AccountsDb: make write_accounts_to_cache() never block sending to the…

… bg hasher (#3814)

Avoid commit_transactions() => store() => store_accounts_to() callers
having to notify the bg hasher thread by doing an explicit sleep instead
of sleeping on channel.recv() when the channel is empty. This avoids the
case in which multiple replay/banking threads call commit_transactions()
at the same time and end up... sleeping themselves acquiring the mutex
to wake up the bg hasher.

(cherry picked from commit f2e3e39)

accounts-db/src/accounts_db.rs

@@ -72,7 +72,7 @@ use {
         u64_align, utils,
         verify_accounts_hash_in_background::VerifyAccountsHashInBackground,
     },
-    crossbeam_channel::{unbounded, Receiver, Sender},
+    crossbeam_channel::{unbounded, Receiver, Sender, TryRecvError},
     dashmap::{DashMap, DashSet},
     log::*,
     rand::{thread_rng, Rng},
@@ -2260,7 +2260,7 @@ impl AccountsDb {
     fn background_hasher(receiver: Receiver<Vec<CachedAccount>>) {
         info!("Background account hasher has started");
         loop {
-            let result = receiver.recv();
+            let result = receiver.try_recv();
             match result {
                 Ok(accounts) => {
                     for account in accounts {
@@ -2271,7 +2271,10 @@ impl AccountsDb {
                         };
                     }
                 }
-                Err(err) => {
+                Err(TryRecvError::Empty) => {
+                    sleep(Duration::from_millis(5));
+                }
+                Err(err @ TryRecvError::Disconnected) => {
                     info!("Background account hasher is stopping because: {err}");
                     break;
                 }