[watermarkstat] Add unit tests for watermarkstat show commands

[neethajohn]

- What I did
Added unit tests for watermarkstat show commands

- How to verify it

platform linux2 -- Python 2.7.16, pytest-3.10.1, py-1.7.0, pluggy-0.8.0 -- /usr/bin/python2
cachedir: .pytest_cache
rootdir: /sonic/src/sonic-utilities/tests, inifile: pytest.ini
plugins: cov-2.6.0
collected 10 items                                                                                                                                                                                                                      

tests/watermarkstat_test.py::TestWatermarkstat::test_show_pg_shared_wm PASSED                                                                                                                                                     [ 10%]
tests/watermarkstat_test.py::TestWatermarkstat::test_show_pg_headroom_wm PASSED                                                                                                                                                   [ 20%]
tests/watermarkstat_test.py::TestWatermarkstat::test_show_queue_unicast_wm PASSED                                                                                                                                                 [ 30%]
tests/watermarkstat_test.py::TestWatermarkstat::test_show_queue_multicast_wm PASSED                                                                                                                                               [ 40%]
tests/watermarkstat_test.py::TestWatermarkstat::test_show_buffer_pool_wm PASSED                                                                                                                                                   [ 50%]
tests/watermarkstat_test.py::TestWatermarkstat::test_show_pg_shared_peristent_wm PASSED                                                                                                                                           [ 60%]
tests/watermarkstat_test.py::TestWatermarkstat::test_show_pg_headroom_persistent_wm PASSED                                                                                                                                        [ 70%]
tests/watermarkstat_test.py::TestWatermarkstat::test_show_queue_unicast_persistent_wm PASSED                                                                                                                                      [ 80%]
tests/watermarkstat_test.py::TestWatermarkstat::test_show_queue_multicast_persistent_wm PASSED                                                                                                                                    [ 90%]
tests/watermarkstat_test.py::TestWatermarkstat::test_show_buffer_pool_persistent_wm PASSED                                                                                                                                        [100%]

======================================================================================================= 10 passed in 1.76 seconds =======================================================================================================

[lgtm-com]

This pull request introduces 1 alert when merging 0bccf33 into e1244a5 - view on LGTM.com

new alerts:

• 1 for Unused import

[lgtm-com]

This pull request introduces 1 alert when merging 7ec4c19 into e1244a5 - view on LGTM.com

new alerts:

• 1 for Unused import

[jleveque]

Why change this?

[neethajohn]

Ran into this error in PR tests after #1151 was merged,
/usr/bin/env: 'python': No such file or directory
https://sonic-jenkins.westus2.cloudapp.azure.com/job/common/job/sonic-utilities-build-pr/2537/console

Had to change the path to fix it

[jleveque]

That's strange. #1151 shouldn't have had an impact on this. Also, all other executable scripts in the repo still use #!/usr/bin/env python. I feel like this change shouldn't be necessary.

[neethajohn]

Not sure how to fix the build failure. seems to be failing every time utilities wheel is built. But if I retain the docker and run the test again using 'python setup.py test', I don't see the issue

[jleveque]

It sounds like it may be a PATH difference with the sonic-utilities build job in sonic-build-tools. But I'm surprised no other tests are failing like this. You're executing the watermarkstat script explicitly in the tests. Are other unit tests doing this? If so, I would expect them to fail, also.

[neethajohn]

I am not executing the command directly. I am using 'show' commands to invoke that script. The same format is used in many of the utilities tests which are invoking modules under 'scripts' path and some of them also use "/usr/bin/env python". I fail to understand what is special about this particular script

[lguohan]

what is pending here? can we merge this?

[jleveque]

@lguohan: I find it odd that Neetha needed to modify the shebang in this file. I would like to understand this better, as it shouldn't be necessary. If you'd like, we can merge this as-is, and @neethajohn can continue to investigate offline.

[neethajohn]

@lguohan, can I go ahead with the merge?

[jleveque]

I'm OK with merging this, but I'd like to understand further what changed to require this shebang to change.

NJ> I will investigate offline

[abdosi]

@neethajohn Please create PR for 201911

[jleveque]

@neethajohn, @lguohan: I believe I found the root cause of the shebang issue. I have opened a PR to fix here: #1233