1  // Copyright (c) 2011 The LevelDB Authors. All rights reserved.
  2  // Use of this source code is governed by a BSD-style license that can be
  3  // found in the LICENSE file. See the AUTHORS file for names of contributors.
  4  
  5  #ifndef STORAGE_LEVELDB_DB_LOG_READER_H_
  6  #define STORAGE_LEVELDB_DB_LOG_READER_H_
  7  
  8  #include <stdint.h>
  9  
 10  #include "db/log_format.h"
 11  #include "leveldb/slice.h"
 12  #include "leveldb/status.h"
 13  
 14  namespace leveldb {
 15  
 16  class SequentialFile;
 17  
 18  namespace log {
 19  
 20  class Reader {
 21   public:
 22    // Interface for reporting errors.
 23    class Reporter {
 24     public:
 25      virtual ~Reporter();
 26  
 27      // Some corruption was detected.  "size" is the approximate number
 28      // of bytes dropped due to the corruption.
 29      virtual void Corruption(size_t bytes, const Status& status) = 0;
 30    };
 31  
 32    // Create a reader that will return log records from "*file".
 33    // "*file" must remain live while this Reader is in use.
 34    //
 35    // If "reporter" is non-null, it is notified whenever some data is
 36    // dropped due to a detected corruption.  "*reporter" must remain
 37    // live while this Reader is in use.
 38    //
 39    // If "checksum" is true, verify checksums if available.
 40    //
 41    // The Reader will start reading at the first record located at physical
 42    // position >= initial_offset within the file.
 43    Reader(SequentialFile* file, Reporter* reporter, bool checksum,
 44           uint64_t initial_offset);
 45  
 46    Reader(const Reader&) = delete;
 47    Reader& operator=(const Reader&) = delete;
 48  
 49    ~Reader();
 50  
 51    // Read the next record into *record.  Returns true if read
 52    // successfully, false if we hit end of the input.  May use
 53    // "*scratch" as temporary storage.  The contents filled in *record
 54    // will only be valid until the next mutating operation on this
 55    // reader or the next mutation to *scratch.
 56    bool ReadRecord(Slice* record, std::string* scratch);
 57  
 58    // Returns the physical offset of the last record returned by ReadRecord.
 59    //
 60    // Undefined before the first call to ReadRecord.
 61    uint64_t LastRecordOffset();
 62  
 63   private:
 64    // Extend record types with the following special values
 65    enum {
 66      kEof = kMaxRecordType + 1,
 67      // Returned whenever we find an invalid physical record.
 68      // Currently there are three situations in which this happens:
 69      // * The record has an invalid CRC (ReadPhysicalRecord reports a drop)
 70      // * The record is a 0-length record (No drop is reported)
 71      // * The record is below constructor's initial_offset (No drop is reported)
 72      kBadRecord = kMaxRecordType + 2
 73    };
 74  
 75    // Skips all blocks that are completely before "initial_offset_".
 76    //
 77    // Returns true on success. Handles reporting.
 78    bool SkipToInitialBlock();
 79  
 80    // Return type, or one of the preceding special values
 81    unsigned int ReadPhysicalRecord(Slice* result);
 82  
 83    // Reports dropped bytes to the reporter.
 84    // buffer_ must be updated to remove the dropped bytes prior to invocation.
 85    void ReportCorruption(uint64_t bytes, const char* reason);
 86    void ReportDrop(uint64_t bytes, const Status& reason);
 87  
 88    SequentialFile* const file_;
 89    Reporter* const reporter_;
 90    bool const checksum_;
 91    char* const backing_store_;
 92    Slice buffer_;
 93    bool eof_;  // Last Read() indicated EOF by returning < kBlockSize
 94  
 95    // Offset of the last record returned by ReadRecord.
 96    uint64_t last_record_offset_;
 97    // Offset of the first location past the end of buffer_.
 98    uint64_t end_of_buffer_offset_;
 99  
100    // Offset at which to start looking for the first record to return
101    uint64_t const initial_offset_;
102  
103    // True if we are resynchronizing after a seek (initial_offset_ > 0). In
104    // particular, a run of kMiddleType and kLastType records can be silently
105    // skipped in this mode
106    bool resyncing_;
107  };
108  
109  }  // namespace log
110  }  // namespace leveldb
111  
112  #endif  // STORAGE_LEVELDB_DB_LOG_READER_H_