Question

我正在努力将我的备份脚本从shell转换为Python。我的旧脚本的一个功能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。

似乎唯一的方法是通过读取tarfile中的每个压缩的TarInfo对象。

有没有办法检查tarfile的完整性，不提取到磁盘，或将其保存在内存中（完整地）？

freenode上的#python好人建议我应该逐块读取每个TarInfo对象，丢弃每个块读取。

我必须承认，我不知道如何做到这一点，因为我刚刚开始使用Python。

想象一下，我有一个30GB的tarfile，其中包含1kb到10GB的文件......

这是我开始编写的解决方案：

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()

此代码远未完成。我不敢在一个巨大的30GB tar存档上运行它，因为在某一点上，检查将是10 + GB的对象（如果我在tar存档中有这么大的文件）

加成：我试过手动破坏zero.tar.gz（十六进制编辑器 - 编辑几个字节midfile）。第一个除了没有捕获IOError ...这是输出：

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

我试过手动破坏zero.tar.gz（十六进制编辑器 - 编辑几个字节 midfile）。第一个除了没有捕获IOError ...

如果你看一下回溯，当你拨打tardude.getmembers()时，你会看到它被抛出，所以你需要像......一样......

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()

至于原来的问题，你几乎就在那里。您只需要使用类似......

的内容读取check对象中的数据

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()

...这应该确保你一次不会使用超过BLOCK_SIZE个字节的内存。

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。尝试只捕获您实际打算处理的异常，例如......

try:
    do_something()
except IOError:
    do_something_else()

...否则你会发现检测代码中的错误更加困难。

Answer 2

PreferenceFragmentCompat答案只是略微改进，使事情变得更加惯用（尽管我删除了一些错误检查以使机制更加明显）：

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while 1:（有时被认为是次要代码气味）和if not data:检查。另请注意，使用with会将此限制为Python 2.7 +

Answer 3

您可以使用subprocess模块在文件中调用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不为0，则有些不对劲。但是，您可能想检查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)

检查Python中的tarfile完整性

3 个答案: