Class: PEROBS::FlatFile

Inherits:
Object
  • Object
show all
Defined in:
lib/perobs/FlatFile.rb

Overview

The FlatFile class manages the storage file of the FlatFileDB. It contains a sequence of blobs Each blob consists of header and the actual blob data bytes.

Constant Summary collapse

INDEX_BTREE_ORDER =

The number of entries in a single BTree node of the index file.

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Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(dir, progressmeter) ⇒ FlatFile

Create a new FlatFile object for a database in the given path.

Parameters:

  • dir (String) —

    Directory path for the data base file



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# File 'lib/perobs/FlatFile.rb', line 49

def initialize(dir, progressmeter)
  @db_dir = dir
  @progressmeter = progressmeter
  @f = nil
  @marks = nil
  @index = BTree.new(@db_dir, 'index', INDEX_BTREE_ORDER, @progressmeter)
  old_spaces_file = File.join(@db_dir, 'database_spaces.blobs')
  if File.exist?(old_spaces_file)
    # PEROBS version 4.1.0 and earlier used this space list format. It is
    # deprecated now. Newly created DBs use the SpaceManager format.
    @space_list = SpaceTree.new(@db_dir, @progressmeter)
  else
    @space_list = SpaceManager.new(@db_dir, @progressmeter)
  end
end

Class Method Details

.insert_header_checksums(db_dir) ⇒ Object 



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# File 'lib/perobs/FlatFile.rb', line 803

def FlatFile::insert_header_checksums(db_dir)
  old_file_name = File.join(db_dir, 'database.blobs')
  new_file_name = File.join(db_dir, 'database_v4.blobs')
  bak_file_name = File.join(db_dir, 'database_v3.blobs')

  old_file = File.open(old_file_name, 'rb')
  new_file = File.open(new_file_name, 'wb')

  entries = 0
  while (buf = old_file.read(21))
    flags, length, id, crc = *buf.unpack('CQQL')
    blob_data = old_file.read(length)

    # Some basic sanity checking to ensure all reserved bits are 0. Older
    # versions of PEROBS used to set bit 1 despite it being reserved now.
    unless flags & 0xF0 == 0
      PEROBS.log.fatal "Blob file #{old_file_name} contains illegal " +
        "flag byte #{'%02x' % flags} at #{old_file.pos - 21}"
    end

    # Check if the blob is valid and current.
    if flags & 0x1 == 1 && flags & 0x8 == 0
      # Make sure the bit 1 is not set anymore.
      flags = flags & 0x05
      header_str = [ flags, length, id, crc ].pack('CQQL')
      header_crc = Zlib.crc32(header_str, 0)
      header_str += [ header_crc ].pack('L')

      new_file.write(header_str + blob_data)
      entries += 1
    end
  end
  PEROBS.log.info "Header checksum added to #{entries} entries"

  old_file.close
  new_file.close

  File.rename(old_file_name, bak_file_name)
  File.rename(new_file_name, old_file_name)
end

Instance Method Details

#check(repair = false) ⇒ Integer

Check (and repair) the FlatFile.

Parameters:

  • repair (Boolean) (defaults to: false) —

    True if errors should be fixed.

Returns:

  • (Integer) —

    Number of errors found



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# File 'lib/perobs/FlatFile.rb', line 458

def check(repair = false)
  errors = 0
  return errors unless @f

  t = Time.now
  PEROBS.log.info "Checking FlatFile database" +
    "#{repair ? ' in repair mode' : ''}..."

