Merge pull request #11657 from hashicorp/f-dynamic-walk

dag: new walker, supports walk-time updates

dag/dag.go

@@ -2,11 +2,8 @@ package dag
 
 import (
 	"fmt"
-	"log"
 	"sort"
 	"strings"
-	"sync"
-	"time"
 
 	"github.com/hashicorp/go-multierror"
 )
@@ -169,94 +166,9 @@ func (g *AcyclicGraph) Cycles() [][]Vertex {
 func (g *AcyclicGraph) Walk(cb WalkFunc) error {
 	defer g.debug.BeginOperation(typeWalk, "").End("")
 
-	// Cache the vertices since we use it multiple times
+	w := &Walker{Callback: cb, Reverse: true}
-	vertices := g.Vertices()
+	w.Update(g)
-
+	return w.Wait()
-	// Build the waitgroup that signals when we're done
-	var wg sync.WaitGroup
-	wg.Add(len(vertices))
-	doneCh := make(chan struct{})
-	go func() {
-		defer close(doneCh)
-		wg.Wait()
-	}()
-
-	// The map of channels to watch to wait for vertices to finish
-	vertMap := make(map[Vertex]chan struct{})
-	for _, v := range vertices {
-		vertMap[v] = make(chan struct{})
-	}
-
-	// The map of whether a vertex errored or not during the walk
-	var errLock sync.Mutex
-	var errs error
-	errMap := make(map[Vertex]bool)
-	for _, v := range vertices {
-		// Build our list of dependencies and the list of channels to
-		// wait on until we start executing for this vertex.
-		deps := AsVertexList(g.DownEdges(v))
-		depChs := make([]<-chan struct{}, len(deps))
-		for i, dep := range deps {
-			depChs[i] = vertMap[dep]
-		}
-
-		// Get our channel so that we can close it when we're done
-		ourCh := vertMap[v]
-
-		// Start the goroutine to wait for our dependencies
-		readyCh := make(chan bool)
-		go func(v Vertex, deps []Vertex, chs []<-chan struct{}, readyCh chan<- bool) {
-			// First wait for all the dependencies
-			for i, ch := range chs {
-			DepSatisfied:
-				for {
-					select {
-					case <-ch:
-						break DepSatisfied
-					case <-time.After(time.Second * 5):
-						log.Printf("[DEBUG] vertex %q, waiting for: %q",
-							VertexName(v), VertexName(deps[i]))
-					}
-				}
-				log.Printf("[DEBUG] vertex %q, got dep: %q",
-					VertexName(v), VertexName(deps[i]))
-			}
-
-			// Then, check the map to see if any of our dependencies failed
-			errLock.Lock()
-			defer errLock.Unlock()
-			for _, dep := range deps {
-				if errMap[dep] {
-					errMap[v] = true
-					readyCh <- false
-					return
-				}
-			}
-
-			readyCh <- true
-		}(v, deps, depChs, readyCh)
-
-		// Start the goroutine that executes
-		go func(v Vertex, doneCh chan<- struct{}, readyCh <-chan bool) {
-			defer close(doneCh)
-			defer wg.Done()
-
-			var err error
-			if ready := <-readyCh; ready {
-				err = cb(v)
-			}
-
-			errLock.Lock()
-			defer errLock.Unlock()
-			if err != nil {
-				errMap[v] = true
-				errs = multierror.Append(errs, err)
-			}
-		}(v, ourCh, readyCh)
-	}
-
-	<-doneCh
-	return errs
 }
 
 // simple convenience helper for converting a dag.Set to a []Vertex

dag/set.go

@@ -48,6 +48,9 @@ func (s *Set) Include(v interface{}) bool {
 // Intersection computes the set intersection with other.
 func (s *Set) Intersection(other *Set) *Set {
 	result := new(Set)
+	if s == nil {
+		return result
+	}
 	if other != nil {
 		for _, v := range s.m {
 			if other.Include(v) {
@@ -59,6 +62,25 @@ func (s *Set) Intersection(other *Set) *Set {
 	return result
 }
 
