Fix a bug in compaction reads causing checksum mismatches and asan errors (#5531)

Summary:
Fixed a bug in compaction reads due to which incorrect number of bytes were being read/utilized. The bug was introduced in https://github.com/facebook/rocksdb/issues/5498 , resulting in "Corruption: block checksum mismatch" and "heap-buffer-overflow" asan errors in our tests.

https://github.com/facebook/rocksdb/issues/5498 was introduced recently and is not in any released versions.

ASAN:
```
> ==2280939==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x6250005e83da at pc 0x000000d57f62 bp 0x7f954f483770 sp 0x7f954f482f20
> === How to use this, how to get the raw stack trace, and more: fburl.com/ASAN ===
> READ of size 4 at 0x6250005e83da thread T4
> SCARINESS: 27 (4-byte-read-heap-buffer-overflow-far-from-bounds)

>      #0 tests+0xd57f61                           __asan_memcpy
>      https://github.com/facebook/rocksdb/issues/1 rocksdb/src/util/coding.h:124            rocksdb::DecodeFixed32(char const*)
>      https://github.com/facebook/rocksdb/issues/2 rocksdb/src/table/block_fetcher.cc:39    rocksdb::BlockFetcher::CheckBlockChecksum()
>      https://github.com/facebook/rocksdb/issues/3 rocksdb/src/table/block_fetcher.cc:99    rocksdb::BlockFetcher::TryGetFromPrefetchBuffer()
>      https://github.com/facebook/rocksdb/issues/4 rocksdb/src/table/block_fetcher.cc:209   rocksdb::BlockFetcher::ReadBlockContents()
>      https://github.com/facebook/rocksdb/issues/5 rocksdb/src/table/block_based/block_based_table_reader.cc:93 rocksdb::(anonymous namespace)::ReadBlockFromFile(rocksdb::RandomAccessFileReader*, rocksdb::FilePrefetchBuffer*, rocksdb::Footer const&, rocksdb::ReadOptions const&, rocksdb::BlockHandle const&, std::unique_ptr<...>*, rocksdb::ImmutableCFOptions const&, bool, bool, rocksdb::UncompressionDict
 const&, rocksdb::PersistentCacheOptions const&, unsigned long, unsigned long, rocksdb::MemoryAllocator*, bool)
>      https://github.com/facebook/rocksdb/issues/6 rocksdb/src/table/block_based/block_based_table_reader.cc:2331 rocksdb::BlockBasedTable::RetrieveBlock(rocksdb::FilePrefetchBuffer*, rocksdb::ReadOptions const&, rocksdb::BlockHandle const&, rocksdb::UncompressionDict const&, rocksdb::CachableEntry<...>*, rocksdb::BlockType, rocksdb::GetContext*, rocksdb::BlockCacheLookupContext*, bool) const
>      https://github.com/facebook/rocksdb/issues/7 rocksdb/src/table/block_based/block_based_table_reader.cc:2090 rocksdb::DataBlockIter* rocksdb::BlockBasedTable::NewDataBlockIterator<...>(rocksdb::ReadOptions const&, rocksdb::BlockHandle const&, rocksdb::DataBlockIter*, rocksdb::BlockType, bool, bool, rocksdb::GetContext*, rocksdb::BlockCacheLookupContext*, rocksdb::Status, rocksdb::FilePrefetchBuffe
r*, bool) const
>      https://github.com/facebook/rocksdb/issues/8 rocksdb/src/table/block_based/block_based_table_reader.cc:2720 rocksdb::BlockBasedTableIterator<...>::InitDataBlock()
>      https://github.com/facebook/rocksdb/issues/9 rocksdb/src/table/block_based/block_based_table_reader.cc:2607 rocksdb::BlockBasedTableIterator<...>::SeekToFirst()
>     https://github.com/facebook/rocksdb/issues/10 rocksdb/src/table/iterator_wrapper.h:83  rocksdb::IteratorWrapperBase<...>::SeekToFirst()
>     https://github.com/facebook/rocksdb/issues/11 rocksdb/src/table/merging_iterator.cc:100 rocksdb::MergingIterator::SeekToFirst()
>     https://github.com/facebook/rocksdb/issues/12 rocksdb/compaction/compaction_job.cc:877 rocksdb::CompactionJob::ProcessKeyValueCompaction(rocksdb::CompactionJob::SubcompactionState*)
>     https://github.com/facebook/rocksdb/issues/13 rocksdb/compaction/compaction_job.cc:590 rocksdb::CompactionJob::Run()
>     https://github.com/facebook/rocksdb/issues/14 rocksdb/db_impl/db_impl_compaction_flush.cc:2689 rocksdb::DBImpl::BackgroundCompaction(bool*, rocksdb::JobContext*, rocksdb::LogBuffer*, rocksdb::DBImpl::PrepickedCompaction*, rocksdb::Env::Priority)
>     https://github.com/facebook/rocksdb/issues/15 rocksdb/db_impl/db_impl_compaction_flush.cc:2248 rocksdb::DBImpl::BackgroundCallCompaction(rocksdb::DBImpl::PrepickedCompaction*, rocksdb::Env::Priority)
>     https://github.com/facebook/rocksdb/issues/16 rocksdb/db_impl/db_impl_compaction_flush.cc:2024 rocksdb::DBImpl::BGWorkCompaction(void*)
>     https://github.com/facebook/rocksdb/issues/23 rocksdb/src/util/threadpool_imp.cc:266   rocksdb::ThreadPoolImpl::Impl::BGThread(unsigned long)
>     https://github.com/facebook/rocksdb/issues/24 rocksdb/src/util/threadpool_imp.cc:307   rocksdb::ThreadPoolImpl::Impl::BGThreadWrapper(void*)
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5531

Test Plan: Verified that this fixes the fb-internal Logdevice test which caught the issue.

Differential Revision: D16109702

Pulled By: sagar0

fbshipit-source-id: 1fc08549cf7b553e338a133ae11eb9f4d5011914

util/file_reader_writer.cc

@@ -842,7 +842,7 @@ bool FilePrefetchBuffer::TryReadFromCache(uint64_t offset, size_t n,
       assert(max_readahead_size_ >= readahead_size_);
       Status s;
       if (for_compaction) {
-        s = Prefetch(file_reader_, offset, readahead_size_, for_compaction);
+        s = Prefetch(file_reader_, offset, std::max(n, readahead_size_), for_compaction);
       } else {
         s = Prefetch(file_reader_, offset, n + readahead_size_, for_compaction);
       }