Detect OS X sed and fail client generation script

[roycaihw]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

In the client generation script, fail early if OS X sed is being used and suggest using GNU sed instead. Both the python repo script and the gen repo script use sed in format that OS X sed may not support: https://stackoverflow.com/a/4247319.

Address issues like:

--- Patching generated code...
sed: 1: "/Users/scott/Projects/k ...": bad flag in substitute command: 'l'

Does this PR introduce a user-facing change?

NONE

/cc @scottilee @yliaog

[k8s-ci-robot]

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In response to this:

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

In the client generation script, fail early if OS X sed is being used and suggest using GNU sed instead. Both the python repo script and the gen repo script use sed in format that OS X sed may not support: https://stackoverflow.com/a/4247319.

Does this PR introduce a user-facing change?

NONE

/cc @scottilee @yliaog

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: roycaihw

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[roycaihw]

For more context, here are my local test results:

OS X sed doesn't work with -i and -i''

$ sw_vers
ProductName:	macOS
ProductVersion:	11.2.3
BuildVersion:	20D91
$ /usr/bin/sed -i 's/a/b/g' test
sed: 1: "test": undefined label 'est'
$ /usr/bin/sed -i'' 's/a/b/g' test
sed: 1: "test": undefined label 'est'

GNU sed doesn't work with -i ''

$ sed --version
sed (GNU sed) 4.7
$ sed -i '' 's/a/b/g' test
sed: can't read s/a/b/g: No such file or directory

[yliaog]

/lgtm