+// Difference returns a set with the elements that s has but
+// other doesn't.
+func (s *Set) Difference(other *Set) *Set {
+	result := new(Set)
+	if s != nil {
+		for k, v := range s.m {
+			var ok bool
+			if other != nil {
+				_, ok = other.m[k]
+			}
+			if !ok {
+				result.Add(v)
+			}
+		}
+	}
+
+	return result
+}
+
 // Len is the number of items in the set.
 func (s *Set) Len() int {
 	if s == nil {

dag/set_test.go

@@ -0,0 +1,56 @@
+package dag
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestSetDifference(t *testing.T) {
+	cases := []struct {
+		Name     string
+		A, B     []interface{}
+		Expected []interface{}
+	}{
+		{
+			"same",
+			[]interface{}{1, 2, 3},
+			[]interface{}{3, 1, 2},
+			[]interface{}{},
+		},
+
+		{
+			"A has extra elements",
+			[]interface{}{1, 2, 3},
+			[]interface{}{3, 2},
+			[]interface{}{1},
+		},
+
+		{
+			"B has extra elements",
+			[]interface{}{1, 2, 3},
+			[]interface{}{3, 2, 1, 4},
+			[]interface{}{},
+		},
+	}
+
+	for i, tc := range cases {
+		t.Run(fmt.Sprintf("%d-%s", i, tc.Name), func(t *testing.T) {
+			var one, two, expected Set
+			for _, v := range tc.A {
+				one.Add(v)
+			}
+			for _, v := range tc.B {
+				two.Add(v)
+			}
+			for _, v := range tc.Expected {
+				expected.Add(v)
+			}
+
+			actual := one.Difference(&two)
+			match := actual.Intersection(&expected)
+			if match.Len() != expected.Len() {
+				t.Fatalf("bad: %#v", actual.List())
+			}
+		})
+	}
+}

