address comments from code review

scheduler/context_test.go

@@ -14,6 +14,7 @@ import (
 func testContext(t testing.TB) (*state.StateStore, *EvalContext) {
 	state := state.TestStateStore(t)
 	plan := &structs.Plan{
+		EvalID:          uuid.Generate(),
 		NodeUpdate:      make(map[string][]*structs.Allocation),
 		NodeAllocation:  make(map[string][]*structs.Allocation),
 		NodePreemptions: make(map[string][]*structs.Allocation),

scheduler/feasible.go

@@ -122,7 +122,7 @@ func (iter *StaticIterator) SetNodes(nodes []*structs.Node) {
 // is applied in-place
 func NewRandomIterator(ctx Context, nodes []*structs.Node) *StaticIterator {
 	// shuffle with the Fisher-Yates algorithm
-	shuffleNodes(ctx.Plan().EvalID, nodes)
+	shuffleNodes(ctx.Plan(), nodes)
 
 	// Create a static iterator
 	return NewStaticIterator(ctx, nodes)

scheduler/stack.go

@@ -70,7 +70,7 @@ type GenericStack struct {
 
 func (s *GenericStack) SetNodes(baseNodes []*structs.Node) {
 	// Shuffle base nodes
-	shuffleNodes(s.ctx.Plan().EvalID, baseNodes)
+	shuffleNodes(s.ctx.Plan(), baseNodes)
 
 	// Update the set of base nodes
 	s.source.SetNodes(baseNodes)

scheduler/util.go

@@ -381,9 +381,20 @@ func taintedNodes(state State, allocs []*structs.Allocation) (map[string]*struct
 // algorithm. We seed the random source with the eval ID (which is
 // random) to aid in postmortem debuggging of specific evaluations and
 // state snapshots.
-func shuffleNodes(evalID string, nodes []*structs.Node) {
+func shuffleNodes(plan *structs.Plan, nodes []*structs.Node) {
+
+	// use the last 4 bytes because those are the random bits
+	// if we have sortable IDs
+	buf := []byte(plan.EvalID)
+	seed := int64(binary.BigEndian.Uint32(buf[len(buf)-4:]))
+
+	// include the plan progress so that we don't retry with
+	// the exact same shuffle
+	seed = seed |
+		int64(len(plan.NodeAllocation)) |
+		int64(len(plan.NodeUpdate)) |
+		int64(len(plan.DeploymentUpdates))
 
-	seed, _ := binary.Varint([]byte(evalID))
 	r := rand.New(rand.NewSource(seed))
 
 	n := len(nodes)

scheduler/util_test.go

@@ -507,7 +507,9 @@ func TestShuffleNodes(t *testing.T) {
 	}
 	orig := make([]*structs.Node, len(nodes))
 	copy(orig, nodes)
-	shuffleNodes(uuid.Generate(), nodes)
+	eval := mock.Eval() // will have random EvalID
+	plan := eval.MakePlan(mock.Job())
+	shuffleNodes(plan, nodes)
 	require.False(t, reflect.DeepEqual(nodes, orig))
 }