Free rendered JSON output when resetting server tests

[MikeeI]

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• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies: master at c9b74229d0d9bfec6d2307b66b43c29a7665ad0b

• Issues fixed (if any): None.

• Brief description of code changes (suitable for use as a commit message):
  Release the test-owned rendered JSON string when resetting a server test.

Summary

A persistent server reuses one iperf_test across runs. Full JSON rendering stores a heap copy in test->json_output_string, but reset did not release it before the next result replaced the pointer. This change frees and clears that test-owned string in iperf_reset_test.

Evidence

• iperf_json_finish duplicates the rendered document into json_output_string.
• iperf_reset_test did not release that string before persistent-server reuse.
• On Ubuntu 24.04.4/glibc, a persistent -J server receiving sequential one-second, 32-stream localhost tests retained 628 KiB over warm runs 20–40, or 31.4 KiB/run. A representative rendered document was 32,134 bytes.

Changes

• Free and clear json_output_string at the reset lifecycle boundary.
• Preserve the rendered result through the completed run and leave JSON serialization, emitted output, callbacks, protocol behavior, and the public getter unchanged.

Risks and boundaries

• iperf_reset_test already invalidates test-owned result state for the next run; this applies the same lifetime to the rendered string.
• The correction uses the destructor's existing free-and-NULL pattern and adds no new abstraction or platform-specific behavior.

Verification

• Persistent full JSON candidate: warm RSS growth fell to 40 KiB over runs 20–40 and 16 KiB over runs 40–60, versus 628 KiB over baseline runs 20–40.
• Baseline and candidate one-off documents had identical sorted JSON structure and scalar types after normalizing live values.
• Two --json-stream --json-stream-full-output runs emitted JSON accepted by jq empty.
• make -s check — 5/5 tests passed.
• test_commands.sh 127.0.0.1 — completed with status 0 in an isolated network namespace; IPv6 was unavailable for the IPv4-only target.

I checked the relevant issues, comments, pull requests, discussions, and iperf-dev search results; this pull request is not a duplicate.

Disclosure

Investigated thoroughly with GPT-5.6 (extra high reasoning effort), using Oh My Pi as the agent framework.

This report is not generic or unreviewed AI-generated output. Its claims were checked against the cited evidence, and it includes the relevant detail intended to help maintainers resolve the issue.

If reports like this are not useful to the project, please let me know and I will refrain from submitting similar ones. My intent is to help without wasting maintainer time or energy or discouraging their work.

Thank you for your work.