dag/walk.go

@@ -0,0 +1,445 @@
+package dag
+
+import (
+	"errors"
+	"fmt"
+	"log"
+	"sync"
+	"time"
+
+	"github.com/hashicorp/go-multierror"
+)
+
+// Walker is used to walk every vertex of a graph in parallel.
+//
+// A vertex will only be walked when the dependencies of that vertex have
+// been walked. If two vertices can be walked at the same time, they will be.
+//
+// Update can be called to update the graph. This can be called even during
+// a walk, cahnging vertices/edges mid-walk. This should be done carefully.
+// If a vertex is removed but has already been executed, the result of that
+// execution (any error) is still returned by Wait. Changing or re-adding
+// a vertex that has already executed has no effect. Changing edges of
+// a vertex that has already executed has no effect.
+//
+// Non-parallelism can be enforced by introducing a lock in your callback
+// function. However, the goroutine overhead of a walk will remain.
+// Walker will create V*2 goroutines (one for each vertex, and dependency
+// waiter for each vertex). In general this should be of no concern unless
+// there are a huge number of vertices.
+//
+// The walk is depth first by default. This can be changed with the Reverse
+// option.
+//
+// A single walker is only valid for one graph walk. After the walk is complete
+// you must construct a new walker to walk again. State for the walk is never
+// deleted in case vertices or edges are changed.
+type Walker struct {
+	// Callback is what is called for each vertex
+	Callback WalkFunc
+
+	// Reverse, if true, causes the source of an edge to depend on a target.
+	// When false (default), the target depends on the source.
+	Reverse bool
+
+	// changeLock must be held to modify any of the fields below. Only Update
+	// should modify these fields. Modifying them outside of Update can cause
+	// serious problems.
+	changeLock sync.Mutex
+	vertices   Set
+	edges      Set
+	vertexMap  map[Vertex]*walkerVertex
+
+	// wait is done when all vertices have executed. It may become "undone"
+	// if new vertices are added.
+	wait sync.WaitGroup
+
+	// errMap contains the errors recorded so far for execution. Reading
+	// and writing should hold errLock.
+	errMap  map[Vertex]error
+	errLock sync.Mutex
+}
+
+type walkerVertex struct {
+	// These should only be set once on initialization and never written again.
+	// They are not protected by a lock since they don't need to be since
+	// they are write-once.
+
+	// DoneCh is closed when this vertex has completed execution, regardless
+	// of success.
+	//
+	// CancelCh is closed when the vertex should cancel execution. If execution
+	// is already complete (DoneCh is closed), this has no effect. Otherwise,
+	// execution is cancelled as quickly as possible.
+	DoneCh   chan struct{}
+	CancelCh chan struct{}
+
+	// Dependency information. Any changes to any of these fields requires
+	// holding DepsLock.
+	//
+	// DepsCh is sent a single value that denotes whether the upstream deps
+	// were successful (no errors). Any value sent means that the upstream
+	// dependencies are complete. No other values will ever be sent again.
+	//
+	// DepsUpdateCh is closed when there is a new DepsCh set.
+	DepsCh       chan bool
+	DepsUpdateCh chan struct{}
+	DepsLock     sync.Mutex
+
+	// Below is not safe to read/write in parallel. This behavior is
+	// enforced by changes only happening in Update. Nothing else should
+	// ever modify these.
+	deps         map[Vertex]chan struct{}
+	depsCancelCh chan struct{}
+}
+
+// errWalkUpstream is used in the errMap of a walk to note that an upstream
+// dependency failed so this vertex wasn't run. This is not shown in the final
+// user-returned error.
+var errWalkUpstream = errors.New("upstream dependency failed")
+
+// Wait waits for the completion of the walk and returns any errors (
+// in the form of a multierror) that occurred. Update should be called
+// to populate the walk with vertices and edges prior to calling this.
+//
+// Wait will return as soon as all currently known vertices are complete.
+// If you plan on calling Update with more vertices in the future, you
+// should not call Wait until after this is done.
+func (w *Walker) Wait() error {
+	// Wait for completion
+	w.wait.Wait()
+
+	// Grab the error lock
