Allow dual channel full sync in plain failover

[enjoy-binbin]

PSYNC_FULLRESYNC_DUAL_CHANNEL is also a full sync, as the comment says,
we need to allow it. I also tested it by modifying some code locally.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.22%. Comparing base (6c1bb73) to head (130d028).
⚠️ Report is 1 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #2659      +/-   ##
============================================
+ Coverage     72.10%   72.22%   +0.12%     
============================================
  Files           127      127              
  Lines         70935    70936       +1     
============================================
+ Hits          51146    51235      +89     
+ Misses        19789    19701      -88     

Files with missing lines	Coverage Δ
src/replication.c	85.83% <100.00%> (-0.22%)	⬇️

... and 12 files with indirect coverage changes

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[zuiderkwast]

Which scenario is this exactly? If you could test it locally, I guess a TCL test would also be possible to write...

Who is more suited to review dual-channel replication stuff? Not me...

[enjoy-binbin]

Which scenario is this exactly? If you could test it locally, I guess a TCL test would also be possible to write...

it is another full sync, just in this code, PSYNC_FULLRESYNC with dual-channel enabled scenario. We did not add the test before, when i test it locally, i use some debug code (like a debug config just refuse the psync). It is hard to reproduce in a real world i think (since the failover and psync are happen in the same time).

    if (server.failover_state == FAILOVER_IN_PROGRESS) {
        if (psync_result == PSYNC_CONTINUE || psync_result == PSYNC_FULLRESYNC) {
            clearFailoverState();
        } else {
            abortFailover("Failover target rejected psync request");
            return;
        }
    }

[hpatro]

Looks good to me.