auth: Überbackend code cleanup

[miodvallat]

Short description

This apparently simple PR reworks the behaviour of the handle internal object of UeberBackend to:

• factor the code performing backend selection and lookup call.
• make the current backend pointer a private field to guarantee that the above factored logic is the only thing which decides which backend to use.

It is not supposed to introduce any behaviour change. But it will help make further (possibly breaking) changes in UeberBackend with confidence.

P.S.: do we really need to keep the instances atomic counter? It doesn't seem to be used by anything and its value is not retrievable either.

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 19757600681

Details

• 32 of 32 (100.0%) changed or added relevant lines in 2 files are covered.
• 35 unchanged lines in 9 files lost coverage.
• Overall coverage increased (+0.04%) to 73.146%

---

Files with Coverage Reduction	New Missed Lines	%
pdns/pollmplexer.cc	1	84.97%
modules/godbcbackend/sodbc.cc	2	71.43%
pdns/recursordist/rec-tcpout.cc	2	79.53%
pdns/axfr-retriever.cc	3	66.94%
pdns/recursordist/test-syncres_cc2.cc	3	89.12%
pdns/tsigverifier.cc	3	77.22%
pdns/recursordist/syncres.cc	6	81.28%
pdns/recursordist/test-syncres_cc1.cc	7	90.23%
pdns/packethandler.cc	8	72.44%

Totals
Change from base Build 19741851688:	0.04%
Covered Lines:	128173
Relevant Lines:	164538

---

💛 - Coveralls

[Habbie]

P.S.: do we really need to keep the instances atomic counter? It doesn't seem to be used by anything and its value is not retrievable either.

the only thing using it appears to be a // commented out log message. I think it can go.

[Habbie]

nice cleanup