[bhuvan-somisetty]

Description

In apisix/patch.lua, monkey-patched ngx.req.set_body_data only cleared _request_body_table and _request_body_type, leaving api_ctx.req_post_args populated with stale cached form arguments from the pre-rewrite body. In addition, invalidation of post_arg.* variables in var._cache was nested inside if api_ctx and api_ctx._request_body_table then, which skipped variable cache invalidation whenever _request_body_table was not yet populated or was nil.

This PR updates ngx.req.set_body_data:

1. Always clears api_ctx.req_post_args so that subsequent calls to core.request.get_post_args(ctx) re-parse arguments from the updated body.
2. Unconditionally clears post_arg.* variables in api_ctx.var._cache whenever ngx.req.set_body_data is invoked.
3. Adds comprehensive test coverage in t/core/request.t (TEST 20).

Which issue(s) this PR fixes:

Fixes #14042

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[hanzhenfang]

Thanks for the PR, @bhuvan-somisetty!

Could we also invalidate the legacy post_arg_ variables here? core.ctx caches post_arg_foo in the same var._cache, but the invalidation loop only clears the post_arg. prefix.

After reading ctx.var.post_arg_foo and then calling ngx.req.set_body_data("foo=updated"), the cached variable can still return the original value even though req_post_args has been cleared. This appears to be another path of the existing cache-invalidation issue, rather than a regression introduced by this PR.

Please cover both prefixes and add a regression case that reads post_arg_foo before and after the rewrite. This observation is based on source review of f408d4a6; I have not run this case locally.

[bhuvan-somisetty]

Good point! I've updated the invalidation logic in apisix/patch.lua to clear both post_arg. and post_arg_ prefixes from var._cache, and extended the regression test in t/core/request.t to verify post_arg_foo before and after the rewrite.