Correctly set log_dir default in vtcombo

[ajm188]

Description

I accidentally made it so the old default of $VTDATAROOT/tmp was never set for vtcombo.

Now we manually provide this default by first checking that the flag exists, and that the user did not explicitly set a value for it.

Demo

➜  vitess git:(andrew/vtcombo/log-dir-default) ✗ ./bin/vtcombo --port 33574 --bind-address localhost --log_dir /tmp/vtdataroot/vttest445905202/logs --alsologtostderr --grpc_port 33575 --db_charset utf8mb4 --db_app_user vt_dba 2>/dev/null
log_dir=/tmp/vtdataroot/vttest445905202/logs%  

➜  vitess git:(andrew/vtcombo/log-dir-default) ✗ ./bin/vtcombo --port 33574 --bind-address localhost --alsologtostderr --grpc_port 33575 --db_charset utf8mb4 --db_app_user vt_dba 2>/dev/null 
log_dir=$VTDATAROOT/tmp% 

Related Issue(s)

#15120 (comment)

Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c189cf4) 70.62% compared to head (e81fc92) 70.62%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15153      +/-   ##
==========================================
- Coverage   70.62%   70.62%   -0.01%     
==========================================
  Files        1377     1377              
  Lines      182794   182794              
==========================================
- Hits       129102   129098       -4     
- Misses      53692    53696       +4     

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[mattlord]

Does this fix #15120 ? Seems like it would. If so, we should mark it as Fixing it in the description.

[ajm188]

Does this fix #15120 ? Seems like it would. If so, we should mark it as Fixing it in the description.

I thought so at first, but I think it only resolves @frouioui's one comment that I linked

[frouioui]

@mattlord @ajm188, in #15120 i found that we are setting the flag correctly when starting vtcombo, in the logs I can see:

/vt/bin/vtcombo --port 33574 --bind-address localhost --log_dir /vt/vtdataroot/vttest445905202/logs ..

Meaning we wouldn't reach that default, but I might be wrong

[frouioui]

I will run a quick test locally with Docker though, we never know

[frouioui]

I can confirm this does not fix the issue:

vitess@787d46414584:/$ ls /vt/vtdataroot/vttest3208539665/logs/
vitess@787d46414584:/$