Fix for ReadaheadSequentialFile crash in ldb_cmd_test (#5586)

Summary:
Fixing a corner case crash when there was no data read from file, but status is still OK
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5586

Differential Revision: D16348117

Pulled By: elipoz

fbshipit-source-id: f97973308024f020d8be79ca3c56466b84d80656

util/file_reader_writer.cc

@@ -738,7 +738,7 @@ private:
     if (s.ok()) {
       buffer_offset_ = offset;
       buffer_.Size(result.size());
-      assert(buffer_.BufferStart() == result.data());
+      assert(result.size() == 0 || buffer_.BufferStart() == result.data());
     }
     return s;
   }
@@ -886,7 +886,7 @@ class ReadaheadSequentialFile : public SequentialFile {
     if (s.ok()) {
       buffer_offset_ = read_offset_;
       buffer_.Size(result.size());
-      assert(buffer_.BufferStart() == result.data());
+      assert(result.size() == 0 || buffer_.BufferStart() == result.data());
     }
     return s;
   }
@@ -1027,6 +1027,11 @@ std::unique_ptr<RandomAccessFile> NewReadaheadRandomAccessFile(
 std::unique_ptr<SequentialFile>
 SequentialFileReader::NewReadaheadSequentialFile(
     std::unique_ptr<SequentialFile>&& file, size_t readahead_size) {
+  if (file->GetRequiredBufferAlignment() >= readahead_size) {
+    // Short-circuit and return the original file if readahead_size is
+    // too small and hence doesn't make sense to be used for prefetching.
+    return std::move(file);
+  }
   std::unique_ptr<SequentialFile> result(
       new ReadaheadSequentialFile(std::move(file), readahead_size));
   return result;

util/file_reader_writer_test.cc

@@ -325,7 +325,7 @@ class ReadaheadSequentialFileTest : public testing::Test,
                                     public testing::WithParamInterface<size_t> {
  public:
   static std::vector<size_t> GetReadaheadSizeList() {
-    return {1lu << 12, 1lu << 16};
+    return {1lu << 8, 1lu << 12, 1lu << 16, 1lu << 18};
   }
   void SetUp() override {
     readahead_size_ = GetParam();