go/runtime/scheduling/simple: Only dispatch one batch at a time

Since batches only get deleted after a successful dispatch, dispatching
a batch while one is in progress will duplicate transactions that were
already dispatched but not yet deleted. Note: this doesn't change
current behaviour as the executor already enforces this. The added check only
prevents potential future misuse of the scheduler.

go/runtime/scheduling/simple/simple.go

@@ -3,6 +3,7 @@ package simple
 
 import (
 	"fmt"
+	"sync/atomic"
 
 	"github.com/oasisprotocol/oasis-core/go/common/crypto/hash"
 	"github.com/oasisprotocol/oasis-core/go/common/logging"
@@ -26,10 +27,17 @@ type scheduler struct {
 
 	incomingQueue txpoolApi.TxPool
 
-	dispatcher api.TransactionDispatcher
+	dispatchInProgress uint32
+	dispatcher         api.TransactionDispatcher
 }
 
 func (s *scheduler) scheduleBatch(force bool) error {
+	if !atomic.CompareAndSwapUint32(&s.dispatchInProgress, 0, 1) {
+		// Dispatch already in progress.
+		return nil
+	}
+	defer atomic.StoreUint32(&s.dispatchInProgress, 0)
+
 	batch := s.incomingQueue.GetBatch(force)
 	if len(batch) > 0 {
 		// Try to dispatch batch.