DAOS-19018 kv: fix reference leak in daos_kv2objhandle()

[knard38]

Description

kv_hdl2ptr() increments the dc_kv reference count via daos_hhash_link_lookup(). daos_kv2objhandle() was returning without calling kv_decref(), leaking one reference per call. Add the missing kv_decref() after copying the object handle.

Also add a regression test in the DAOS_KV_API suite that calls daos_kv2objhandle() with both invalid and valid handles, exercises the function repeatedly to surface any leak under ASAN/LeakSanitizer, and verifies the KV handle can be cleanly closed afterwards.

Dependency

This PR depends on PR #18423 (DAOS-19017: fix d_aligned_alloc size constraint violation). Without that fix, ASAN aborts immediately on the misaligned allocation, which prevents it from reaching the leak-detection phase. PR #18423 must be merged first for the ASAN/LeakSanitizer validation below to be reproducible.

Validation

Both runs used daos_test -K with filter kv_obj_handle, built with SANITIZERS=address.

Without the fix — LeakSanitizer detects the dc_kv leak:

==2573095==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 80 byte(s) in 1 object(s) allocated from:
    #0 in calloc
    #1 in d_calloc src/gurt/misc.c:126
    #2 in kv_alloc src/client/kv/dc_kv.c:60
    #3 in open_handle_cb src/client/kv/dc_kv.c:130
    ...
    #16 in kv_obj_handle src/tests/suite/daos_kv.c:373

SUMMARY: AddressSanitizer: 17019 byte(s) leaked in 54 allocation(s).

With the fix — no kv_alloc leak, 80 bytes and 1 allocation fewer:

SUMMARY: AddressSanitizer: 16939 byte(s) leaked in 53 allocation(s).

The remaining leaks in both runs (openmpi, isal, hwloc) are pre-existing third-party leaks unrelated to this change.

Steps for the author:

• Commit message follows the guidelines.
• Appropriate Features or Test-tag pragmas were used.
• Appropriate Functional Test Stages were run.
• At least two positive code reviews including at least one code owner from each category referenced in the PR.
• Testing is complete. If necessary, forced-landing label added and a reason added in a comment.

After all prior steps are complete:

• Gatekeeper requested (daos-gatekeeper added as a reviewer).

[github-actions]

Ticket title is 'reference leak in daos_kv2objhandle()'
Status is 'In Review'
Labels: 'asan'
https://daosio.atlassian.net/browse/DAOS-19018

[daosbuild3]

Test stage NLT completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-18412/1/testReport/

[daosbuild3]

Test stage Functional on EL 9 completed with status FAILURE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net//job/daos-stack/job/daos/view/change-requests/job/PR-18412/1/execution/node/983/log

The base branch was changed.

[daosbuild3]

Test stage Functional on EL 9 completed with status FAILURE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net//job/daos-stack/job/daos/view/change-requests/job/PR-18412/4/execution/node/1040/log

[knard38]

Apologies for the force push, but I was not able to properly merge as the initial target branch was landed on master.
The code of the PR was not updated, just cleaning of the git tree of the PR.

[wangshilong]

Good catch