Fix UnmarkEOF for partial blocks

Summary:
Blocks in the transaction log are a fixed size, but the last block in the transaction log file is usually a partial block. When a new record is added after the reader hit the end of the file, a new physical record will be appended to the last block. ReadPhysicalRecord can only read full blocks and assumes that the file position indicator is aligned to the start of a block. If the reader is forced to read further by simply clearing the EOF flag, ReadPhysicalRecord will read a full block starting from somewhere in the middle of a real block, causing it to lose alignment and to have a partial physical record at the end of the read buffer. This will result in length mismatches and checksum failures. When the log file is tailed for replication this will cause the log iterator to become invalid, necessitating the creation of a new iterator which will have to read the log file from scratch.

This diff fixes this issue by reading the remaining portion of the last block we read from. This is done when the reader is forced to read further (UnmarkEOF is called).

Test Plan:
- Added unit tests
- Stress test (with replication). Check dbdir/LOG file for corruptions.
- Test on test tier

Reviewers: emayanke, haobo, dhruba

Reviewed By: haobo

CC: vamsi, sheki, dhruba, kailiu, igor

Differential Revision: https://reviews.facebook.net/D15249

db/log_test.cc

@@ -47,36 +47,93 @@ class LogTest {
    public:
     std::string contents_;
 
+    explicit StringDest(Slice& reader_contents) :
+      WritableFile(),
+      contents_(""),
+      reader_contents_(reader_contents),
+      last_flush_(0) {
+      reader_contents_ = Slice(contents_.data(), 0);
+    };
+
     virtual Status Close() { return Status::OK(); }
-    virtual Status Flush() { return Status::OK(); }
+    virtual Status Flush() {
+      ASSERT_TRUE(reader_contents_.size() <= last_flush_);
+      size_t offset = last_flush_ - reader_contents_.size();
+      reader_contents_ = Slice(
+          contents_.data() + offset,
+          contents_.size() - offset);
+      last_flush_ = contents_.size();
+
+      return Status::OK();
+    }
     virtual Status Sync() { return Status::OK(); }
     virtual Status Append(const Slice& slice) {
       contents_.append(slice.data(), slice.size());
       return Status::OK();
     }
+    void Drop(size_t bytes) {
+      contents_.resize(contents_.size() - bytes);
+      reader_contents_ = Slice(
+          reader_contents_.data(), reader_contents_.size() - bytes);
+      last_flush_ = contents_.size();
+    }
+
+   private:
+    Slice& reader_contents_;
+    size_t last_flush_;
   };
 
   class StringSource : public SequentialFile {
    public:
-    Slice contents_;
+    Slice& contents_;
     bool force_error_;
+    size_t force_error_position_;
+    bool force_eof_;
+    size_t force_eof_position_;
     bool returned_partial_;
-    StringSource() : force_error_(false), returned_partial_(false) { }
+    explicit StringSource(Slice& contents) :
+      contents_(contents),
+      force_error_(false),
+      force_error_position_(0),
+      force_eof_(false),
+      force_eof_position_(0),
+      returned_partial_(false) { }
 
     virtual Status Read(size_t n, Slice* result, char* scratch) {
       ASSERT_TRUE(!returned_partial_) << "must not Read() after eof/error";
 
       if (force_error_) {
-        force_error_ = false;
-        returned_partial_ = true;
-        return Status::Corruption("read error");
+        if (force_error_position_ >= n) {
+          force_error_position_ -= n;
+        } else {
+          *result = Slice(contents_.data(), force_error_position_);
+          contents_.remove_prefix(force_error_position_);
+          force_error_ = false;
+          returned_partial_ = true;
+          return Status::Corruption("read error");
+        }
       }
 
       if (contents_.size() < n) {
         n = contents_.size();
         returned_partial_ = true;
       }
-      *result = Slice(contents_.data(), n);
+
+      if (force_eof_) {
+        if (force_eof_position_ >= n) {
+          force_eof_position_ -= n;
+        } else {
+          force_eof_ = false;
+          n = force_eof_position_;
+          returned_partial_ = true;
+        }
+      }
+
+      // By using scratch we ensure that caller has control over the
+      // lifetime of result.data()
+      memcpy(scratch, contents_.data(), n);
+      *result = Slice(scratch, n);
+
       contents_.remove_prefix(n);
       return Status::OK();
     }
@@ -123,10 +180,10 @@ class LogTest {
     src->contents_ = dest_contents();
   }
 
+  Slice reader_contents_;
   unique_ptr<StringDest> dest_holder_;
   unique_ptr<StringSource> source_holder_;
   ReportCollector report_;
-  bool reading_;
   Writer writer_;
   Reader reader_;
 
@@ -135,16 +192,15 @@ class LogTest {
   static uint64_t initial_offset_last_record_offsets_[];
 
  public:
-  LogTest() : dest_holder_(new StringDest),
-              source_holder_(new StringSource),
-              reading_(false),
+  LogTest() : reader_contents_(),
+              dest_holder_(new StringDest(reader_contents_)),
+              source_holder_(new StringSource(reader_contents_)),
               writer_(std::move(dest_holder_)),
               reader_(std::move(source_holder_), &report_, true/*checksum*/,
                       0/*initial_offset*/) {
   }
 
   void Write(const std::string& msg) {
-    ASSERT_TRUE(!reading_) << "Write() after starting to read";
     writer_.AddRecord(Slice(msg));
   }
 
