rework CancelError into Cancel

That way, cancelling a context while checking the error is much simpler.
The context data already holds onto the cancel func, so this requires no
internal changes.

This renders the Browser.Shutdown API obsolete, even though it doesn't
do exactly the same. If we want Cancel to do a proper shutdown action
before cancelling a browser context and killing the process, we could do
that change within the cancel logic.

browser.go

@@ -7,8 +7,6 @@ import (
 	"os"
 	"sync/atomic"
 
-	"github.com/mailru/easyjson"
-
 	"github.com/chromedp/cdproto"
 	"github.com/chromedp/cdproto/cdp"
 	"github.com/chromedp/cdproto/runtime"
@@ -87,27 +85,6 @@ func NewBrowser(ctx context.Context, urlstr string, opts ...BrowserOption) (*Bro
 	return b, nil
 }
 
-// Shutdown shuts down the browser.
-func (b *Browser) Shutdown() error {
-	if b.conn != nil {
-		if err := b.send(cdproto.CommandBrowserClose, nil); err != nil {
-			b.errf("could not close browser: %v", err)
-		}
-		return b.conn.Close()
-	}
-	return nil
-}
-
-// send writes the supplied message and params.
-func (b *Browser) send(method cdproto.MethodType, params easyjson.RawMessage) error {
-	msg := &cdproto.Message{
-		ID:     atomic.AddInt64(&b.next, 1),
-		Method: method,
-		Params: params,
-	}
-	return b.conn.Write(msg)
-}
-
 func (b *Browser) newExecutorForTarget(ctx context.Context, targetID target.ID, sessionID target.SessionID) *Target {
 	if targetID == "" {
 		panic("empty target ID")

chromedp.go

@@ -133,11 +133,19 @@ func FromContext(ctx context.Context) *Context {
 	return c
 }
 
-func CancelError(ctx context.Context) error {
+// Cancel cancels a chromedp context, waits for its resources to be cleaned up,
+// and returns any error encountered during that process.
+//
+// Usually a "defer cancel()" will be enough for most use cases. This API is
+// useful if you want to catch underlying cancel errors, such as when a
+// temporary directory cannot be deleted.
+func Cancel(ctx context.Context) error {
 	c := FromContext(ctx)
 	if c == nil {
 		return ErrInvalidContext
 	}
+	c.cancel()
+	c.wg.Wait()
 	return c.cancelErr
 }
 

chromedp_test.go

@@ -28,7 +28,7 @@ var (
 func testAllocate(t *testing.T, path string) (_ context.Context, cancel func()) {
 	// Same browser, new tab; not needing to start new chrome browsers for
 	// each test gives a huge speed-up.
-	ctx, cancel_ := NewContext(browserCtx)
+	ctx, _ := NewContext(browserCtx)
 
 	// Only navigate if we want a path, otherwise leave the blank page.
 	if path != "" {
@@ -38,8 +38,7 @@ func testAllocate(t *testing.T, path string) (_ context.Context, cancel func())
 	}
 
 	cancelErr := func() {
-		cancel_() // not cancel(); that's the returned one
+		if err := Cancel(ctx); err != nil {
-		if err := CancelError(ctx); err != nil {
 			t.Error(err)
 		}
 	}
@@ -180,8 +179,7 @@ func TestCancelError(t *testing.T) {
 	if err := Run(ctx2); err != nil {
 		t.Fatal(err)
 	}
-	cancel2()
+	if err := Cancel(ctx2); err != nil {
-	if err := CancelError(ctx2); err != nil {
 		t.Fatalf("expected a nil error, got %v", err)
 	}
 
@@ -192,9 +190,7 @@ func TestCancelError(t *testing.T) {
 		t.Fatal(err)
 	}
 	FromContext(ctx3).Target.TargetID = "wrong"
-	cancel3()
+	if err := Cancel(ctx3); err == nil {
-	time.Sleep(time.Second)
-	if err := CancelError(ctx3); err == nil {
 		t.Fatalf("expected a non-nil error, got %v", err)
 	}
 }