Return root when bank not found

[garious]

Problem

The bank at a given CommitmentLevel may not be in BankForks. See #11078

Summary of Changes

• Band-aid: Return the root bank instead of panicking
• Add a comment explaining how to remove the band-aid

[carllin]

If we wanted to be extra careful, we could also disable the Commitment::Max until the "cluster confirmed root" queries are actually supported. If not, for that commitment level at least (just b/c of the implications) it feels safer to return a failure rather than a local root, as critical clients that are expecting Commitment::Max getting a local root are getting confidence than they asked for.

Local root and cluster confirmed root should only diverge if there's some critical bug/attack that causes a few validators to commit down the wrong path. It hasn't happened to date, but just being extra careful here.

Maybe @mvines can illuminate if there are further implications for restart any current partners using this call?

[mvines]

Maybe @mvines can illuminate if there are further implications for restart any current partners using this call?

Turning "max" into "root" for a time will be transparent to downstream users AFAICT. We made the distinction unilaterally to improve the safely of the results that "max" returns, not due to any partner request

[carllin]

Maybe @mvines can illuminate if there are further implications for restart any current partners using this call?

Turning "max" into "root" for a time will be transparent to downstream users AFAICT. We made the distinction unilaterally to improve the safely of the results that "max" returns, not due to any partner request

Awesome, thanks!

[codecov]

Codecov Report

Merging #11188 into master will increase coverage by 0.0%.
The diff coverage is 16.6%.

@@           Coverage Diff           @@
##           master   #11188   +/-   ##
=======================================
  Coverage    82.1%    82.1%           
=======================================
  Files         318      318           
  Lines       74002    74007    +5     
=======================================
+ Hits        60804    60816   +12     
+ Misses      13198    13191    -7     

Pull request has been modified.