@@ -153,10 +209,6 @@ class LogTest {
   }
 
   std::string Read() {
-    if (!reading_) {
-      reading_ = true;
-      reset_source_contents();
-    }
     std::string scratch;
     Slice record;
     if (reader_.ReadRecord(&record, &scratch)) {
@@ -175,7 +227,9 @@ class LogTest {
   }
 
   void ShrinkSize(int bytes) {
-    dest_contents().resize(dest_contents().size() - bytes);
+    auto dest = dynamic_cast<StringDest*>(writer_.file());
+    assert(dest);
+    dest->Drop(bytes);
   }
 
   void FixChecksum(int header_offset, int len) {
@@ -185,9 +239,10 @@ class LogTest {
     EncodeFixed32(&dest_contents()[header_offset], crc);
   }
 
-  void ForceError() {
+  void ForceError(size_t position = 0) {
     auto src = dynamic_cast<StringSource*>(reader_.file());
     src->force_error_ = true;
+    src->force_error_position_ = position;
   }
 
   size_t DroppedBytes() const {
@@ -198,6 +253,22 @@ class LogTest {
     return report_.message_;
   }
 
+  void ForceEOF(size_t position = 0) {
+    auto src = dynamic_cast<StringSource*>(reader_.file());
+    src->force_eof_ = true;
+    src->force_eof_position_ = position;
+  }
+
+  void UnmarkEOF() {
+    auto src = dynamic_cast<StringSource*>(reader_.file());
+    src->returned_partial_ = false;
+    reader_.UnmarkEOF();
+  }
+
+  bool IsEOF() {
+    return reader_.IsEOF();
+  }
+
   // Returns OK iff recorded error message contains "msg"
   std::string MatchError(const std::string& msg) const {
     if (report_.message_.find(msg) == std::string::npos) {
@@ -217,9 +288,7 @@ class LogTest {
 
   void CheckOffsetPastEndReturnsNoRecords(uint64_t offset_past_end) {
     WriteInitialOffsetLog();
-    reading_ = true;
-    unique_ptr<StringSource> source(new StringSource);
-    source->contents_ = dest_contents();
+    unique_ptr<StringSource> source(new StringSource(reader_contents_));
     unique_ptr<Reader> offset_reader(
       new Reader(std::move(source), &report_, true/*checksum*/,
                  WrittenBytes() + offset_past_end));
@@ -231,9 +300,7 @@ class LogTest {
   void CheckInitialOffsetRecord(uint64_t initial_offset,
                                 int expected_record_offset) {
     WriteInitialOffsetLog();
-    reading_ = true;
-    unique_ptr<StringSource> source(new StringSource);
-    source->contents_ = dest_contents();
+    unique_ptr<StringSource> source(new StringSource(reader_contents_));
     unique_ptr<Reader> offset_reader(
       new Reader(std::move(source), &report_, true/*checksum*/,
                  initial_offset));
@@ -520,6 +587,70 @@ TEST(LogTest, ReadPastEnd) {
   CheckOffsetPastEndReturnsNoRecords(5);
 }
 
+TEST(LogTest, ClearEofSingleBlock) {
+  Write("foo");
+  Write("bar");
+  ForceEOF(3 + kHeaderSize + 2);
+  ASSERT_EQ("foo", Read());
+  UnmarkEOF();
+  ASSERT_EQ("bar", Read());
+  ASSERT_TRUE(IsEOF());
+  ASSERT_EQ("EOF", Read());
+  Write("xxx");
+  UnmarkEOF();
+  ASSERT_EQ("xxx", Read());
+  ASSERT_TRUE(IsEOF());
+}
+
+TEST(LogTest, ClearEofMultiBlock) {
+  size_t num_full_blocks = 5;
+  size_t n = (kBlockSize - kHeaderSize) * num_full_blocks + 25;
+  Write(BigString("foo", n));
+  Write(BigString("bar", n));
+  ForceEOF(n + num_full_blocks * kHeaderSize + 10);
+  ASSERT_EQ(BigString("foo", n), Read());
+  ASSERT_TRUE(IsEOF());
+  UnmarkEOF();
+  ASSERT_EQ(BigString("bar", n), Read());
+  ASSERT_TRUE(IsEOF());
+  Write(BigString("xxx", n));
+  UnmarkEOF();
+  ASSERT_EQ(BigString("xxx", n), Read());
+  ASSERT_TRUE(IsEOF());
+}
+
+TEST(LogTest, ClearEofError) {
+  // If an error occurs during Read() in UnmarkEOF(), the records contained
+  // in the buffer should be returned on subsequent calls of ReadRecord()
+  // until no more full records are left, whereafter ReadRecord() should return
+  // false to indicate that it cannot read any further.
+
+  Write("foo");
+  Write("bar");
+  UnmarkEOF();
+  ASSERT_EQ("foo", Read());
+  ASSERT_TRUE(IsEOF());
+  Write("xxx");
+  ForceError(0);
+  UnmarkEOF();
+  ASSERT_EQ("bar", Read());
+  ASSERT_EQ("EOF", Read());
+}
+
+TEST(LogTest, ClearEofError2) {
+  Write("foo");
+  Write("bar");
+  UnmarkEOF();
+  ASSERT_EQ("foo", Read());
+  Write("xxx");
+  ForceError(3);
+  UnmarkEOF();
+  ASSERT_EQ("bar", Read());
+  ASSERT_EQ("EOF", Read());
+  ASSERT_EQ(3U, DroppedBytes());
+  ASSERT_EQ("OK", MatchError("read error"));
+}
+
 }  // namespace log
 }  // namespace rocksdb