Make ShredFetchStage refresh root bank even if no shreds arrive

[steviez]

Problem

#31576 introduced a QUIC receiver for turbine. From the networking perspective, this code is currently dormant and doesn't really do anything given that we aren't broadcasting shreds over QUIC / to QUIC TVU port.

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 QUIC turbine packets, 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 by the solTvuFetchQuic thread that is spawned below for the duration of the process.

solana/core/src/shred_fetch_stage.rs

Lines 224 to 237 in b13589b

	Builder::new()
	.name("solTvuFetchQuic".to_string())
	.spawn(move || {
	Self::modify_packets(
	packet_receiver,
	sender,
	&bank_forks,
	shred_version,
	"shred_fetch_quic",
	PacketFlags::empty(),
	None, // repair_context
	turbine_disabled,
	)
	})

Summary of Changes

Rework the code to timeout on reading from the crossbeam channel so that the root bank is updated even if no shreds are actually showing up.

I have some scaffolding code that tracks creation / drop of new banks.

• Prior to this change, I could see the bank that the load-snapshot-slot bank was never dropped
• With this change, I do see that bank getting dropped

[behzadnouri]

Wondering where this is causing problem, because this is a master-only code.

That aside, the feature activation check can be removed, and we don't need a root-bank anymore: #33088
So I believe we can just remove the root-bank here. Once #33088 is merged I can do that or you do it, whichever you prefer.

[steviez]

Wondering where this is causing problem, because this is a master-only code.

Yep, master only. Was looking to do some profiling and saw this, so tracked this down prior for cleaner results.

I can do that or you do it, whichever you prefer.

Either or. If you have time and want to knock it out real quick, feel free. If you're more preoccupied with other stuff, happy to scoop it up tomorrow too

[steviez]

Closing in lieu of #33088 - no need to rework this function since the root bank even being pulled out will be going away