CentOS 7.0.1406上yum install提示exiting on user cancel错误的解决方法

问题

公司一台老CentOS 7机器上要安装某个软件，yum install后，提示安装失败，提示Exiting on user cancel。

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File "/usr/libexec/urlgrabber-ext-down", line 75, in <module>
main()
File "/usr/libexec/urlgrabber-ext-down", line 61, in main
fo = PyCurlFileObject(opts.url, opts.filename, opts)
File "/usr/lib/python2.7/site-packages/urlgrabber/grabber.py", line 1258, in __init__
self._do_open()
File "/usr/lib/python2.7/site-packages/urlgrabber/grabber.py", line 1589, in _do_open
self._do_grab()
File "/usr/lib/python2.7/site-packages/urlgrabber/grabber.py", line 1723, in _do_grab
self._do_perform()
File "/usr/lib/python2.7/site-packages/urlgrabber/grabber.py", line 1517, in _do_perform
raise KeyboardInterrupt
KeyboardInterrupt
Exiting on user cancel

试了多次都是这样，一脸诧异。
查看了下CentOS版本是CentOS Linux release 7.0.1406 (Core), 网上搜一通，发现是CentOS 7.0.1406中yum的一个Bug。摘录解决方法如下。

解决方法

CentOS社区论坛上有相关描述，参见SSH and Yum don’t work in clean CentOS 7

后续版本中已经解决了。但在CentOS 7.0.1406中的解决方法，需要修改/usr/lib/python2.7/site-packages/urlgrabber/grabber.py文件的1510行。
将

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elif errcode in (42, 55, 56):

修改为

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elif errcode == 42:

修改后再yum install就解决了。

有趣的一点是，1510行下面的注释。从中就可以看出，即使是操作系统的基础包，很多时候也是包含了妥协的成分。

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elif errcode in (42, 55, 56):
    # this is probably wrong but ultimately this is what happens
    # we have a legit http code and a pycurl 'writer failed' code
    # which almost always means something aborted it from outside
    # since we cannot know what it is -I'm banking on it being
    # a ctrl-c. XXXX - if there's a way of going back two raises to
    # figure out what aborted the pycurl process FIXME
    raise KeyboardInterrupt

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