Fix a rare segfault

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.

news/5366.bugfix

@@ -0,0 +1 @@
+Check for file existence and unlink first when clobbering existing files during a wheel install.

src/pip/_internal/wheel.py

@@ -285,6 +285,17 @@ def move_wheel_files(name, req, wheeldir, user=False, home=None, root=None,
                 # uninstalled.
                 ensure_dir(destdir)
 
+                # copyfile (called below) truncates the destination if it
+                # exists and then writes the new contents. This is fine in most
+                # cases, but can cause a segfault if pip has loaded a shared
+                # object (e.g. from pyopenssl through its vendored urllib3)
+                # Since the shared object is mmap'd an attempt to call a
+                # symbol in it will then cause a segfault. Unlinking the file
+                # allows writing of new contents while allowing the process to
+                # continue to use the old copy.
+                if os.path.exists(destfile):
+                    os.unlink(destfile)
+
                 # We use copyfile (not move, copy, or copy2) to be extra sure
                 # that we are not moving directories over (copyfile fails for
                 # directories) as well as to ensure that we are not copying