在Python中检查tarfile完整性

Question

我正在将备份脚本从shell转换为Python。我的旧脚本的一个特性是通过执行以下操作来检查创建的tarfile的完整性：gzip-t。

这在Python中似乎有点棘手。

似乎唯一的方法就是读取tarfile中的每个压缩TarInfo对象。

有没有一种方法可以检查tar文件的完整性，而不提取到磁盘，或者将其保存在内存中（完整的）？

freenode上python上的好人建议我应该一块一块地读取每个TarInfo对象，放弃读取的每个块。

我必须承认我不知道该怎么做，因为我刚开始使用Python。

假设我有一个30GB的tarfile，其中包含从1kb到10GB的文件。。。

这是我开始写的解决方案：

try:
    tardude = tarfile.open("zero.tar.gz")
except:
    print "There was an error opening tarfile. The file might be corrupt or missing."

for member_info in tardude.getmembers():
    try:
        check = tardude.extractfile(member_info.name)
    except:
        print "File: %r is corrupt." % member_info.name

tardude.close()

这段代码远未完成。我不敢在一个30GB的大tar存档上运行这个，因为在某一点上，check是一个10+GB的对象（如果我在tar存档中有这么大的文件）

奖金： 我试图手动损坏zero.tar.gz（hex editor-编辑几个字节的midfile）。第一个例外是不捕获IOError。。。输出如下：

Traceback (most recent call last):
  File "./test.py", line 31, in <module>
    for member_info in tardude.getmembers():
  File "/usr/lib/python2.7/tarfile.py", line 1805, in getmembers
    self._load()        # all members, we first have to
  File "/usr/lib/python2.7/tarfile.py", line 2380, in _load
    tarinfo = self.next()
  File "/usr/lib/python2.7/tarfile.py", line 2315, in next
    self.fileobj.seek(self.offset)
  File "/usr/lib/python2.7/gzip.py", line 429, in seek
    self.read(1024)
  File "/usr/lib/python2.7/gzip.py", line 256, in read
    self._read(readsize)
  File "/usr/lib/python2.7/gzip.py", line 320, in _read
    self._read_eof()
  File "/usr/lib/python2.7/gzip.py", line 342, in _read_eof
    hex(self.crc)))
IOError: CRC check failed 0xe5384b87 != 0xdfe91e1L

Answer 1

只是对Aya's答案稍加改进，使其更具习惯性（尽管我正在删除一些错误检查，以使机制更直观）：

BLOCK_SIZE = 1024

with tarfile.open("zero.tar.gz") as tardude:
    for member in tardude.getmembers():
        with tardude.extractfile(member.name) as target:
            for chunk in iter(lambda: target.read(BLOCK_SIZE), b''):
                pass

这实际上只是删除了while 1:（有时被认为是一个小代码味道）和if not data:检查。还要注意，with的使用将此限制为Python 2.7+

Answer 2

您可以使用subprocess模块调用文件上的gzip -t。。。

from subprocess import call
import os

with open(os.devnull, 'w') as bb:
    result = call(['gzip', '-t', "zero.tar.gz"], stdout=bb, stderr=bb)

如果result不是0，则有问题。不过，您可能需要检查gzip是否可用。我为此写了一个实用函数

import subprocess
import sys
import os

def checkfor(args, rv = 0):
    """Make sure that a program necessary for using this script is
    available.

    Arguments:
    args  -- string or list of strings of commands. A single string may
             not contain spaces.
    rv    -- expected return value from evoking the command.
    """
    if isinstance(args, str):
        if ' ' in args:
            raise ValueError('no spaces in single command allowed')
        args = [args]
    try:
        with open(os.devnull, 'w') as bb:
            rc = subprocess.call(args, stdout=bb, stderr=bb)
        if rc != rv:
            raise OSError
    except OSError as oops:
        outs = "Required program '{}' not found: {}."
        print(outs.format(args[0], oops.strerror))
        sys.exit(1)

Answer 3

I tried manually corrupting zero.tar.gz (hex editor - edit a few bytes midfile). The first except does not catch IOError...

如果你看一下回溯，你会发现它在你调用tardude.getmembers()时被抛出，所以你需要像。。。

try:
    tardude = tarfile.open("zero.tar.gz")
except:
    print "There was an error opening tarfile. The file might be corrupt or missing."

try:
    members = tardude.getmembers()
except:
    print "There was an error reading tarfile members."

for member_info in members:
    try:
        check = tardude.extractfile(member_info.name)
    except:
        print "File: %r is corrupt." % member_info.name

tardude.close()

至于最初的问题，你就快到了。你只需要从check对象中读取数据，比如。。。

BLOCK_SIZE = 1024

try:
    tardude = tarfile.open("zero.tar.gz")
except:
    print "There was an error opening tarfile. The file might be corrupt or missing."

try:
    members = tardude.getmembers()
except:
    print "There was an error reading tarfile members."

for member_info in members:
    try:            
        check = tardude.extractfile(member_info.name)
        while 1:
            data = check.read(BLOCK_SIZE)
            if not data:
                break
    except:
        print "File: %r is corrupt." % member_info.name

tardude.close()

…这将确保一次使用的内存永远不会超过BLOCK_SIZE字节。

另外，你应该尽量避免使用。。。

try:
    do_something()
except:
    do_something_else()

…因为它会掩盖意外的异常。试着只捕捉你真正想要处理的异常，比如。。。

try:
    do_something()
except IOError:
    do_something_else()

…否则你会发现在你的代码中发现错误更加困难。