+	w.errLock.Lock()
+	defer w.errLock.Unlock()
+
+	// Build the error
+	var result error
+	for v, err := range w.errMap {
+		if err != nil && err != errWalkUpstream {
+			result = multierror.Append(result, fmt.Errorf(
+				"%s: %s", VertexName(v), err))
+		}
+	}
+
+	return result
+}
+
+// Update updates the currently executing walk with the given graph.
+// This will perform a diff of the vertices and edges and update the walker.
+// Already completed vertices remain completed (including any errors during
+// their execution).
+//
+// This returns immediately once the walker is updated; it does not wait
+// for completion of the walk.
+//
+// Multiple Updates can be called in parallel. Update can be called at any
+// time during a walk.
+func (w *Walker) Update(g *AcyclicGraph) {
+	var v, e *Set
+	if g != nil {
+		v, e = g.vertices, g.edges
+	}
+
+	// Grab the change lock so no more updates happen but also so that
+	// no new vertices are executed during this time since we may be
+	// removing them.
+	w.changeLock.Lock()
+	defer w.changeLock.Unlock()
+
+	// Initialize fields
+	if w.vertexMap == nil {
+		w.vertexMap = make(map[Vertex]*walkerVertex)
+	}
+
+	// Calculate all our sets
+	newEdges := e.Difference(&w.edges)
+	oldEdges := w.edges.Difference(e)
+	newVerts := v.Difference(&w.vertices)
+	oldVerts := w.vertices.Difference(v)
+
+	// Add the new vertices
+	for _, raw := range newVerts.List() {
+		v := raw.(Vertex)
+
+		// Add to the waitgroup so our walk is not done until everything finishes
+		w.wait.Add(1)
+
+		// Add to our own set so we know about it already
+		log.Printf("[DEBUG] dag/walk: added new vertex: %q", VertexName(v))
+		w.vertices.Add(raw)
+
+		// Initialize the vertex info
+		info := &walkerVertex{
+			DoneCh:   make(chan struct{}),
+			CancelCh: make(chan struct{}),
+			deps:     make(map[Vertex]chan struct{}),
+		}
+
+		// Add it to the map and kick off the walk
+		w.vertexMap[v] = info
+	}
+
+	// Remove the old vertices
+	for _, raw := range oldVerts.List() {
+		v := raw.(Vertex)
+
+		// Get the vertex info so we can cancel it
+		info, ok := w.vertexMap[v]
+		if !ok {
+			// This vertex for some reason was never in our map. This
+			// shouldn't be possible.
+			continue
+		}
+
+		// Cancel the vertex
+		close(info.CancelCh)
+
+		// Delete it out of the map
+		delete(w.vertexMap, v)
+
+		log.Printf("[DEBUG] dag/walk: removed vertex: %q", VertexName(v))
+		w.vertices.Delete(raw)
+	}
+
+	// Add the new edges
+	var changedDeps Set
+	for _, raw := range newEdges.List() {
+		edge := raw.(Edge)
+		waiter, dep := w.edgeParts(edge)
+
+		// Get the info for the waiter
+		waiterInfo, ok := w.vertexMap[waiter]
+		if !ok {
+			// Vertex doesn't exist... shouldn't be possible but ignore.
+			continue
+		}
+
+		// Get the info for the dep
+		depInfo, ok := w.vertexMap[dep]
+		if !ok {
+			// Vertex doesn't exist... shouldn't be possible but ignore.
+			continue
+		}
+
+		// Add the dependency to our waiter
+		waiterInfo.deps[dep] = depInfo.DoneCh
+
+		// Record that the deps changed for this waiter
+		changedDeps.Add(waiter)
+
+		log.Printf(
+			"[DEBUG] dag/walk: added edge: %q waiting on %q",
+			VertexName(waiter), VertexName(dep))
+		w.edges.Add(raw)
+	}
+
+	// Process reoved edges
+	for _, raw := range oldEdges.List() {
+		edge := raw.(Edge)
+		waiter, dep := w.edgeParts(edge)
+
+		// Get the info for the waiter
+		waiterInfo, ok := w.vertexMap[waiter]
+		if !ok {
+			// Vertex doesn't exist... shouldn't be possible but ignore.
+			continue
+		}
+
+		// Delete the dependency from the waiter
+		delete(waiterInfo.deps, dep)
+
+		// Record that the deps changed for this waiter
+		changedDeps.Add(waiter)
+
+		log.Printf(
+			"[DEBUG] dag/walk: removed edge: %q waiting on %q",
+			VertexName(waiter), VertexName(dep))
+		w.edges.Delete(raw)
+	}
+
+	// For each vertex with changed dependencies, we need to kick off
+	// a new waiter and notify the vertex of the changes.
+	for _, raw := range changedDeps.List() {
+		v := raw.(Vertex)
+		info, ok := w.vertexMap[v]
+		if !ok {
+			// Vertex doesn't exist... shouldn't be possible but ignore.
+			continue
+		}
+
+		// Create a new done channel
+		doneCh := make(chan bool, 1)
+
+		// Create the channel we close for cancellation
+		cancelCh := make(chan struct{})
