log_reader.h

//  Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
//  This source code is licensed under both the GPLv2 (found in the
//  COPYING file in the root directory) and Apache 2.0 License
//  (found in the LICENSE.Apache file in the root 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"
#include "rocksdb/options.h"

namespace rocksdb {

class SequentialFileReader;
class Logger;

namespace log {

/**
 * Reader is a general purpose log stream reader implementation. The actual job
 * of reading from the device is implemented by the SequentialFile interface.
 *
 * Please see Writer for details on the file and record layout.
 */
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.
  Reader(std::shared_ptr<Logger> info_log,
         // @lint-ignore TXT2 T25377293 Grandfathered in
         std::unique_ptr<SequentialFileReader>&& file, Reporter* reporter,
         bool checksum, uint64_t log_num);

  virtual ~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.
  virtual bool ReadRecord(Slice* record, std::string* scratch,
                          WALRecoveryMode wal_recovery_mode =
                              WALRecoveryMode::kTolerateCorruptedTailRecords);

  // 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_;
  }

  // returns true if the reader has encountered read error.
  bool hasReadError() const { return read_error_; }

  // 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.
  virtual void UnmarkEOF();

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

  Reporter* GetReporter() const { return reporter_; }

  uint64_t GetLogNumber() const { return log_number_; }

 protected:
  std::shared_ptr<Logger> info_log_;
  const std::unique_ptr<SequentialFileReader> file_;
  Reporter* const reporter_;
  bool const checksum_;
  char* const backing_store_;

  // Internal state variables used for reading records
  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_;

  // which log number this is
  uint64_t const log_number_;

  // Whether this is a recycled log file
  bool recycled_;

  // 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)
    kBadRecord = kMaxRecordType + 2,
    // Returned when we fail to read a valid header.
    kBadHeader = kMaxRecordType + 3,
    // Returned when we read an old record from a previous user of the log.
    kOldRecord = kMaxRecordType + 4,
    // Returned when we get a bad record length
    kBadRecordLen = kMaxRecordType + 5,
    // Returned when we get a bad record checksum
    kBadRecordChecksum = kMaxRecordType + 6,
  };

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

  // Read some more
  bool ReadMore(size_t* drop_size, int *error);

  void UnmarkEOFInternal();

  // 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);

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

class FragmentBufferedReader : public Reader {
 public:
  FragmentBufferedReader(std::shared_ptr<Logger> info_log,
                         // @lint-ignore TXT2 T25377293 Grandfathered in
                         std::unique_ptr<SequentialFileReader>&& _file,
                         Reporter* reporter, bool checksum, uint64_t log_num)
      : Reader(info_log, std::move(_file), reporter, checksum, log_num),
        fragments_(),
        in_fragmented_record_(false) {}
  ~FragmentBufferedReader() override {}
  bool ReadRecord(Slice* record, std::string* scratch,
                  WALRecoveryMode wal_recovery_mode =
                      WALRecoveryMode::kTolerateCorruptedTailRecords) override;
  void UnmarkEOF() override;

 private:
  std::string fragments_;
  bool in_fragmented_record_;

  bool TryReadFragment(Slice* result, size_t* drop_size,
                       unsigned int* fragment_type_or_err);

  bool TryReadMore(size_t* drop_size, int* error);

  // No copy allowed
  FragmentBufferedReader(const FragmentBufferedReader&);
  void operator=(const FragmentBufferedReader&);
};

}  // namespace log
}  // namespace rocksdb