fix: file conversion from 2.8 to 3.4 for post-function plugin

[harshadixit12]

Summary

Referring to https://developer.konghq.com/gateway/upgrade/lts-upgrade-28-34/, if the 2.8 config has both functions (deprecated) and access keys in the plugin config, we will modify logic to replace the legacy key with new key.

New approach:

• Both exist, old field is empty: Choose new
• Both exist, both have values:: Error
• Both exist, new field is empty: Choose old
• Only old exists: Choose old

How 2.8 used to work with functions and access

Based on https://legacy-gateway-2-8--kongdocs.netlify.app/hub/kong-inc/post-function/how-to/phases/ - the fields functions and access refer to the different phases in which we can configure the plugin to execute.
In kong 3.0, functions was removed, and access phase was to be used instead.

Even in 2.8 - functions and access are treated as mutually exclusive - sync fails if both are present in config and are non-empty.

Fixes #1933

[codecov-commenter]

Codecov Report

❌ Patch coverage is 27.47253% with 66 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.72%. Comparing base (1e62866) to head (f743756).
⚠️ Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
convert/plugin_updates.go	9.09%	39 Missing and 1 partial ⚠️
convert/plugin_updates_310.go	34.48%	11 Missing and 8 partials ⚠️
convert/convert.go	30.00%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1974      +/-   ##
==========================================
- Coverage   32.88%   32.72%   -0.16%     
==========================================
  Files          77       77              
  Lines        6988     7046      +58     
==========================================
+ Hits         2298     2306       +8     
- Misses       4506     4545      +39     
- Partials      184      195      +11     

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[Prashansa-K]

This config should have resulted in an error, right?
We have set errorExpected: false and the tests pass.

[harshadixit12]

Yes - I wanted to check with you about this - during conversion of plugin config as of now in other functions, the errors are printed to console, but execution is not halted - https://github.com/Kong/deck/blob/main/convert/plugin_updates.go#L243
Shall I modify the function signature and return error instead?

This is what the output looks like

> go run main.go file convert --from 2.8 --to 3.4 --input-file x.yaml -o y.yaml

From the 'x.yaml' config file,
the _format_version field has been migrated from '1.1' to '3.0'.

Error: both legacy configuration field "functions" and new field "access" have values in plugin post-function, please remove the legacy field
Automatically converted legacy configuration field "functions" to the new field "access" in plugin post-function

[Prashansa-K]

We should halt the conversion and show the error. Otherwise, the error output would get mixed with other information printed. It would be hard to spot the error as the file size increases and there's too much info on the screen and a non-zero exit code won't give any feedback to our users to check for the printed warnings..

Shall I modify the function signature and return error instead?

Yes.

[harshadixit12]

Makes sense, will update.