Use current root slot to disambiguate pruning of old programs

[pgarg66]

Problem

ForkGraph returns BlockRelation::Unknown for programs deployed before current root slot. That causes pruning of such programs from the cache.

Summary of Changes

The bank_forks retains slot relationship history till the current root. The pruning has the following logic.

1. BankForks::set_root() gets called when a new root slot is identified.
2. set_root() calls cache's prune() method before actually storing the new root slot (i.e. cache's view of ForkGraph still has the old root slot).
3. prune() calls ForkGraph::relationship() trait method on the BankForks object with new root, and program's deployment slot.
   • If the deployment slot is descendant of the new root, it's retained in the cache.
   • If the deployment slot is an ancestor of the new root, it's retained in the cache (and first ancestor found flag is set).
   • Otherwise, it's assumed that the cache entry is on a fork that doesn't connect to the new root, and the entry is pruned.

This logic doesn't work if the deployment slot is older than the current root. Ideally it should be an ancestor of the new root. But the BankForks doesn't have enough history to calculate this relationship, and returns Unknown.

3. prune() calls ForkGraph::relationship() trait method on the BankForks object with new root, and program's deployment slot.
   • If the deployment slot is descendant of the new root, it's retained in the cache.
   • If the deployment slot is an ancestor of the new root,
     or, the deployment slot is older than the previous root,
     it's retained in the cache (and first ancestor found flag is set).
   • Otherwise, it's assumed that the cache entry is on a fork that doesn't connect to the new root, and the entry is pruned.

Fixes #

[codecov]

Codecov Report

Merging #31548 (eae13f0) into master (b20024c) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

@@            Coverage Diff            @@
##           master   #31548     +/-   ##
=========================================
- Coverage    81.3%    81.3%   -0.1%     
=========================================
  Files         734      734             
  Lines      205284   205287      +3     
=========================================
- Hits       167077   167062     -15     
- Misses      38207    38225     +18     

[pgarg66]

This PR is causing consensus issues when applied on top of #31462
But, the issue is valid. So we need a solution to this.

[pgarg66]

This PR is causing consensus issues when applied on top of #31462
But, the issue is valid. So we need a solution to this.