+
+		// Build a new deps copy
+		deps := make(map[Vertex]<-chan struct{})
+		for k, v := range info.deps {
+			deps[k] = v
+		}
+
+		// Update the update channel
+		info.DepsLock.Lock()
+		if info.DepsUpdateCh != nil {
+			close(info.DepsUpdateCh)
+		}
+		info.DepsCh = doneCh
+		info.DepsUpdateCh = make(chan struct{})
+		info.DepsLock.Unlock()
+
+		// Cancel the older waiter
+		if info.depsCancelCh != nil {
+			close(info.depsCancelCh)
+		}
+		info.depsCancelCh = cancelCh
+
+		log.Printf(
+			"[DEBUG] dag/walk: dependencies changed for %q, sending new deps",
+			VertexName(v))
+
+		// Start the waiter
+		go w.waitDeps(v, deps, doneCh, cancelCh)
+	}
+
+	// Start all the new vertices. We do this at the end so that all
+	// the edge waiters and changes are setup above.
+	for _, raw := range newVerts.List() {
+		v := raw.(Vertex)
+		go w.walkVertex(v, w.vertexMap[v])
+	}
+}
+
+// edgeParts returns the waiter and the dependency, in that order.
+// The waiter is waiting on the dependency.
+func (w *Walker) edgeParts(e Edge) (Vertex, Vertex) {
+	if w.Reverse {
+		return e.Source(), e.Target()
+	}
+
+	return e.Target(), e.Source()
+}
+
+// walkVertex walks a single vertex, waiting for any dependencies before
+// executing the callback.
+func (w *Walker) walkVertex(v Vertex, info *walkerVertex) {
+	// When we're done executing, lower the waitgroup count
+	defer w.wait.Done()
+
+	// When we're done, always close our done channel
+	defer close(info.DoneCh)
+
+	// Wait for our dependencies. We create a [closed] deps channel so
+	// that we can immediately fall through to load our actual DepsCh.
+	var depsSuccess bool
+	var depsUpdateCh chan struct{}
+	depsCh := make(chan bool, 1)
+	depsCh <- true
+	close(depsCh)
+	for {
+		select {
+		case <-info.CancelCh:
+			// Cancel
+			return
+
+		case depsSuccess = <-depsCh:
+			// Deps complete! Mark as nil to trigger completion handling.
+			depsCh = nil
+
+		case <-depsUpdateCh:
+			// New deps, reloop
+		}
+
+		// Check if we have updated dependencies. This can happen if the
+		// dependencies were satisfied exactly prior to an Update occuring.
+		// In that case, we'd like to take into account new dependencies
+		// if possible.
+		info.DepsLock.Lock()
+		if info.DepsCh != nil {
+			depsCh = info.DepsCh
+			info.DepsCh = nil
+		}
+		if info.DepsUpdateCh != nil {
+			depsUpdateCh = info.DepsUpdateCh
+		}
+		info.DepsLock.Unlock()
+
+		// If we still have no deps channel set, then we're done!
+		if depsCh == nil {
+			break
+		}
+	}
+
+	// If we passed dependencies, we just want to check once more that
+	// we're not cancelled, since this can happen just as dependencies pass.
+	select {
+	case <-info.CancelCh:
+		// Cancelled during an update while dependencies completed.
+		return
+	default:
+	}
+
+	// Run our callback or note that our upstream failed
+	var err error
+	if depsSuccess {
+		log.Printf("[DEBUG] dag/walk: walking %q", VertexName(v))
+		err = w.Callback(v)
+	} else {
+		log.Printf("[DEBUG] dag/walk: upstream errored, not walking %q", VertexName(v))
+		err = errWalkUpstream
+	}
+
+	// Record the error
+	if err != nil {
+		w.errLock.Lock()
+		defer w.errLock.Unlock()
+
+		if w.errMap == nil {
+			w.errMap = make(map[Vertex]error)
+		}
+		w.errMap[v] = err
+	}
+}
+
+func (w *Walker) waitDeps(
+	v Vertex,
+	deps map[Vertex]<-chan struct{},
+	doneCh chan<- bool,
+	cancelCh <-chan struct{}) {
+	// For each dependency given to us, wait for it to complete
+	for dep, depCh := range deps {
+	DepSatisfied:
+		for {
+			select {
+			case <-depCh:
+				// Dependency satisfied!
+				break DepSatisfied
+
+			case <-cancelCh:
+				// Wait cancelled. Note that we didn't satisfy dependencies
+				// so that anything waiting on us also doesn't run.
+				doneCh <- false
+				return
+
+			case <-time.After(time.Second * 5):
+				log.Printf("[DEBUG] dag/walk: vertex %q, waiting for: %q",
+					VertexName(v), VertexName(dep))
+			}
+		}
+	}
+
+	// Dependencies satisfied! We need to check if any errored
+	w.errLock.Lock()
+	defer w.errLock.Unlock()
+	for dep, _ := range deps {
+		if w.errMap[dep] != nil {
+			// One of our dependencies failed, so return false
+			doneCh <- false
+			return
+		}
+	}
+
+	// All dependencies satisfied and successful
+	doneCh <- true
+}

