Compute the next nonce from the tipset, instead of actually executing the tipset

We could also do this in the message pool itself, but I'm not sure if it's worth it?

node/modules/mpoolnonceapi.go

@@ -4,6 +4,7 @@ import (
 	"context"
 
 	"go.uber.org/fx"
+	"golang.org/x/xerrors"
 
 	"github.com/filecoin-project/lotus/node/impl/full"
 
@@ -21,13 +22,41 @@ type MpoolNonceAPI struct {
 	StateAPI full.StateAPI
 }
 
-// GetNonce gets the nonce from actor state
+// GetNonce gets the nonce from current chain head.
 func (a *MpoolNonceAPI) GetNonce(addr address.Address) (uint64, error) {
-	act, err := a.StateAPI.StateGetActor(context.Background(), addr, types.EmptyTSK)
+	ts := a.StateAPI.Chain.GetHeaviestTipSet()
+
+	// make sure we have a key address so we can compare with messages
+	keyAddr, err := a.StateAPI.StateManager.ResolveToKeyAddress(context.TODO(), addr, ts)
 	if err != nil {
 		return 0, err
 	}
-	return act.Nonce, nil
+
+	// Load the last nonce from the state, if it exists.
+	highestNonce := uint64(0)
+	if baseActor, err := a.StateAPI.StateManager.LoadActorRaw(context.TODO(), addr, ts.ParentState()); err != nil {
+		if !xerrors.Is(err, types.ErrActorNotFound) {
+			return 0, err
+		}
+	} else {
+		highestNonce = baseActor.Nonce
+	}
+
+	// Otherwise, find the highest nonce in the tipset.
+	msgs, err := a.StateAPI.Chain.MessagesForTipset(ts)
+	if err != nil {
+		return 0, err
+	}
+	for _, msg := range msgs {
+		vmmsg := msg.VMMessage()
+		if vmmsg.From != keyAddr {
+			continue
+		}
+		if vmmsg.Nonce > highestNonce {
+			highestNonce = vmmsg.Nonce
+		}
+	}
+	return highestNonce, nil
 }
 
 var _ messagesigner.MpoolNonceAPI = (*MpoolNonceAPI)(nil)