  # First check the database blob file. Each entry should be readable and
  # correct and all IDs must be unique. We use a shadow index to keep
  # track of the already found IDs.
  new_index = BTree.new(@db_dir, 'new-index', INDEX_BTREE_ORDER,
                        @progressmeter)
  new_index.erase
  new_index.open

  corrupted_blobs = 0
  end_of_last_healthy_blob = nil
  @progressmeter.start('Checking blobs file', @f.size) do |pm|
    corrupted_blobs = each_blob_header do |header|
      if header.is_valid?
        # We have a non-deleted entry.
        begin
          @f.seek(header.addr + FlatFileBlobHeader::LENGTH)
          buf = @f.read(header.length)
          if buf.bytesize != header.length
            PEROBS.log.error "Premature end of file in blob with ID " +
              "#{header.id}."
            discard_damaged_blob(header) if repair
            errors += 1
            next
          end

          # Uncompress the data if the compression bit is set in the mark
          # byte.
          if header.is_compressed?
            begin
              buf = Zlib.inflate(buf)
            rescue Zlib::BufError, Zlib::DataError
              PEROBS.log.error "Corrupted compressed block with ID " +
                "#{header.id} found."
              discard_damaged_blob(header) if repair
              errors += 1
              next
            end
          end

          if header.crc && checksum(buf) != header.crc
            PEROBS.log.error "Checksum failure while checking blob " +
              "with ID #{header.id}"
            discard_damaged_blob(header) if repair
            errors += 1
            next
          end
        rescue IOError => e
          PEROBS.log.fatal "Check of blob with ID #{header.id} failed: " +
            e.message
        end

        # Check if the ID has already been found in the file.
        if (previous_address = new_index.get(header.id))
          PEROBS.log.error "Multiple blobs for ID #{header.id} found. " +
            "Addresses: #{previous_address}, #{header.addr}"
          errors += 1
          previous_header = FlatFileBlobHeader.read(@f, previous_address,
                                                    header.id)
          if repair
            # We have two blobs with the same ID and we must discard one of
            # them.
            if header.is_outdated?
              discard_damaged_blob(header)
            elsif previous_header.is_outdated?
              discard_damaged_blob(previous_header)
            else
              PEROBS.log.error "None of the blobs with same ID have " +
                "the outdated flag set. Deleting the smaller one."
              errors += 1
              discard_damaged_blob(header.length < previous_header.length ?
                                   header : previous_header)
            end
            next
          end
        else
          # ID is unique so far. Add it to the shadow index.
          new_index.insert(header.id, header.addr)
        end
      end
      end_of_last_healthy_blob = header.addr +
        FlatFileBlobHeader::LENGTH + header.length

      pm.update(header.addr)
    end

    if end_of_last_healthy_blob && end_of_last_healthy_blob != @f.size
      # The blob file ends with a corrupted blob header.
      PEROBS.log.error "#{@f.size - end_of_last_healthy_blob} corrupted " +
        'bytes found at the end of FlatFile.'
      corrupted_blobs += 1
      if repair
        PEROBS.log.error "Truncating FlatFile to " +
          "#{end_of_last_healthy_blob} bytes by discarding " +
          "#{@f.size - end_of_last_healthy_blob} bytes"
        @f.truncate(end_of_last_healthy_blob)
      end
    end

    errors += corrupted_blobs
  end

  # We no longer need the new index.
  new_index.close
  new_index.erase

  if repair && corrupted_blobs > 0
    erase_index_files
    defragmentize
    regenerate_index_and_spaces
  elsif corrupted_blobs == 0
    # Now we check the index data. It must be correct and the entries must
    # match the blob file. All entries in the index must be in the blob file
    # and vise versa.
    begin
      index_ok = @index.check do |id, address|
        has_id_at?(id, address)
      end
      x_check_errs = 0
      space_check_ok = true
      unless index_ok && (space_check_ok = @space_list.check(self)) &&
        (x_check_errs = cross_check_entries) == 0
        errors += 1 unless index_ok && space_check_ok
        errors += x_check_errs
        regenerate_index_and_spaces if repair
      end
    rescue PEROBS::FatalError
      errors += 1
      regenerate_index_and_spaces if repair
    end
  end

  sync if repair
  PEROBS.log.info "check_db completed in #{Time.now - t} seconds. " +
    "#{errors} errors found."

  errors
end

#clear_all_marks ⇒ Object

Clear alls marks.



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# File 'lib/perobs/FlatFile.rb', line 338

def clear_all_marks
  if @marks
    @marks.clear
  else
    @marks = IDList.new(@db_dir, 'marks', 8)
  end
end

#close ⇒ Object

Close the flat file. This method must be called to ensure that all data is really written into the filesystem.