dag/walk_test.go

@@ -0,0 +1,281 @@
+package dag
+
+import (
+	"fmt"
+	"reflect"
+	"sync"
+	"testing"
+	"time"
+)
+
+func TestWalker_basic(t *testing.T) {
+	var g AcyclicGraph
+	g.Add(1)
+	g.Add(2)
+	g.Connect(BasicEdge(1, 2))
+
+	// Run it a bunch of times since it is timing dependent
+	for i := 0; i < 50; i++ {
+		var order []interface{}
+		w := &Walker{Callback: walkCbRecord(&order)}
+		w.Update(&g)
+
+		// Wait
+		if err := w.Wait(); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+
+		// Check
+		expected := []interface{}{1, 2}
+		if !reflect.DeepEqual(order, expected) {
+			t.Fatalf("bad: %#v", order)
+		}
+	}
+}
+
+func TestWalker_updateNilGraph(t *testing.T) {
+	var g AcyclicGraph
+	g.Add(1)
+	g.Add(2)
+	g.Connect(BasicEdge(1, 2))
+
+	// Run it a bunch of times since it is timing dependent
+	for i := 0; i < 50; i++ {
+		var order []interface{}
+		w := &Walker{Callback: walkCbRecord(&order)}
+		w.Update(&g)
+		w.Update(nil)
+
+		// Wait
+		if err := w.Wait(); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
+}
+
+func TestWalker_error(t *testing.T) {
+	var g AcyclicGraph
+	g.Add(1)
+	g.Add(2)
+	g.Add(3)
+	g.Add(4)
+	g.Connect(BasicEdge(1, 2))
+	g.Connect(BasicEdge(2, 3))
+	g.Connect(BasicEdge(3, 4))
+
+	// Record function
+	var order []interface{}
+	recordF := walkCbRecord(&order)
+
+	// Build a callback that delays until we close a channel
+	cb := func(v Vertex) error {
+		if v == 2 {
+			return fmt.Errorf("error!")
+		}
+
+		return recordF(v)
+	}
+
+	w := &Walker{Callback: cb}
+	w.Update(&g)
+
+	// Wait
+	if err := w.Wait(); err == nil {
+		t.Fatal("expect error")
+	}
+
+	// Check
+	expected := []interface{}{1}
+	if !reflect.DeepEqual(order, expected) {
+		t.Fatalf("bad: %#v", order)
+	}
+}
+
+func TestWalker_newVertex(t *testing.T) {
+	// Run it a bunch of times since it is timing dependent
+	for i := 0; i < 50; i++ {
+		var g AcyclicGraph
+		g.Add(1)
+		g.Add(2)
+		g.Connect(BasicEdge(1, 2))
+
+		var order []interface{}
+		w := &Walker{Callback: walkCbRecord(&order)}
+		w.Update(&g)
+
+		// Wait a bit
+		time.Sleep(10 * time.Millisecond)
+
+		// Update the graph
+		g.Add(3)
+		w.Update(&g)
+
+		// Update the graph again but with the same vertex
+		g.Add(3)
+		w.Update(&g)
+
+		// Wait
+		if err := w.Wait(); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+
+		// Check
+		expected := []interface{}{1, 2, 3}
+		if !reflect.DeepEqual(order, expected) {
+			t.Fatalf("bad: %#v", order)
+		}
+	}
+}
+
+func TestWalker_removeVertex(t *testing.T) {
+	// Run it a bunch of times since it is timing dependent
+	for i := 0; i < 50; i++ {
+		var g AcyclicGraph
+		g.Add(1)
+		g.Add(2)
+		g.Connect(BasicEdge(1, 2))
+
+		// Record function
+		var order []interface{}
+		recordF := walkCbRecord(&order)
+
+		// Build a callback that delays until we close a channel
+		var w *Walker
+		cb := func(v Vertex) error {
+			if v == 1 {
+				g.Remove(2)
