Introduce KEEP_OS_PIP_PKG flag

[Noel McLoughlin]

The tools/install_pip.sh script uninstalls the system pip package (i.e. python-pip).

Could this be optional?

Removing python-pip breaks IT automation where devstack is part of a bigger stack which depend on python modules like docker, compose, etc.

[Dr. Jens Harbott (j-harbott)]

Devstack isn't meant to be part of a bigger stack, if you need that, please have a look at some other deployment project.

[Noel McLoughlin (noelmcloughlin)]

Here are two valid reasons-
- The OS provided pip package is supported by OS vendor but PyPi is not.
- Devstack deletes system python pip but does not migrate system python modules - thats data loss.

[Noel McLoughlin (noelmcloughlin)]

I have changed state to new because its not "spam".

[Noel McLoughlin (noelmcloughlin)]

Right now it sucks devstack has hard dependency on OS vendor-unsupported pip2.7 binary.
Running these commands on production systems is sickening from quality/security perspective.

    $ wget https://bootstrap.pypa.io/get-pip.py
    $ sudo python2.7 get-pip.py

----
2019-08-05 18:26:44.779 | [ERROR] /opt/stack/inc/python:37 Unable to find pip2.7; cannot continue
2019-08-05 18:26:45.787 | ++^[[38;5;242mstack.sh:main:766 ^[(B^[[m err_trap
2019-08-05 18:26:45.792 | ++^[[38;5;242mstack.sh:err_trap:562 ^[(B^[[m local r=1
2019-08-05 18:26:45.797 | stack.sh failed: full log in /tmp/devstack/stack.sh.log.2019-08-05-112633
2019-08-05 18:26:45.798 | Error on exit
2019-08-05 18:26:45.871 | Traceback (most recent call last):
2019-08-05 18:26:45.871 | File "/opt/stack/tools/worlddump.py", line 253, in <module>
2019-08-05 18:26:45.871 | sys.exit(main())
2019-08-05 18:26:45.871 | File "/opt/stack/tools/worlddump.py", line 237, in main
2019-08-05 18:26:45.871 | sys.stdout = os.fdopen(sys.stdout.fileno(), 'w', 0)
2019-08-05 18:26:45.871 | File "/usr/lib/python3.6/os.py", line 1017, in fdopen
2019-08-05 18:26:45.871 | return io.open(fd, *args, **kwargs)
2019-08-05 18:26:45.871 | ValueError: can't have unbuffered text I/O
2019-08-05 18:26:45.871 | Exception ignored in: <_io.TextIOWrapper name='<stdout>' mode='w' encoding='UTF-8'>
2019-08-05 18:26:45.871 | OSError: [Errno 9] Bad file descriptor
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This ticket can be closed if devstack project believes this is okay. I don't!

[Noel McLoughlin (noelmcloughlin)]

py-pip must indeed be hard dependency. Why not python-pip and python3-pip?
---
2019-08-05 19:02:23.140 | Found existing installation: PyYAML 3.12
2019-08-05 19:02:23.149 | ERROR: Cannot uninstall 'PyYAML'. It is a distutils installed project and thus we cannot accurately determine which files belong to it which would lead to only a partial uninstall.
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