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# File 'lib/perobs/FlatFile.rb', line 92

def close
  @space_list.close if @space_list.is_open?
  @index.close if @index.is_open?

  if @marks
    @marks.erase
    @marks = nil
  end

  if @f
    @f.flush
    @f.flock(File::LOCK_UN)
    @f.fsync
    @f.close
    @f = nil
  end
end

#defragmentize ⇒ Object

Eliminate all the holes in the file. This is an in-place implementation. No additional space will be needed on the file system.



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# File 'lib/perobs/FlatFile.rb', line 348

def defragmentize
  distance = 0
  new_file_size = 0
  deleted_blobs = 0
  corrupted_blobs = 0
  valid_blobs = 0

  # Iterate over all entries.
  @progressmeter.start('Defragmentizing blobs file', @f.size) do |pm|
    each_blob_header do |header|
      # If we have stumbled over a corrupted blob we treat it similar to a
      # deleted blob and reuse the space.
      if header.corruption_start
        distance += header.addr - header.corruption_start
        corrupted_blobs += 1
      end

      # Total size of the current entry
      entry_bytes = FlatFileBlobHeader::LENGTH + header.length
      if header.is_valid?
        # We have found a valid entry.
        valid_blobs += 1
        if distance > 0
          begin
            # Read current entry into a buffer
            @f.seek(header.addr)
            buf = @f.read(entry_bytes)
            # Write the buffer right after the end of the previous entry.
            @f.seek(header.addr - distance)
            @f.write(buf)
            # Mark the space between the relocated current entry and the
            # next valid entry as deleted space.
            FlatFileBlobHeader.new(@f, @f.pos, 0,
                                   distance - FlatFileBlobHeader::LENGTH,
                                   0, 0).write
            @f.flush
          rescue IOError => e
            PEROBS.log.fatal "Error while moving blob for ID " +
              "#{header.id}: #{e.message}"
          end
        end
        new_file_size = header.addr - distance +
          FlatFileBlobHeader::LENGTH + header.length
      else
        deleted_blobs += 1
        distance += entry_bytes
      end

      pm.update(header.addr)
    end
  end

  PEROBS.log.info "#{distance / 1000} KiB/#{deleted_blobs} blobs of " +
    "#{@f.size / 1000} KiB/#{valid_blobs} blobs or " +
    "#{'%.1f' % (distance.to_f / @f.size * 100.0)}% reclaimed"
  if corrupted_blobs > 0
    PEROBS.log.info "#{corrupted_blobs} corrupted blob(s) found. Space " +
      "was recycled."
  end

  @f.flush
  @f.truncate(new_file_size)
  @f.flush

  sync
end

#delete_obj_by_address(addr, id) ⇒ Object

Delete the blob that is stored at the specified address.

Parameters:

  • addr (Integer) —

    Address of the blob to delete

  • id (Integer) —

    ID of the blob to delete



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# File 'lib/perobs/FlatFile.rb', line 137

def delete_obj_by_address(addr, id)
  @index.remove(id) if @index.is_open?
  header = FlatFileBlobHeader.read(@f, addr, id)
  header.clear_flags
  @space_list.add_space(addr, header.length) if @space_list.is_open?
end

#delete_obj_by_id(id) ⇒ Boolean

Delete the blob for the specified ID.

Parameters:

  • id (Integer) —

    ID of the object to be deleted

Returns:

  • (Boolean) —

    True if object was deleted, false otherwise



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# File 'lib/perobs/FlatFile.rb', line 125

def delete_obj_by_id(id)
  if (pos = find_obj_addr_by_id(id))
    delete_obj_by_address(pos, id)
    return true
  end

  return false
end

#delete_unmarked_objects(&block) ⇒ Object

Delete all unmarked objects.



