diff --git a/go/buildutil/allpackages.go b/go/buildutil/allpackages.go
index c95db42c35..0f909eeb20 100644
--- a/go/buildutil/allpackages.go
+++ b/go/buildutil/allpackages.go
@@ -30,11 +30,8 @@ import (
 //
 func AllPackages(ctxt *build.Context) []string {
 	var list []string
-	var mu sync.Mutex
 	ForEachPackage(ctxt, func(pkg string, _ error) {
-		mu.Lock()
 		list = append(list, pkg)
-		mu.Unlock()
 	})
 	sort.Strings(list)
 	return list
@@ -47,27 +44,42 @@ func AllPackages(ctxt *build.Context) []string {
 // If the package directory exists but could not be read, the second
 // argument to the found function provides the error.
 //
-// The found function and the build.Context file system interface
-// accessors must be concurrency safe.
+// All I/O is done via the build.Context file system interface,
+// which must be concurrency-safe.
 //
 func ForEachPackage(ctxt *build.Context, found func(importPath string, err error)) {
 	// We use a counting semaphore to limit
 	// the number of parallel calls to ReadDir.
 	sema := make(chan bool, 20)
 
+	ch := make(chan item)
+
 	var wg sync.WaitGroup
 	for _, root := range ctxt.SrcDirs() {
 		root := root
 		wg.Add(1)
 		go func() {
-			allPackages(ctxt, sema, root, found)
+			allPackages(ctxt, sema, root, ch)
 			wg.Done()
 		}()
 	}
-	wg.Wait()
+	go func() {
+		wg.Wait()
+		close(ch)
+	}()
+
+	// All calls to found occur in the caller's goroutine.
+	for i := range ch {
+		found(i.importPath, i.err)
+	}
 }
 
-func allPackages(ctxt *build.Context, sema chan bool, root string, found func(string, error)) {
+type item struct {
+	importPath string
+	err        error // (optional)
+}
+
+func allPackages(ctxt *build.Context, sema chan bool, root string, ch chan<- item) {
 	root = filepath.Clean(root) + string(os.PathSeparator)
 
 	var wg sync.WaitGroup
@@ -92,7 +104,7 @@ func allPackages(ctxt *build.Context, sema chan bool, root string, found func(st
 		files, err := ReadDir(ctxt, dir)
 		<-sema
 		if pkg != "" || err != nil {
-			found(pkg, err)
+			ch <- item{pkg, err}
 		}
 		for _, fi := range files {
 			fi := fi
diff --git a/refactor/rename/mvpkg.go b/refactor/rename/mvpkg.go
index 4d922f76fd..bf8edd6f5a 100644
--- a/refactor/rename/mvpkg.go
+++ b/refactor/rename/mvpkg.go
@@ -26,7 +26,6 @@ import (
 	"runtime"
 	"strconv"
 	"strings"
-	"sync"
 	"text/template"
 
 	"golang.org/x/tools/go/buildutil"
@@ -133,13 +132,12 @@ func srcDir(ctxt *build.Context, pkg string) (string, error) {
 // subpackages returns the set of packages in the given srcDir whose
 // import paths start with dir.
 func subpackages(ctxt *build.Context, srcDir string, dir string) map[string]bool {
-	var mu sync.Mutex
 	subs := map[string]bool{dir: true}
 
 	// Find all packages under srcDir whose import paths start with dir.
 	buildutil.ForEachPackage(ctxt, func(pkg string, err error) {
 		if err != nil {
-			log.Fatalf("unexpected error in ForEackPackage: %v", err)
+			log.Fatalf("unexpected error in ForEachPackage: %v", err)
 		}
 
 		if !strings.HasPrefix(pkg, path.Join(dir, "")) {
@@ -157,9 +155,7 @@ func subpackages(ctxt *build.Context, srcDir string, dir string) map[string]bool
 			return
 		}
 
-		mu.Lock()
 		subs[pkg] = true
-		mu.Unlock()
 	})
 
 	return subs
diff --git a/refactor/rename/mvpkg_test.go b/refactor/rename/mvpkg_test.go
index b9f29d6b6a..3c915b44a2 100644
--- a/refactor/rename/mvpkg_test.go
+++ b/refactor/rename/mvpkg_test.go
@@ -15,7 +15,6 @@ import (
 	"path/filepath"
 	"regexp"
 	"strings"
-	"sync"
 	"testing"
 
 	"golang.org/x/tools/go/buildutil"
@@ -212,7 +211,6 @@ var _ a.T
 	for _, test := range tests {
 		ctxt := test.ctxt
 
-		var mu sync.Mutex
 		got := make(map[string]string)
 		// Populate got with starting file set. rewriteFile and moveDirectory
 		// will mutate got to produce resulting file set.
@@ -225,19 +223,17 @@ var _ a.T
 				return
 			}
 			f, err := ctxt.OpenFile(path)
-			defer f.Close()
 			if err != nil {
 				t.Errorf("unexpected error opening file: %s", err)
 				return
 			}
 			bytes, err := ioutil.ReadAll(f)
+			f.Close()
 			if err != nil {
 				t.Errorf("unexpected error reading file: %s", err)
 				return
 			}
-			mu.Lock()
 			got[path] = string(bytes)
-			defer mu.Unlock()
 		})
 		rewriteFile = func(fset *token.FileSet, f *ast.File, orig string) error {
 			var out bytes.Buffer