server, sessionctx: workaround var initialization

[morgo]

What problem does this PR solve?

This provides a workaround for #9382

I suggest a longer-term fix here: #9381

What is changed and how it works?

This makes various sysvars work:

• port
• datadir
• socket

Port was previously added in #9365 , but it turns out this did not work correctly since it retrieved the 'default' port prior to parsing the config file. I have changed the value to be "4000" in variables so it becomes less confusing since this doesn't work.

Check List

Tests

• Manual test (add detailed scripts or steps below)

The test-suite contains the server variables from the default config only, and so it is difficult to assert for these changes.

Code changes

• Has exported variable/fields change

Side effects

• Increased code complexity (workaround)

Related changes

• Need to update the documentation
• Need to be included in the release note

[morgo]

/run-all-tests

[codecov-io]

Codecov Report

Merging #9383 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9383      +/-   ##
==========================================
+ Coverage   67.16%   67.17%   +0.01%     
==========================================
  Files         373      373              
  Lines       78269    78269              
==========================================
+ Hits        52567    52580      +13     
+ Misses      20995    20986       -9     
+ Partials     4707     4703       -4

Impacted Files	Coverage Δ
sessionctx/variable/sysvar.go	100% <ø> (ø)	⬆️
store/tikv/gcworker/gc_worker.go	41.14% <0%> (-0.43%)	⬇️
executor/join.go	79.42% <0%> (+0.52%)	⬆️
util/filesort/filesort.go	76.48% <0%> (+1.25%)	⬆️
infoschema/infoschema.go	77.63% <0%> (+1.31%)	⬆️
ddl/delete_range.go	77.77% <0%> (+2.64%)	⬆️
util/systimemon/systime_mon.go	100% <0%> (+20%)	⬆️

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[tiancaiamao]

LGTM

[morgo]

@jackysp thanks for the suggestion, I've moved it. PTAL again.

[jackysp]

LGTM

[jackysp]

/run-all-tests