+				w.Update(&g)
+			}
+
+			return recordF(v)
+		}
+
+		// Add the initial vertices
+		w = &Walker{Callback: cb}
+		w.Update(&g)
+
+		// Wait
+		if err := w.Wait(); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+
+		// Check
+		expected := []interface{}{1}
+		if !reflect.DeepEqual(order, expected) {
+			t.Fatalf("bad: %#v", order)
+		}
+	}
+}
+
+func TestWalker_newEdge(t *testing.T) {
+	// Run it a bunch of times since it is timing dependent
+	for i := 0; i < 50; i++ {
+		var g AcyclicGraph
+		g.Add(1)
+		g.Add(2)
+		g.Connect(BasicEdge(1, 2))
+
+		// Record function
+		var order []interface{}
+		recordF := walkCbRecord(&order)
+
+		// Build a callback that delays until we close a channel
+		var w *Walker
+		cb := func(v Vertex) error {
+			if v == 1 {
+				g.Add(3)
+				g.Connect(BasicEdge(3, 2))
+				w.Update(&g)
+			}
+
+			return recordF(v)
+		}
+
+		// Add the initial vertices
+		w = &Walker{Callback: cb}
+		w.Update(&g)
+
+		// Wait
+		if err := w.Wait(); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+
+		// Check
+		expected := []interface{}{1, 3, 2}
+		if !reflect.DeepEqual(order, expected) {
+			t.Fatalf("bad: %#v", order)
+		}
+	}
+}
+
+func TestWalker_removeEdge(t *testing.T) {
+	// Run it a bunch of times since it is timing dependent
+	for i := 0; i < 50; i++ {
+		var g AcyclicGraph
+		g.Add(1)
+		g.Add(2)
+		g.Add(3)
+		g.Connect(BasicEdge(1, 2))
+		g.Connect(BasicEdge(3, 2))
+
+		// Record function
+		var order []interface{}
+		recordF := walkCbRecord(&order)
+
+		// The way this works is that our original graph forces
+		// the order of 1 => 3 => 2. During the execution of 1, we
+		// remove the edge forcing 3 before 2. Then, during the execution
+		// of 3, we wait on a channel that is only closed by 2, implicitly
+		// forcing 2 before 3 via the callback (and not the graph). If
+		// 2 cannot execute before 3 (edge removal is non-functional), then
+		// this test will timeout.
+		var w *Walker
+		gateCh := make(chan struct{})
+		cb := func(v Vertex) error {
+			if v == 1 {
+				g.RemoveEdge(BasicEdge(3, 2))
+				w.Update(&g)
+			}
+
+			if v == 2 {
+				close(gateCh)
+			}
+
+			if v == 3 {
+				select {
+				case <-gateCh:
+				case <-time.After(50 * time.Millisecond):
+					return fmt.Errorf("timeout 3 waiting for 2")
+				}
+			}
+
+			return recordF(v)
+		}
+
+		// Add the initial vertices
+		w = &Walker{Callback: cb}
+		w.Update(&g)
+
+		// Wait
+		if err := w.Wait(); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+
+		// Check
+		expected := []interface{}{1, 2, 3}
+		if !reflect.DeepEqual(order, expected) {
+			t.Fatalf("bad: %#v", order)
+		}
+	}
+}
+
+// walkCbRecord is a test helper callback that just records the order called.
+func walkCbRecord(order *[]interface{}) WalkFunc {
+	var l sync.Mutex
+	return func(v Vertex) error {
+		l.Lock()
+		defer l.Unlock()
+		*order = append(*order, v)
+		return nil
+	}
+}