From 9f5cfb8e7142fe7b8fe4668aefd481e881f5bb42 Mon Sep 17 00:00:00 2001
From: Eli Pozniansky <elipoz@gmail.com>
Date: Wed, 17 Jul 2019 17:01:30 -0700
Subject: [PATCH] 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      | 9 +++++++--
 util/file_reader_writer_test.cc | 2 +-
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/util/file_reader_writer.cc b/util/file_reader_writer.cc
index 9175fa502..b6a5eefcf 100644
--- a/util/file_reader_writer.cc
+++ b/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;
diff --git a/util/file_reader_writer_test.cc b/util/file_reader_writer_test.cc
index aa74303b8..1b86f798f 100644
--- a/util/file_reader_writer_test.cc
+++ b/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();
-- 
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