[Impala-ASF-CR] IMPALA-9626: Use Python from the toolchain for Impala

["Laszlo Gaal (Code Review)" <ge...@cloudera.org>]

Hello David Knupp, Tim Armstrong, Joe McDonnell, Impala Public Jenkins, 

I'd like you to reexamine a change. Please visit

    http://gerrit.cloudera.org:8080/15624

to look at the new patch set (#14).

Change subject: IMPALA-9626: Use Python from the toolchain for Impala
......................................................................

IMPALA-9626: Use Python from the toolchain for Impala

Historically Impala used the Python2 version that was available on
the hosting platform, as long as that version was at least v2.6.
This caused constant headache as all Python syntax had to be kept
compatible with Python 2.6 (for Centos 6). It also caused a recent problem
on Centos 8: here the system Python version was compiled with the
system's GCC version (v8.3), which was much more recent than the Impala
standard compiler version (GCC 4.9.2). When the Impala virtualenv was
built, the system Python version supplied C compiler switches for models
containing native code that were unknown for the Impala version of GCC,
thus breaking virtualenv installation.

This patch changes the Impala virtualenv to always use the Python2
version from the toolchain, which is built with the toolchain compiler.

This ensures that
- Impala always has a known Python 2.7 version for all its scripts,
- virtualenv modules based on native code will always be installable, as
  the Python environment and the modules are built with the same compiler
  version.

Additional changes:
- Add an auto-use fixture to conftest.py to check that the tests are
  being run with Python 2.7.x
- Make bootstrap_toolchain.py independent from the Impala virtualenv:
  remove the dependency on the "sh" library

Tests:
- Passed core-mode tests on CentOS 7.4
- Passed core-mode tests in Docker-based mode for centos:7
  and ubuntu:16.04

Most content in this patch was developed but not published earlier
by Tim Armstrong.

Change-Id: Ic7b40cef89cfb3b467b61b2d54a94e708642882b
---
M bin/bootstrap_toolchain.py
M bin/impala-config.sh
M bin/set-pythonpath.sh
M infra/python/bootstrap_virtualenv.py
M tests/conftest.py
5 files changed, 67 insertions(+), 30 deletions(-)


  git pull ssh://gerrit.cloudera.org:29418/Impala-ASF refs/changes/24/15624/14
-- 
To view, visit http://gerrit.cloudera.org:8080/15624
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Gerrit-Project: Impala-ASF
Gerrit-Branch: master
Gerrit-MessageType: newpatchset
Gerrit-Change-Id: Ic7b40cef89cfb3b467b61b2d54a94e708642882b
Gerrit-Change-Number: 15624
Gerrit-PatchSet: 14
Gerrit-Owner: Laszlo Gaal <la...@cloudera.com>
Gerrit-Reviewer: David Knupp <dk...@cloudera.com>
Gerrit-Reviewer: Impala Public Jenkins <im...@cloudera.com>
Gerrit-Reviewer: Joe McDonnell <jo...@cloudera.com>
Gerrit-Reviewer: Laszlo Gaal <la...@cloudera.com>
Gerrit-Reviewer: Tim Armstrong <ta...@cloudera.com>