Bank's call to account::squash() causes assert on testnet

[pgarg66]

Problem

The fullnode is crashing with the following assert

thread 'solana-replay-stage' panicked at 'assertion failed: !self.account_locks.lock().unwrap().contains_key(&fork)', runtime/src/accounts.rs:995:9
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:70
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:58
             at src/libstd/panicking.rs:200
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:215
   4: solana_metrics::metrics::set_panic_hook::{{closure}}::{{closure}}
   5: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:482
   6: std::panicking::begin_panic
   7: solana_runtime::accounts::Accounts::squash
   8: solana_runtime::bank::Bank::squash
   9: solana::bank_forks::BankForks::set_root

This has been observed on V0.12 but should apply to master branch as well.
This PR comments out the offending call in V0.12 for beacons release.
#3424

Proposed Solution

Reassess the crash on master branch. If it's a contained fix, pick it up to v0.12 and revert the change that's commenting out the call to squash.

[pgarg66]

@sakridge , FYI

[carllin]

hmmmm this assert implies that an account for fork wasn't unlocked...so there are still transactions running on an old frozen bank?

[sakridge]

@carllin that seems to be the case.

[carllin]

So PohRecorder should clear its bank once you've voted on the next bank, which means no new transactions should be attempted to be played on that old bank because the reference changes here: https://github.com/solana-labs/solana/blob/master/core/src/banking_stage.rs#L368. It could be possible, however that the queue of transactions is so long that we're still attempting to process them by the time we try to freeze and squash this bank.

[carllin]

So this check here makes sure we don't process transactions after we detect the bank has been cleared from PohRecorder: https://github.com/solana-labs/solana/blob/master/core/src/banking_stage.rs#L363. So for transactions to still be running, that must mean there is one ongoing iteration of this loop here: https://github.com/solana-labs/solana/blob/master/core/src/banking_stage.rs#L362, that isn't completed even after 32 slots have gone by. Ouch.

[sambley]

it doesn't look like the transactions are pending...probably looks like the some accounts are not getting unlocked in certain condition, still looking to narrow down the issue

[aeyakovenko]

#3447 i think covers this crash too

[pgarg66]

I can cherry-pick it and try it on v12

[sambley]

still seeing the crash on tip which has #3447 merged

[aeyakovenko]

@sambley, same here. It fixed another race for me though.

[sambley]

Uploaded fix in #3458