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# File 'lib/perobs/FlatFile.rb', line 145

def delete_unmarked_objects(&block)
  # We don't update the index and the space list during this operation as
  # we defragmentize the blob file at the end. We'll end the operation
  # with an empty space list.
  clear_index_files

  deleted_objects_count = 0
  @progressmeter.start('Sweeping unmarked objects', @f.size) do |pm|
    each_blob_header do |header|
      if header.is_valid? && !@marks.include?(header.id)
        delete_obj_by_address(header.addr, header.id)
        yield(header.id) if block_given?
        deleted_objects_count += 1
      end

      pm.update(header.addr)
    end
  end
  defragmentize

  # Update the index file and create a new, empty space list.
  regenerate_index_and_spaces

  deleted_objects_count
end

#find_obj_addr_by_id(id) ⇒ Integer

Find the address of the object with the given ID.

Parameters:

  • id (Integer) —

    ID of the object

Returns:

  • (Integer) —

    Offset in the flat file or nil if not found



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# File 'lib/perobs/FlatFile.rb', line 268

def find_obj_addr_by_id(id)
  @index.get(id)
end

#has_id_at?(id, address) ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/perobs/FlatFile.rb', line 780

def has_id_at?(id, address)
  begin
    header = FlatFileBlobHeader.read(@f, address)
  rescue PEROBS::FatalError
    return false
  end
  header.is_valid? && header.id == id
end

#has_space?(address, size) ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/perobs/FlatFile.rb', line 775

def has_space?(address, size)
  header = FlatFileBlobHeader.read(@f, address)
  !header.is_valid? && header.length == size
end

#inspect ⇒ Object 



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# File 'lib/perobs/FlatFile.rb', line 789

def inspect
  s = '['
  each_blob_header do |header|
    s << "{ :pos => #{header.addr}, :flags => #{header.flags}, " +
         ":length => #{header.length}, :id => #{header.id}, " +
         ":crc => #{header.crc}"
    if header.is_valid?
      s << ", :value => #{@f.read(header.length)}"
    end
    s << " }\n"
  end
  s + ']'
end

#is_marked_by_id?(id) ⇒ Boolean

Return true if the object with the given ID is marked, false otherwise.

Parameters:

  • id (Integer) —

    ID of the object

Returns:

  • (Boolean) 


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# File 'lib/perobs/FlatFile.rb', line 333

def is_marked_by_id?(id)
  @marks.include?(id)
end

#item_counter ⇒ Integer

Returns Number of items stored in the DB.

Returns:

  • (Integer) —

    Number of items stored in the DB.



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# File 'lib/perobs/FlatFile.rb', line 284

def item_counter
  @index.entries_count
end

#mark_obj_by_id(id) ⇒ Object

Mark the object with the given ID.

Parameters:

  • id (Integer) —

    ID of the object



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# File 'lib/perobs/FlatFile.rb', line 327

def mark_obj_by_id(id)
  @marks.insert(id)
end

#open ⇒ Object

Open the flat file for reading and writing.



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# File 'lib/perobs/FlatFile.rb', line 66

def open
  file_name = File.join(@db_dir, 'database.blobs')
  new_db_created = false
  begin
    if File.exist?(file_name)
      @f = File.open(file_name, 'rb+')
    else
      PEROBS.log.info "New FlatFile database '#{file_name}' created"
      @f = File.open(file_name, 'wb+')
      new_db_created = true
    end
  rescue IOError => e
    PEROBS.log.fatal "Cannot open FlatFile database #{file_name}: " +
      e.message
  end
  unless @f.flock(File::LOCK_NB | File::LOCK_EX)
    PEROBS.log.fatal "FlatFile database '#{file_name}' is locked by " +
      "another process"
  end
  @f.sync = true

  open_index_files(!new_db_created)
end

#read_obj_by_address(addr, id) ⇒ String

Read the object at the specified address.

