node: don't attempt fast sync when InitChain sets self as only validator (#5211)

Fixes #5178.

CHANGELOG_PENDING.md

@@ -17,3 +17,4 @@ Friendly reminder, we have a [bug bounty program](https://hackerone.com/tendermi
 ### BUG FIXES:
 
 - [evidence] [\#5170](https://github.com/tendermint/tendermint/pull/5170) change abci evidence time to the time the infraction happened not the time the evidence was committed on the block (@cmwaters)
+- [node] Don't attempt fast sync when the ABCI application specifies ourself as the only validator via `InitChain` (@erikgrinaker)

node/node.go

@@ -666,9 +666,7 @@ func NewNode(config *cfg.Config,
 		return nil, fmt.Errorf("can't get pubkey: %w", err)
 	}
 
-	// Determine whether we should do state and/or fast sync.
-	// We don't fast-sync when the only validator is us.
-	fastSync := config.FastSyncMode && !onlyValidatorIsUs(state, pubKey)
+	// Determine whether we should attempt state sync.
 	stateSync := config.StateSync.Enable && !onlyValidatorIsUs(state, pubKey)
 	if stateSync && state.LastBlockHeight > 0 {
 		logger.Info("Found local state with non-zero height, skipping state sync")
@@ -689,6 +687,10 @@ func NewNode(config *cfg.Config,
 		state = sm.LoadState(stateDB)
 	}
 
+	// Determine whether we should do fast sync. This must happen after the handshake, since the
+	// app may modify the validator set, specifying ourself as the only validator.
+	fastSync := config.FastSyncMode && !onlyValidatorIsUs(state, pubKey)
+
 	logNodeStartupInfo(state, pubKey, logger, consensusLogger)
 
 	csMetrics, p2pMetrics, memplMetrics, smMetrics := metricsProvider(genDoc.ChainID)