Explicitly free allocated buffer when status is not ok (#6903)

Summary:
Currently we rely on `BlockContents` to implicitly free the allocated scratch buffer, but when IO error happens, it doesn't make sense to construct the `BlockContents` which might be corrupted. In the stress test, we find that `assert(req.result.size() == block_size(handle));` fails because of potential IO errors.

In this PR, we explicitly free the scratch buffer on error without constructing `BlockContents`.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6903

Test Plan: watch stress test

Reviewed By: anand1976

Differential Revision: D21823869

Pulled By: cheng-chang

fbshipit-source-id: 5603fc80e9bf3f44a9d7250ddebd871afe1eb89f

table/block_based/block_based_table_reader.cc

@@ -1727,6 +1727,7 @@ void BlockBasedTable::RetrieveMultipleBlocks(
     }
 
     BlockContents raw_block_contents;
+    if (s.ok()) {
       if (!use_shared_buffer) {
         // We allocated a buffer for this block. Give ownership of it to
         // BlockContents so it can free the memory
@@ -1746,7 +1747,7 @@ void BlockBasedTable::RetrieveMultipleBlocks(
       raw_block_contents.is_raw_block = true;
 #endif
 
-    if (s.ok() && options.verify_checksums) {
+      if (options.verify_checksums) {
         PERF_TIMER_GUARD(block_checksum_time);
         const char* data = req.result.data();
         uint32_t expected =
@@ -1756,11 +1757,14 @@ void BlockBasedTable::RetrieveMultipleBlocks(
         // begin address of each read request, we need to add the offset
         // in each read request. Checksum is stored in the block trailer,
         // which is handle.size() + 1.
-      s = ROCKSDB_NAMESPACE::VerifyChecksum(footer.checksum(),
+        s = ROCKSDB_NAMESPACE::VerifyChecksum(
-                                            data + req_offset,
+            footer.checksum(), data + req_offset, handle.size() + 1, expected);
-                                            handle.size() + 1, expected);
         TEST_SYNC_POINT_CALLBACK("RetrieveMultipleBlocks:VerifyChecksum", &s);
       }
+    } else if (!use_shared_buffer) {
+      // Free the allocated scratch buffer.
+      delete[] req.scratch;
+    }
 
     if (s.ok()) {
       // When the blocks share the same underlying buffer (scratch or direct io