Parameters:

  • addr (Integer) —

    Offset in the flat file

  • id (Integer) —

    ID of the data blob

Returns:

  • (String) —

    Raw object data



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# File 'lib/perobs/FlatFile.rb', line 292

def read_obj_by_address(addr, id)
  header = FlatFileBlobHeader.read(@f, addr, id)
  if header.id != id
    PEROBS.log.fatal "Database index corrupted: Index for object " +
      "#{id} points to object with ID #{header.id}"
  end

  buf = nil

  begin
    @f.seek(addr + FlatFileBlobHeader::LENGTH)
    buf = @f.read(header.length)
  rescue IOError => e
    PEROBS.log.fatal "Cannot read blob for ID #{id}: #{e.message}"
  end

  # Uncompress the data if the compression bit is set in the flags byte.
  if header.is_compressed?
    begin
      buf = Zlib.inflate(buf)
    rescue Zlib::BufError, Zlib::DataError
      PEROBS.log.fatal "Corrupted compressed block with ID " +
        "#{header.id} found."
    end
  end

  if checksum(buf) != header.crc
    PEROBS.log.fatal "Checksum failure while reading blob ID #{id}"
  end

  buf
end

#read_obj_by_id(id) ⇒ String or nil

Read the object with the given ID.

Parameters:

  • id (Integer) —

    ID of the object

Returns:

  • (String or nil) —

    Raw object data if found, otherwise nil



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# File 'lib/perobs/FlatFile.rb', line 275

def read_obj_by_id(id)
  if (addr = find_obj_addr_by_id(id))
    return read_obj_by_address(addr, id)
  end

  nil
end

#refresh ⇒ Object

This method iterates over all entries in the FlatFile and removes the entry and inserts it again. This is useful to update all entries in case the storage format has changed.



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# File 'lib/perobs/FlatFile.rb', line 418

def refresh
  # This iteration might look scary as we iterate over the entries while
  # while we are rearranging them. Re-inserted items may be inserted
  # before or at the current entry and this is fine. They also may be
  # inserted after the current entry and will be re-read again unless they
  # are inserted after the original file end.
  file_size = @f.size

  # We don't update the index and the space list during this operation as
  # we defragmentize the blob file at the end. We'll end the operation
  # with an empty space list.
  clear_index_files

  @progressmeter.start('Converting objects to new storage format',
                       @f.size) do |pm|
    each_blob_header do |header|
      if header.is_valid?
        buf = read_obj_by_address(header.addr, header.id)
        delete_obj_by_address(header.addr, header.id)
        write_obj_by_id(header.id, buf)
      end

      # Some re-inserted blobs may be inserted after the original file end.
      # No need to process those blobs again.
      break if header.addr >= file_size

      pm.update(header.addr)
    end
  end

  # Reclaim the space saved by compressing entries.
  defragmentize

  # Recreate the index file and create an empty space list.
  regenerate_index_and_spaces
end

#regenerate_index_and_spaces ⇒ Object

This method clears the index tree and the free space list and regenerates them from the FlatFile.



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# File 'lib/perobs/FlatFile.rb', line 741

def regenerate_index_and_spaces
  PEROBS.log.warn "Re-generating FlatFileDB index and space files"
  @index.open unless @index.is_open?
  @index.clear
  @space_list.open unless @space_list.is_open?
  @space_list.clear

  @progressmeter.start('Re-generating database index', @f.size) do |pm|
    each_blob_header do |header|
      if header.is_valid?
        if (duplicate_pos = @index.get(header.id))
          PEROBS.log.error "FlatFile contains multiple blobs for ID " +
            "#{header.id}. First blob is at address #{duplicate_pos}. " +
            "Other blob found at address #{header.addr}."
          if header.length > 0
            @space_list.add_space(header.addr, header.length)
          end
          discard_damaged_blob(header)
        else
          @index.insert(header.id, header.addr)
        end
      else
        if header.length > 0
          @space_list.add_space(header.addr, header.length)
        end
      end

      pm.update(header.addr)
    end
  end

  sync
end

#repair ⇒ Integer

Repair the FlatFile. In contrast to the repair functionality in the check() method this method is much faster. It simply re-creates the index and space list from the blob file.

Parameters:

  • repair (Boolean) —

    True if errors should be fixed.

