Fix data racing of BlockBasedTableBuilder::ParallelCompressionRep::first_block (#6640)

Summary:
BlockBasedTableBuilder::ParallelCompressionRep::first_block can be read in
Flush() and written in BGWorkWriteRawBlock() concurrently. This commit fixes
the issue by reading first_block out before pushing the block to compression
and write.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6640

Test Plan: Run all tests concurrently with TSAN.

Reviewed By: cheng-chang

Differential Revision: D20851370

fbshipit-source-id: 6f039222e8319d31e15f1b45e05c106527253f72

table/block_based/block_based_table_builder.cc

@@ -818,6 +818,9 @@ void BlockBasedTableBuilder::Flush() {
             new_blocks_inflight * kBlockTrailerSize,
         std::memory_order_relaxed);
 
+    // Read out first_block here to avoid data race with BGWorkWriteRawBlock
+    bool first_block = r->pc_rep->first_block;
+
     assert(block_rep->status.ok());
     if (!r->pc_rep->write_queue.push(block_rep->slot.get())) {
       return;
@@ -826,7 +829,7 @@ void BlockBasedTableBuilder::Flush() {
       return;
     }
 
-    if (r->pc_rep->first_block) {
+    if (first_block) {
       std::unique_lock<std::mutex> lock(r->pc_rep->first_block_mutex);
       r->pc_rep->first_block_cond.wait(lock,
                                        [=] { return !r->pc_rep->first_block; });
@@ -1140,7 +1143,7 @@ void BlockBasedTableBuilder::BGWorkWriteRawBlock() {
     }
 
     if (r->pc_rep->first_block) {
-      std::unique_lock<std::mutex> lock(r->pc_rep->first_block_mutex);
+      std::lock_guard<std::mutex> lock(r->pc_rep->first_block_mutex);
       r->pc_rep->first_block = false;
       r->pc_rep->first_block_cond.notify_one();
     }