Fix use_after_free bug when underlying FS enables kFSBuffer (#11645)

Summary:
Fix use_after_free bug in async_io MultiReads when underlying FS enabled kFSBuffer. kFSBuffer is when underlying FS pass their own buffer instead of using RocksDB scratch in FSReadRequest
Since it's an experimental feature, added a hack for now to fix the bug.
Planning to make public API change to remove const from the callback as it doesn't make sense to use const.

Pull Request resolved: #11645

Test Plan: tested locally

Reviewed By: ltamasi

Differential Revision: D47819907

Pulled By: akankshamahajan15

fbshipit-source-id: 1faf5ef795bf27e2b3a60960374d91274931df8d

unreleased_history/bug_fixes/fsbuffer_bug_fix.md

@@ -0,0 +1 @@
+Fix use_after_free bug in async_io MultiReads when underlying FS enabled kFSBuffer. kFSBuffer is when underlying FS pass their own buffer instead of using RocksDB scratch in FSReadRequest. Right now it's an experimental feature.

util/async_file_reader.cc

@@ -26,6 +26,11 @@ bool AsyncFileReader::MultiReadAsyncImpl(ReadAwaiter* awaiter) {
           FSReadRequest* read_req = static_cast<FSReadRequest*>(cb_arg);
           read_req->status = req.status;
           read_req->result = req.result;
+          if (req.fs_scratch != nullptr) {
+            // TODO akanksha: Revisit to remove the const in the callback.
+            FSReadRequest& req_tmp = const_cast<FSReadRequest&>(req);
+            read_req->fs_scratch = std::move(req_tmp.fs_scratch);
+          }
         },
         &awaiter->read_reqs_[i], &awaiter->io_handle_[i], &awaiter->del_fn_[i],
         /*aligned_buf=*/nullptr);