Returns:

  • (Integer) —

    Number of errors found



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# File 'lib/perobs/FlatFile.rb', line 609

def repair
  errors = 0
  return errors unless @f

  t = Time.now
  PEROBS.log.info "Repairing FlatFile database"

  # Erase and re-open the index and space list files. We purposely don't
  # close the files at it would trigger needless flushing.
  clear_index_files(true)

  # Now we scan the blob file and re-index all blobs and spaces. Corrupted
  # blobs will be skipped.
  corrupted_blobs = 0
  end_of_last_healthy_blob = nil
  @progressmeter.start('Re-indexing blobs file', @f.size) do |pm|
    corrupted_blobs = each_blob_header do |header|
      if header.corruption_start
        # The blob is preceeded by a corrupted area. We create a new
        # header of a deleted blob for this area and write the new blob
        # over it.
        if (data_length = header.addr - header.corruption_start -
            FlatFileBlobHeader::LENGTH) <= 0
          PEROBS.log.error "Found a corrupted blob that is too small to " +
            "fit a header (#{data_length}). File must be defragmented."
        else
          new_header = FlatFileBlobHeader.new(@f, header.corruption_start,
                                              0, data_length, 0, 0)
          new_header.write
          @space_list.add_space(header.corruption_start, data_length)
        end
      end

      if header.is_valid?
        # We have a non-deleted entry.
        begin
          @f.seek(header.addr + FlatFileBlobHeader::LENGTH)
          buf = @f.read(header.length)
          if buf.bytesize != header.length
            PEROBS.log.error "Premature end of file in blob with ID " +
              "#{header.id}."
            discard_damaged_blob(header)
            errors += 1
            next
          end

          # Uncompress the data if the compression bit is set in the mark
          # byte.
          if header.is_compressed?
            begin
              buf = Zlib.inflate(buf)
            rescue Zlib::BufError, Zlib::DataError
              PEROBS.log.error "Corrupted compressed block with ID " +
                "#{header.id} found."
              discard_damaged_blob(header)
              errors += 1
              next
            end
          end

          if header.crc && checksum(buf) != header.crc
            PEROBS.log.error "Checksum failure while checking blob " +
              "with ID #{header.id}"
            discard_damaged_blob(header)
            errors += 1
            next
          end
        rescue IOError => e
          PEROBS.log.fatal "Check of blob with ID #{header.id} failed: " +
            e.message
        end

        # Check if the ID has already been found in the file.
        if (previous_address = @index.get(header.id))
          PEROBS.log.error "Multiple blobs for ID #{header.id} found. " +
            "Addresses: #{previous_address}, #{header.addr}"
          errors += 1
          previous_header = FlatFileBlobHeader.read(@f, previous_address,
                                                    header.id)
          # We have two blobs with the same ID and we must discard one of
          # them.
          if header.is_outdated?
            discard_damaged_blob(header)
          elsif previous_header.is_outdated?
            discard_damaged_blob(previous_header)
          else
            PEROBS.log.error "None of the blobs with same ID have " +
              "the outdated flag set. Deleting the smaller one."
            errors += 1
            discard_damaged_blob(header.length < previous_header.length ?
                                 header : previous_header)
          end
        else
          # ID is unique so far. Add it to the shadow index.
          @index.insert(header.id, header.addr)
        end

      else
        if header.length > 0
          @space_list.add_space(header.addr, header.length)
        end
      end
      end_of_last_healthy_blob = header.addr +
        FlatFileBlobHeader::LENGTH + header.length

      pm.update(header.addr)
    end

    if end_of_last_healthy_blob && end_of_last_healthy_blob != @f.size
      # The blob file ends with a corrupted blob header.
      PEROBS.log.error "#{@f.size - end_of_last_healthy_blob} corrupted " +
        'bytes found at the end of FlatFile.'
      corrupted_blobs += 1

      PEROBS.log.error "Truncating FlatFile to " +
        "#{end_of_last_healthy_blob} bytes by discarding " +
        "#{@f.size - end_of_last_healthy_blob} bytes"
      @f.truncate(end_of_last_healthy_blob)
    end

    errors += corrupted_blobs
  end

  sync
  PEROBS.log.info "FlatFile repair completed in #{Time.now - t} seconds. " +
    "#{errors} errors found."

  errors
end

#sync ⇒ Object

Force outstanding data to be written to the filesystem.



