log_reader.cc

//  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.

#include "db/log_reader.h"

#include <stdio.h>
#include "rocksdb/env.h"
#include "util/coding.h"
#include "util/crc32c.h"
#include "util/file_reader_writer.h"

namespace rocksdb {
namespace log {

Reader::Reporter::~Reporter() {
}

Reader::Reader(unique_ptr<SequentialFileReader>&& _file, Reporter* reporter,
               bool checksum, uint64_t initial_offset)
    : file_(std::move(_file)),
      reporter_(reporter),
      checksum_(checksum),
      backing_store_(new char[kBlockSize]),
      buffer_(),
      eof_(false),
      read_error_(false),
      eof_offset_(0),
      last_record_offset_(0),
      end_of_buffer_offset_(0),
      initial_offset_(initial_offset) {}

Reader::~Reader() {
  delete[] backing_store_;
}

bool Reader::SkipToInitialBlock() {
  size_t initial_offset_in_block = initial_offset_ % kBlockSize;
  uint64_t block_start_location = initial_offset_ - initial_offset_in_block;

  // Don't search a block if we'd be in the trailer
  if (initial_offset_in_block > kBlockSize - 6) {
    block_start_location += kBlockSize;
  }

  end_of_buffer_offset_ = block_start_location;

  // Skip to start of first block that can contain the initial record
  if (block_start_location > 0) {
    Status skip_status = file_->Skip(block_start_location);
    if (!skip_status.ok()) {
      ReportDrop(static_cast<size_t>(block_start_location), skip_status);
      return false;
    }
  }

  return true;
}

bool Reader::ReadRecord(Slice* record, std::string* scratch,
                        const bool report_eof_inconsistency) {
  if (last_record_offset_ < initial_offset_) {
    if (!SkipToInitialBlock()) {
      return false;
    }
  }

  scratch->clear();
  record->clear();
  bool in_fragmented_record = false;
  // Record offset of the logical record that we're reading
  // 0 is a dummy value to make compilers happy
  uint64_t prospective_record_offset = 0;

  Slice fragment;
  while (true) {
    uint64_t physical_record_offset = end_of_buffer_offset_ - buffer_.size();
    const unsigned int record_type =
        ReadPhysicalRecord(&fragment, report_eof_inconsistency);
    switch (record_type) {
      case kFullType:
        if (in_fragmented_record && !scratch->empty()) {
          // Handle bug in earlier versions of log::Writer where
          // it could emit an empty kFirstType record at the tail end
          // of a block followed by a kFullType or kFirstType record
          // at the beginning of the next block.
          ReportCorruption(scratch->size(), "partial record without end(1)");
        }
        prospective_record_offset = physical_record_offset;
        scratch->clear();
        *record = fragment;
        last_record_offset_ = prospective_record_offset;
        return true;

      case kFirstType:
        if (in_fragmented_record && !scratch->empty()) {
          // Handle bug in earlier versions of log::Writer where
          // it could emit an empty kFirstType record at the tail end
          // of a block followed by a kFullType or kFirstType record
          // at the beginning of the next block.
          ReportCorruption(scratch->size(), "partial record without end(2)");
        }
        prospective_record_offset = physical_record_offset;
        scratch->assign(fragment.data(), fragment.size());
        in_fragmented_record = true;
        break;

      case kMiddleType:
        if (!in_fragmented_record) {
          ReportCorruption(fragment.size(),
                           "missing start of fragmented record(1)");
        } else {
          scratch->append(fragment.data(), fragment.size());
        }
        break;

      case kLastType:
        if (!in_fragmented_record) {
          ReportCorruption(fragment.size(),
                           "missing start of fragmented record(2)");
        } else {
          scratch->append(fragment.data(), fragment.size());
          *record = Slice(*scratch);
          last_record_offset_ = prospective_record_offset;
          return true;
        }
        break;

      case kEof:
        if (in_fragmented_record) {
          if (report_eof_inconsistency) {
            ReportCorruption(scratch->size(), "error reading trailing data");
          }
          // This can be caused by the writer dying immediately after
          //  writing a physical record but before completing the next; don't
          //  treat it as a corruption, just ignore the entire logical record.
          scratch->clear();
        }
        return false;

      case kBadRecord:
        if (in_fragmented_record) {
          ReportCorruption(scratch->size(), "error in middle of record");
          in_fragmented_record = false;
          scratch->clear();
        }
        break;

      default: {
        char buf[40];
        snprintf(buf, sizeof(buf), "unknown record type %u", record_type);
        ReportCorruption(
            (fragment.size() + (in_fragmented_record ? scratch->size() : 0)),
            buf);
        in_fragmented_record = false;
        scratch->clear();
        break;
      }
    }
  }
  return false;
}

uint64_t Reader::LastRecordOffset() {
  return last_record_offset_;
}

void Reader::UnmarkEOF() {
  if (read_error_) {
    return;
  }

  eof_ = false;

  if (eof_offset_ == 0) {
    return;
  }

  // If the EOF was in the middle of a block (a partial block was read) we have
  // to read the rest of the block as ReadPhysicalRecord can only read full
  // blocks and expects the file position indicator to be aligned to the start
  // of a block.
  //
  //      consumed_bytes + buffer_size() + remaining == kBlockSize

  size_t consumed_bytes = eof_offset_ - buffer_.size();
  size_t remaining = kBlockSize - eof_offset_;

