log_reader.h

//  Copyright (c) 2013, Facebook, Inc.  All rights reserved.
//  This source code is licensed under the BSD-style license found in the
//  LICENSE file in the root directory of this source tree. An additional grant
//  of patent rights can be found in the PATENTS file in the same directory.
//
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.

#pragma once
#include <memory>
#include <stdint.h>

#include "db/log_format.h"
#include "rocksdb/slice.h"
#include "rocksdb/status.h"

namespace rocksdb {

class SequentialFile;
using std::unique_ptr;

namespace log {

class Reader {
 public:
  // Interface for reporting errors.
  class Reporter {
   public:
    virtual ~Reporter();

    // Some corruption was detected.  "size" is the approximate number
    // of bytes dropped due to the corruption.
    virtual void Corruption(size_t bytes, const Status& status) = 0;
  };

  // Create a reader that will return log records from "*file".
  // "*file" must remain live while this Reader is in use.
  //
  // If "reporter" is non-nullptr, it is notified whenever some data is
  // dropped due to a detected corruption.  "*reporter" must remain
  // live while this Reader is in use.
  //
  // If "checksum" is true, verify checksums if available.
  //
  // The Reader will start reading at the first record located at physical
  // position >= initial_offset within the file.
  Reader(unique_ptr<SequentialFile>&& file, Reporter* reporter,
         bool checksum, uint64_t initial_offset);

  ~Reader();

  // Read the next record into *record.  Returns true if read
  // successfully, false if we hit end of the input.  May use
  // "*scratch" as temporary storage.  The contents filled in *record
  // will only be valid until the next mutating operation on this
  // reader or the next mutation to *scratch.
  bool ReadRecord(Slice* record, std::string* scratch);

  // Returns the physical offset of the last record returned by ReadRecord.
  //
  // Undefined before the first call to ReadRecord.
  uint64_t LastRecordOffset();

  // returns true if the reader has encountered an eof condition.
  bool IsEOF() {
    return eof_;
  }

  // when we know more data has been written to the file. we can use this
  // function to force the reader to look again in the file.
  // Also aligns the file position indicator to the start of the next block
  // by reading the rest of the data from the EOF position to the end of the
  // block that was partially read.
  void UnmarkEOF();

  SequentialFile* file() { return file_.get(); }

 private:
  const unique_ptr<SequentialFile> file_;
  Reporter* const reporter_;
  bool const checksum_;
  char* const backing_store_;
  Slice buffer_;
  bool eof_;   // Last Read() indicated EOF by returning < kBlockSize
  bool read_error_;   // Error occurred while reading from file

  // Offset of the file position indicator within the last block when an
  // EOF was detected.
  size_t eof_offset_;

  // Offset of the last record returned by ReadRecord.
  uint64_t last_record_offset_;
  // Offset of the first location past the end of buffer_.
  uint64_t end_of_buffer_offset_;

  // Offset at which to start looking for the first record to return
  uint64_t const initial_offset_;

  // Extend record types with the following special values
  enum {
    kEof = kMaxRecordType + 1,
    // Returned whenever we find an invalid physical record.
    // Currently there are three situations in which this happens:
    // * The record has an invalid CRC (ReadPhysicalRecord reports a drop)
    // * The record is a 0-length record (No drop is reported)
    // * The record is below constructor's initial_offset (No drop is reported)
    kBadRecord = kMaxRecordType + 2
  };

  // Skips all blocks that are completely before "initial_offset_".
  //
  // Returns true on success. Handles reporting.
  bool SkipToInitialBlock();

  // Return type, or one of the preceding special values
  unsigned int ReadPhysicalRecord(Slice* result);

  // Reports dropped bytes to the reporter.
  // buffer_ must be updated to remove the dropped bytes prior to invocation.
  void ReportCorruption(size_t bytes, const char* reason);
  void ReportDrop(size_t bytes, const Status& reason);

  // No copying allowed
  Reader(const Reader&);
  void operator=(const Reader&);
};

}  // namespace log
}  // namespace rocksdb