auth Lua health checks: more responsiveness

[miodvallat]

Short description

The current state of the health check thread is to check for statuses checks to launch, every lua-health-checks-interval seconds.

This design has two issues:

• if the thread runs before its work list is filled, then the checks won't be started until lua-health-checks-interval seconds, which will be noticeable if this value is large (as noticed in Lua health checks usually don't trigger after the first resolve but rather wait for lua-health-checks-interval #16369).
• processing Lua records causing new health checks to be started won't cause them to be noticed and work on until the thread wakes up from its lua-health-checks-interval seconds sleep.

This PR attempts to address this in two commits:

• the checker thread was always being started (if not running already) before adding checks to its work list, creating a race where the thread could run, notice its list of checks is empty, and go to sleep, after which its check job is immediately added.
• because further check requests can occur at any time, let it wake up every second to check for more work to do.

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document, including the AI Policy, and added a "Signed-off-by" to my commits
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[coveralls]

Pull Request Test Coverage Report for Build 19332892815

Details

• 0 of 13 (0.0%) changed or added relevant lines in 1 file are covered.
• 175 unchanged lines in 14 files lost coverage.
• Overall coverage decreased (-0.04%) to 73.029%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pdns/lua-record.cc	0	13	0.0%

Files with Coverage Reduction	New Missed Lines	%
modules/geoipbackend/geoipbackend.cc	1	60.22%
pdns/iputils.hh	1	76.11%
modules/gsqlite3backend/gsqlite3backend.cc	2	94.55%
modules/pipebackend/pipebackend.cc	2	60.79%
pdns/recursordist/sortlist.cc	2	72.94%
pdns/recursordist/test-rec-tcounters_cc.cc	2	95.59%
pdns/dnsdistdist/dnsdist-carbon.cc	3	62.01%
pdns/iputils.cc	3	56.99%
pdns/lua-record.cc	3	0.18%
pdns/dnsdistdist/dnsdist-tcp.cc	4	77.05%

Totals
Change from base Build 19332602989:	-0.04%
Covered Lines:	128003
Relevant Lines:	164594

---

💛 - Coveralls

[Habbie]

approved, but please think about the comment i posted

[miodvallat]

Logic slightly updated to try and make sure the checker thread, unless awaken earlier to handle new requests, sleeps the same time as before and not a full interval (i.e. takes into account the time it spent processing its queue). This yields a bit more deterministic timings, which the Lua tests are relying upon.