v1.16: removes outdated check for merkle shreds (backport of #33088)

[mergify]

This is an automatic backport of pull request #33088 done by Mergify.

Motivation for v1.16 Backport

ShredFetchStage implements some basic verification in modify_packets. Some of this verification currently relies on having a (root) bank in order to check feature status as well as to check number of slots in an epoch (number of slots in an epoch could be calculated and then Arc<Bank> could be dropped). The root bank is fetched here:

solana/core/src/shred_fetch_stage.rs

Lines 57 to 59 in b13589b

	let (mut root_bank, mut last_slot) = {
	let bank_forks_r = bank_forks.read().unwrap();
	(bank_forks_r.root_bank(), bank_forks_r.highest_slot())

At the time that the bank is fetched, the node just unpacked its' snapshot. So, this initial root bank corresponds to the load-snapshot-slot. Later in the function, we currently use a blocking method to pull shred out of the crossbeam receiver:

solana/core/src/shred_fetch_stage.rs

Line 65 in b13589b

for mut packet_batch in recvr {

Since we're not getting any packets from TVU forwards, execution is blocked waiting for a shred to show up, and the following code that updates the root bank is never executed:

solana/core/src/shred_fetch_stage.rs

Lines 69 to 70 in b13589b

	let bank_forks_r = bank_forks.read().unwrap();
	root_bank = bank_forks_r.root_bank();

As a result, the initial root bank at startup (ie the snapshot slot) is held for the duration of the process. Holding this bank is a waste of resources and could interfere with background cleanup that AccountsDB would otherwise be performing.

In #33078, I identified a very similar problem on master with the QUIC receiver. Instead of adjusting the impl, we realized that we could cleanup the code that needed the root bank. Hence, we shipped #33088 to remove outdated feature status check, and then #33105 to remove holding the root bank altogether.

Additional v1.16 Backport

In order to fully fix the bug of the process holding the initial root bank for process lifetime, we will also need a backport of #33105. I plan on pushing that BP after this one lands. I'll also add that I recently performed some separate cleanup on that function in #33066. If we want to optimize for 1-to-1 mapping between master and v1.16 commits, backporting that one first will make #33105 go in cleaner.

Same bug exists on v1.14

Lastly, this same problem exists on v1.14. At the time of writing this, we're on the cusp of upgrading mnb to v1.16 pending some debugging efforts. This isn't a critical bug (ie it will not crash nodes) so I think we can probably skip the v1.14 backport. However, this bank being present will obfuscate other potential debugging efforts such as mem profiling.

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Cherry-pick of 1431275 has failed:

On branch mergify/bp/v1.16/pr-33088
Your branch is up to date with 'origin/v1.16'.

You are currently cherry-picking commit 1431275328.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   ledger/src/shred.rs
	modified:   sdk/src/feature_set.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   core/src/cluster_nodes.rs
	both modified:   core/src/shred_fetch_stage.rs

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[codecov]

Codecov Report

Merging #33136 (208d80b) into v1.16 (0c211bd) will increase coverage by 0.0%.
The diff coverage is 66.6%.

@@           Coverage Diff           @@
##            v1.16   #33136   +/-   ##
=======================================
  Coverage    81.9%    81.9%           
=======================================
  Files         762      762           
  Lines      207829   207798   -31     
=======================================
+ Hits       170370   170393   +23     
+ Misses      37459    37405   -54     

[steviez]

Double checked that this branch was feature "compatible" (ie same feature set id) with the last couple v1.16 releases:

$ git status
On branch mergify/bp/v1.16/pr-33088
Your branch is up to date with 'origin/mergify/bp/v1.16/pr-33088'.
$ cargo run --release -- feature status
    Finished release [optimized] target(s) in 0.54s
     Running `/home/sol/src/solana/target/release/solana feature status`
...
Tool Feature Set: 3949673676
Software Version                                                        Feature Set   Stake     RPC
1.17.0                                                                   4115909905   0.02%   3.14%  
1.17.0                                                                    461617547   0.00%   0.31%  
1.16.13, 1.16.12, 1.16.11                                                3949673676   9.24%   6.60%  <-- me
...