blockstore: Parent must be full to set previous_blockhash

[steviez]

Problem

The Blockstore:: do_get_complete_block_with_entries() method is in the call stack for an RPC getBlock call. One of the fields in the result is the blockhash of the parent block. The current logic grabs the last entry from the parent block and uses this. However, the parent block could be non-full which could result in an invalid value being set for previous_blockhash.

In normal operation, this is quite unlikely as rooting the current block implies that we'll have rooted the parent block. This bug was discovered debugging a devnet bigtable issue that stems from weirdness around deactivating features (see #7287 for more on that).

Summary of Changes

• The first commit reworks the existing unit test to more exhaustively test scenarios
  • Additionally, there is an assert that is commented out that will fail with the tip of master
• The second commit adjusts do_get_complete_block_with_entries() to ensure the previous blockhash is only populated when the parent is full
  • The commented out assert is uncommented and the full unit test passes

I plan on squashing commits prior to merge to clean up commit history + add details from above message into commit

[steviez]

A couple notes on this removal:

• Directly modifying the meta muddies the water on what insertion logic does
• Insertion logic inserts an empty slot meta for the empty slot - 1 as part of chaining so this is redundant / unnecessary
• If this was modifying a meta for a non-empty slot, it'd be creating a column inconsistency that isn't reachable with normal logic

[steviez]

If reviewers want to verify, they can:

• Pull the branch down
• Revert the second commit
• Uncomment this assert out
• Run cargo test test_get_rooted_block to see this test fail
• Pull down the tip of the branch again (to get both commits)
• Run cargo test test_get_rooted_block again to see the test succeed since the second commit adjusts logic in the actual prod code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.59259% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.2%. Comparing base (039baa1) to head (9600df3).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #12012   +/-   ##
=======================================
  Coverage    83.2%    83.2%           
=======================================
  Files         860      860           
  Lines      321570   321579    +9     
=======================================
+ Hits       267774   267784   +10     
+ Misses      53796    53795    -1     

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