Speed up nominator state checks in staking pallet (#2153)

Should help #234.
Related to #2020 and
#2108.

Refactors and improves running time for try runtime checks for staking
pallet.

Tested on westend on my M2 pro: running time drops from 90 seconds to 7
seconds.

substrate/frame/staking/src/pallet/impls.rs

@@ -1864,6 +1864,13 @@ impl<T: Config> Pallet<T> {
  // a check per nominator to ensure their entire stake is correctly distributed. Will only
  // kick-in if the nomination was submitted before the current era.
  let era = Self::active_era().unwrap().index;
+
+ // cache era exposures to avoid too many db reads.
+ let era_exposures = T::SessionInterface::validators()
+ .iter()
+ .map(|v| Self::eras_stakers(era, v))
+ .collect::<Vec<_>>();
+
  <Nominators<T>>::iter()
  .filter_map(
  |(nominator, nomination)| {
@@ -1878,9 +1885,8 @@ impl<T: Config> Pallet<T> {
  // must be bonded.
  Self::ensure_is_stash(&nominator)?;
  let mut sum = BalanceOf::<T>::zero();
- T::SessionInterface::validators()
+ era_exposures
  .iter()
- .map(|v| Self::eras_stakers(era, v))
  .map(|e| -> Result<(), TryRuntimeError> {
  let individual =
  e.others.iter().filter(|e| e.who == nominator).collect::<Vec<_>>();
@@ -1896,6 +1902,14 @@ impl<T: Config> Pallet<T> {
  Ok(())
  })
  .collect::<Result<Vec<_>, _>>()?;
+
+ // We take total instead of active as the nominator might have requested to unbond
+ // some of their stake that is still exposed in the current era.
+ if sum <= Self::ledger(Stash(nominator.clone()))?.total {
+ // This can happen when there is a slash in the current era so we only warn.
+ log!(warn, "nominator stake exceeds what is bonded.");
+ }
+
  Ok(())
  })
  .collect::<Result<Vec<_>, _>>()?;