core: dag errors should cascade to all descendents

We weren't marking skipped nodes as failing, so any
grandchild-and-deeper dependencies would still evaluate.

For example:

    A -> B -> C -> D

If B failed, C would be skipped, but D would still be evaluated.

This fixes the behavior so C, D, and any further descendents will all be
skipped when B fails.

Addresses crashing aspect of #2955 and likely a lot of other confusing
failure modes.

dag/dag.go

@@ -195,7 +195,7 @@ func (g *AcyclicGraph) Walk(cb WalkFunc) error {
 
  // Start the goroutine to wait for our dependencies
  readyCh := make(chan bool)
- go func(deps []Vertex, chs []<-chan struct{}, readyCh chan<- bool) {
+ go func(v Vertex, deps []Vertex, chs []<-chan struct{}, readyCh chan<- bool) {
  // First wait for all the dependencies
  for _, ch := range chs {
  <-ch
@@ -206,13 +206,14 @@ func (g *AcyclicGraph) Walk(cb WalkFunc) error {
  defer errLock.Unlock()
  for _, dep := range deps {
  if errMap[dep] {
+ errMap[v] = true
  readyCh <- false
  return
  }
  }
 
  readyCh <- true
- }(deps, depChs, readyCh)
+ }(v, deps, depChs, readyCh)
 
  // Start the goroutine that executes
  go func(v Vertex, doneCh chan<- struct{}, readyCh <-chan bool) {

dag/dag_test.go

@@ -226,8 +226,10 @@ func TestAcyclicGraphWalk_error(t *testing.T) {
  g.Add(1)
  g.Add(2)
  g.Add(3)
+ g.Add(4)
+ g.Connect(BasicEdge(4, 3))
  g.Connect(BasicEdge(3, 2))
- g.Connect(BasicEdge(3, 1))
+ g.Connect(BasicEdge(2, 1))
 
  var visits []Vertex
  var lock sync.Mutex