  // backing_store_ is used to concatenate what is left in buffer_ and
  // the remainder of the block. If buffer_ already uses backing_store_,
  // we just append the new data.
  if (buffer_.data() != backing_store_ + consumed_bytes) {
    // Buffer_ does not use backing_store_ for storage.
    // Copy what is left in buffer_ to backing_store.
    memmove(backing_store_ + consumed_bytes, buffer_.data(), buffer_.size());
  }

  Slice read_buffer;
  Status status = file_->Read(remaining, &read_buffer,
    backing_store_ + eof_offset_);

  size_t added = read_buffer.size();
  end_of_buffer_offset_ += added;

  if (!status.ok()) {
    if (added > 0) {
      ReportDrop(added, status);
    }

    read_error_ = true;
    return;
  }

  if (read_buffer.data() != backing_store_ + eof_offset_) {
    // Read did not write to backing_store_
    memmove(backing_store_ + eof_offset_, read_buffer.data(),
      read_buffer.size());
  }

  buffer_ = Slice(backing_store_ + consumed_bytes,
    eof_offset_ + added - consumed_bytes);

  if (added < remaining) {
    eof_ = true;
    eof_offset_ += added;
  } else {
    eof_offset_ = 0;
  }
}

void Reader::ReportCorruption(size_t bytes, const char* reason) {
  ReportDrop(bytes, Status::Corruption(reason));
}

void Reader::ReportDrop(size_t bytes, const Status& reason) {
  if (reporter_ != nullptr &&
      end_of_buffer_offset_ - buffer_.size() - bytes >= initial_offset_) {
    reporter_->Corruption(bytes, reason);
  }
}

unsigned int Reader::ReadPhysicalRecord(Slice* result,
                                        const bool report_eof_inconsistency) {
  while (true) {
    if (buffer_.size() < (size_t)kHeaderSize) {
      if (!eof_ && !read_error_) {
        // Last read was a full read, so this is a trailer to skip
        buffer_.clear();
        Status status = file_->Read(kBlockSize, &buffer_, backing_store_);
        end_of_buffer_offset_ += buffer_.size();
        if (!status.ok()) {
          buffer_.clear();
          ReportDrop(kBlockSize, status);
          read_error_ = true;
          return kEof;
        } else if (buffer_.size() < (size_t)kBlockSize) {
          eof_ = true;
          eof_offset_ = buffer_.size();
        }
        continue;
      } else {
        // Note that if buffer_ is non-empty, we have a truncated header at the
        //  end of the file, which can be caused by the writer crashing in the
        //  middle of writing the header. Unless explicitly requested we don't
        //  considering this an error, just report EOF.
        if (buffer_.size() && report_eof_inconsistency) {
          ReportCorruption(buffer_.size(), "truncated header");
        }
        buffer_.clear();
        return kEof;
      }
    }

    // Parse the header
    const char* header = buffer_.data();
    const uint32_t a = static_cast<uint32_t>(header[4]) & 0xff;
    const uint32_t b = static_cast<uint32_t>(header[5]) & 0xff;
    const unsigned int type = header[6];
    const uint32_t length = a | (b << 8);
    if (kHeaderSize + length > buffer_.size()) {
      size_t drop_size = buffer_.size();
      buffer_.clear();
      if (!eof_) {
        ReportCorruption(drop_size, "bad record length");
        return kBadRecord;
      }
      // If the end of the file has been reached without reading |length| bytes
      // of payload, assume the writer died in the middle of writing the record.
      // Don't report a corruption unless requested.
      if (drop_size && report_eof_inconsistency) {
        ReportCorruption(drop_size, "truncated header");
      }
      return kEof;
    }

    if (type == kZeroType && length == 0) {
      // Skip zero length record without reporting any drops since
      // such records are produced by the mmap based writing code in
      // env_posix.cc that preallocates file regions.
      // NOTE: this should never happen in DB written by new RocksDB versions,
      // since we turn off mmap writes to manifest and log files
      buffer_.clear();
      return kBadRecord;
    }

    // Check crc
    if (checksum_) {
      uint32_t expected_crc = crc32c::Unmask(DecodeFixed32(header));
      uint32_t actual_crc = crc32c::Value(header + 6, 1 + length);
      if (actual_crc != expected_crc) {
        // Drop the rest of the buffer since "length" itself may have
        // been corrupted and if we trust it, we could find some
        // fragment of a real log record that just happens to look
        // like a valid log record.
        size_t drop_size = buffer_.size();
        buffer_.clear();
        ReportCorruption(drop_size, "checksum mismatch");
        return kBadRecord;
      }
    }

    buffer_.remove_prefix(kHeaderSize + length);

    // Skip physical record that started before initial_offset_
    if (end_of_buffer_offset_ - buffer_.size() - kHeaderSize - length <
        initial_offset_) {
      result->clear();
      return kBadRecord;
    }

    *result = Slice(header + kHeaderSize, length);
    return type;
  }
}

}  // namespace log
}  // namespace rocksdb