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# File 'lib/perobs/FlatFile.rb', line 111

def sync
  begin
    @f.flush
    @f.fsync
  rescue IOError => e
    PEROBS.log.fatal "Cannot sync flat file database: #{e.message}"
  end
  @index.sync
  @space_list.sync
end

#write_obj_by_id(id, raw_obj) ⇒ Integer

Write the given object into the file. This method never uses in-place updates for existing objects. A new copy is inserted first and only when the insert was successful, the old copy is deleted and the index updated.

Parameters:

  • id (Integer) —

    ID of the object

  • raw_obj (String) —

    Raw object as String

Returns:

  • (Integer) —

    position of the written blob in the blob file



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# File 'lib/perobs/FlatFile.rb', line 178

def write_obj_by_id(id, raw_obj)
  # Check if we have already an object with the given ID. We'll mark it as
  # outdated and save the header for later deletion. In case this
  # operation is aborted or interrupted we ensure that we either have the
  # old or the new version available.
  if (old_addr = find_obj_addr_by_id(id))
    old_header = FlatFileBlobHeader.read(@f, old_addr)
    old_header.set_outdated_flag
  end

  crc = checksum(raw_obj)

  # If the raw_obj is larger then 256 characters we will compress it to
  # safe some space in the database file. For smaller strings the
  # performance impact of compression is not compensated by writing
  # less data to the storage.
  compressed = false
  raw_obj_bytesize = raw_obj.bytesize
  if raw_obj_bytesize > 256
    raw_obj = Zlib.deflate(raw_obj)
    raw_obj_bytesize = raw_obj.bytesize
    compressed = true
  end

  addr, length = find_free_blob(raw_obj_bytesize)
  begin
    if length != -1
      # Just a safeguard so we don't overwrite current data.
      header = FlatFileBlobHeader.read(@f, addr)
      if header.length != length
        PEROBS.log.fatal "Length in free list (#{length}) and header " +
          "(#{header.length}) for address #{addr} don't match."
      end
      if raw_obj_bytesize > header.length
        PEROBS.log.fatal "Object (#{raw_obj_bytesize}) is longer than " +
          "blob space (#{header.length})."
      end
      if header.is_valid?
        PEROBS.log.fatal "Entry at address #{addr} with flags: " +
          "#{header.flags} is already used for ID #{header.id}."
      end
    end
    flags = 1 << FlatFileBlobHeader::VALID_FLAG_BIT
    flags |= (1 << FlatFileBlobHeader::COMPRESSED_FLAG_BIT) if compressed
    FlatFileBlobHeader.new(@f, addr, flags, raw_obj_bytesize, id, crc).write
    @f.write(raw_obj)
    if length != -1 && raw_obj_bytesize < length
      # The new object was not appended and it did not completely fill the
      # free space. So we have to write a new header to mark the remaining
      # empty space.
      unless length - raw_obj_bytesize >= FlatFileBlobHeader::LENGTH
        PEROBS.log.fatal "Not enough space to append the empty space " +
          "header (space: #{length} bytes, object: #{raw_obj_bytesize} " +
          "bytes)."
      end
      space_address = @f.pos
      space_length = length - FlatFileBlobHeader::LENGTH - raw_obj_bytesize
      FlatFileBlobHeader.new(@f, space_address, 0, space_length,
                             0, 0).write
      # Register the new space with the space list.
      if @space_list.is_open? && space_length > 0
        @space_list.add_space(space_address, space_length)
      end
    end

    # Once the blob has been written we can update the index as well.
    @index.insert(id, addr) if @index.is_open?

    if old_addr
      # If we had an existing object stored for the ID we have to mark
      # this entry as deleted now.
      old_header.clear_flags
      # And register the newly freed space with the space list.
      if @space_list.is_open?
        @space_list.add_space(old_addr, old_header.length)
      end
    else
      @f.flush
    end
  rescue IOError => e
    PEROBS.log.fatal "Cannot write blob for ID #{id} to FlatFileDB: " +
      e.message
  end

  addr
end