Fix sourcenetwork#19

Fixing now, as it is quick, and it gets in the way of the primary change coming in a later commit.

badger/iter.go

@@ -109,6 +109,12 @@ func (it *iterator) Next() (bool, error) {
 		return it.restart()
 	}
 
+	if !it.i.Valid() {
+		// `it.i.Next()` will panic if we attempt to call it having already reached
+		// the iterator end, so we much return before trying to make that call if we can.
+		return false, nil
+	}
+
 	it.i.Next()
 	return it.valid(), nil
 }

test/integration/iterator/reverse_next_test.go

@@ -6,8 +6,6 @@ import (
 	"github.com/sourcenetwork/corekv"
 	"github.com/sourcenetwork/corekv/test/action"
 	"github.com/sourcenetwork/corekv/test/integration"
-	"github.com/sourcenetwork/corekv/test/state"
-	"github.com/stretchr/testify/require"
 )
 
 func TestIteratorReverseNext(t *testing.T) {
@@ -28,48 +26,8 @@ func TestIteratorReverseNext(t *testing.T) {
 	test.Execute(t)
 }
 
-// This test documents undesirable behaviour, issue:
-// https://github.com/sourcenetwork/corekv/issues/19
-func TestIteratorReverseNext_BeyondEnd_Badger(t *testing.T) {
+func TestIteratorReverseNext_BeyondEnd(t *testing.T) {
 	test := &integration.Test{
-		SupportedStoreTypes: []state.StoreType{
-			state.BadgerStoreType,
-		},
-		Actions: []action.Action{
-			action.Set([]byte("k1"), []byte("v1")),
-			action.Set([]byte("k3"), nil),
-			action.Set([]byte("k4"), []byte("v4")),
-			action.Set([]byte("k2"), []byte("v2")),
-			&action.Iterator{
-				IterOptions: corekv.IterOptions{
-					Reverse: true,
-				},
-				ChildActions: []action.IteratorAction{
-					action.Next(true),
-					action.Next(true),
-					action.Next(true),
-					action.Next(true),
-					action.Next(false),
-					action.Next(false),
-				},
-			},
-		},
-	}
-
-	require.PanicsWithError(
-		t,
-		"runtime error: invalid memory address or nil pointer dereference",
-		func() {
-			test.Execute(t)
-		},
-	)
-}
-
-func TestIteratorReverseNext_BeyondEnd_Memory(t *testing.T) {
-	test := &integration.Test{
-		SupportedStoreTypes: []state.StoreType{
-			state.MemoryStoreType,
-		},
 		Actions: []action.Action{
 			action.Set([]byte("k1"), []byte("v1")),
 			action.Set([]byte("k3"), nil),