Fix a rare segfault

[reaperhulk]

If pip10 is installed and pyopenssl+cryptography are present in the
environment pip's vendored urllib3 will automatically activate its
pyopenssl adapter. This is by design so users can potentially opt-in to
more up-to-date TLS versions than what are provided by their copy of
Python. However, if a user has these packages & subsequently runs
pip install --ignore-installed cryptography then a segfault will occur
due to overwriting an mmap'd shared object.

Fixes #5366

[njsmith]

Another case that this PR fixes, maybe, I think: conda uses hardlinks to share files between different environments, so if you use pip inside a conda environment and pip rewrites a file in place then it will corrupt other unrelated conda environments. Adding the unlink here fixes that too.

(I'm basing this on a stray comment made by either @msarahan or @jjhelmus today.)

[jjhelmus]

Thanks for this fix @reaperhulk. We were applying a similar patch to avoid breaking hard links in the version of pip shipped with Anaconda. Glad to hear that we can drop this in the next release.

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.

[chrahunt]

Also applies to hiredis for pip 9.0.1 as observed in #6233.