BT源码阅读兼移植三：种子文件的生成（1）

知道了bencoding编码，我们来看一下种子文件是如何生成的。

第一篇文章中提到Bittorrent/makematafle.py中的函数实现了生成种子功能。

生成种子的总的思路就是将文件切片后用sha1取得摘要，摘要和目录结构、TrackerUrl等信息bencoding后生成一个文件。

 

下面说明细节：

 

先看一下sha1是啥：

sha1是security hash algrithom 的缩写，其实跟MD5类似，也是一种单向哈希映射。我们一般把它用作数字签名（因为雪崩效应）。对于长度小于2^64位的消息，SHA1会产生一个160位的消息摘要。 在BT种子中，把文件按照2^n位进行切分，每段用sha1生成一个160位的摘要信息。这段摘要信息能够帮助确定下载的片段是否正确。

 

再看一下官方协议：

http://wiki.theory.org/BitTorrentSpecification 的MetaInfo File Structure部分。

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All data in a metainfo file is bencoded. The specification for bencoding is defined above.

The content of a metainfo file (the file ending in ".torrent") is a bencoded dictionary, containing the keys listed below. All character string values are UTF-8 encoded.

• info: a dictionary that describes the file(s) of the torrent. There are two possible forms: one for the case of a 'single-file' torrent with no directory structure, and one for the case of a 'multi-file' torrent (see below for details)
• announce: The announce URL of the tracker (string)
• announce-list: (optional) this is an extention to the official specification, offering backwards-compatibility. (list of lists of strings).
  • The official request for a specification change is here.
• creation date: (optional) the creation time of the torrent, in standard UNIX epoch format (integer, seconds since 1-Jan-1970 00:00:00 UTC)
• comment: (optional) free-form textual comments of the author (string)
• created by: (optional) name and version of the program used to create the .torrent (string)
• encoding: (optional) the string encoding format used to generate the pieces part of the info dictionary in the .torrent metafile (string)

Info Dictionary

This section contains the field which are common to both mode, "single file" and "multiple file".

• piece length: number of bytes in each piece (integer)
• pieces: string consisting of the concatenation of all 20-byte SHA1 hash values, one per piece (byte string, i.e. not urlencoded)
• private: (optional) this field is an integer. If it is set to "1", the client MUST publish its presence to get other peers ONLY via the trackers explicitly described in the metainfo file. If this field is set to "0" or is not present, the client may obtain peer from other means, e.g. PEX peer exchange, dht. Here, "private" may be read as "no external peer source".
  • NOTE: There is much debate surrounding private trackers.
  • The official request for a specification change is here.
  • Azureus was the first client to respect private trackers, see their wiki for more details.

Info in Single File Mode

For the case of the single-file mode, the info dictionary contains the following structure:

• name: the filename. This is purely advisory. (string)
• length: length of the file in bytes (integer)
• md5sum: (optional) a 32-character hexadecimal string corresponding to the MD5 sum of the file. This is not used by BitTorrent at all, but it is included by some programs for greater compatibility.

Info in Multiple File Mode

For the case of the multi-file mode, the info dictionary contains the following structure:

• name: the filename of the directory in which to store all the files. This is purely advisory. (string)
• files: a list of dictionaries, one for each file. Each dictionary in this list contains the following keys:
  • length: length of the file in bytes (integer)
  • md5sum: (optional) a 32-character hexadecimal string corresponding to the MD5 sum of the file. This is not used by BitTorrent at all, but it is included by some programs for greater compatibility.
  • path: a list containing one or more string elements that together represent the path and filename. Each element in the list corresponds to either a directory name or (in the case of the final element) the filename. For example, a the file "dir1/dir2/file.ext" would consist of three string elements: "dir1", "dir2", and "file.ext". This is encoded as a bencoded list of strings such as l4:dir14:dir28:file.exte

Notes

• The piece length specifies the nominal piece size, and is usually a power of 2. The piece size is typically chosen based on the total amount of file data in the torrent, and is constrained by the fact that too-large piece sizes cause inefficiency, and too-small piece sizes cause large .torrent metadata file. Historically, piece size was chosen to result in a .torrent file no greater than approx. 50 - 75 kB (presumably to ease the load on the server hosting the torrent files).
  • Current best-practice is to keep the piece size to 512KB or less, for torrents around 8-10GB, even if that results in a larger .torrent file. This results in a more efficient swarm for sharing files. The most common sizes are 256 kB, 512 kB, and 1 MB.
  • Every piece is of equal length except for the final piece, which is irregular. The number of pieces is thus determined by 'ceil( total length / piece size )'.
  • For the purposes of piece boundaries in the multi-file case, consider the file data as one long continuous stream, composed of the concatenation of each file in the order listed in the files list. The number of pieces and their boundaries are then determined in the same manner as the case of a single file. Pieces may overlap file boundaries.
• Each piece has a corresponding SHA1 hash of the data contained within that piece. These hashes are concatenated to form the pieces value in the above info dictionary. Note that this is not a list but rather a single string. The length of the string must be a multiple of 20.

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协议明确的规定出了文件的结构，可以直接看一下python的代码了。

makematafle.py 的主函数 makematafile 实现了单个文件的torrent文件的制作，makemetafiles实现了多个文件的torrent文件制作。 makeinfo完成了infodictionary的制作。

 

先看一下makeinfo函数