartTrace(env_, trace_opt, std::move(trace_writer)));
+  ASSERT_OK(env_->FileExists(trace_file_path_));
+}
+
 TEST_F(BlockCacheTracerTest, AtomicNoWriteAfterEndTrace) {
   BlockCacheTraceRecord record = GenerateAccessRecord();
   {