used cjson.safe instead of pcall.

[moonming]

What this PR does / why we need it:
better performance and readable.
Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

[ElvinEfendi]

@moonming thanks for the PR! As far as I can tell cjson.safe will return error message as well in case of error, can you include that in log messages as well (like we were doing with pcall)?

Also it would be great if you add a new test case to https://github.com/kubernetes/ingress-nginx/blob/master/rootfs/etc/nginx/lua/test/configuration_test.lua to cover the case where JSON payload is not correct - to make sure the error handled properly without blowing up.

better performance

How's it better performance? Because we eliminate an extra function (pcall) call?

[moonming]

@ElvinEfendi added the error message to log, thx.

How's it better performance? Because we eliminate an extra function (pcall) call?

yes, it can eliminate an extra Lua function call.

[ElvinEfendi]

rootfs/etc/nginx/lua/configuration.lua:62:22: (W421) shadowing definition of variable err on line 52

CI is failing because of this.

Once you fix the please squash your commits into one.

[moonming]

fixed and squashed commits into one.

[ElvinEfendi]

/lgtm
/approve

[k8s-ci-robot]

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