cmd/gogio: remove sleeps in x11 e2e test

We can instead synchronize with the gio app via stdout. We need three
states, since we need to first invalidate a frame and then print when
the next frame is drawn.

This is not happening on the JS test yet, because stdout printing
crashes in that case. See the comment.

This change should make the X11 test a bit faster on fast machines,
while making it more stable in small or headless machines like CI.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>

cmd/gogio/x11_test.go

@@ -7,6 +7,7 @@
 package main_test
 
 import (
+	"bufio"
 	"bytes"
 	"context"
 	"fmt"
@@ -26,10 +27,13 @@ import (
 type X11TestDriver struct {
 	t *testing.T
 
+	frameNotifs chan bool
+
 	display string
 }
 
 func (d *X11TestDriver) Start(t_ *testing.T, path string, width, height int) (cleanups []func()) {
+	d.frameNotifs = make(chan bool, 1)
 	d.t = t_
 
 	// Pick a random display number between 1 and 100,000. Most machines
@@ -125,10 +129,14 @@ func (d *X11TestDriver) Start(t_ *testing.T, path string, width, height int) (cl
 	{
 		ctx, cancel := context.WithCancel(context.Background())
 		cmd := exec.CommandContext(ctx, bin)
-		out := &bytes.Buffer{}
 		cmd.Env = []string{"DISPLAY=" + d.display}
-		cmd.Stdout = out
-		cmd.Stderr = out
+		stdout, err := cmd.StdoutPipe()
+		if err != nil {
+			d.t.Fatal(err)
+		}
+		stderr := &bytes.Buffer{}
+		cmd.Stderr = stderr
+
 		if err := cmd.Start(); err != nil {
 			d.t.Fatal(err)
 		}
@@ -136,17 +144,26 @@ func (d *X11TestDriver) Start(t_ *testing.T, path string, width, height int) (cl
 		wg.Add(1)
 		go func() {
 			if err := cmd.Wait(); err != nil && ctx.Err() == nil {
-				// Print all output and error.
-				io.Copy(os.Stdout, out)
+				// Print stderr and error.
+				io.Copy(os.Stdout, stderr)
 				d.t.Error(err)
 			}
 			wg.Done()
 		}()
+		go func() {
+			scanner := bufio.NewScanner(stdout)
+			for scanner.Scan() {
+				line := scanner.Text()
+				if line == "frame ready" {
+					d.frameNotifs <- true
+				}
+			}
+		}()
+
 	}
 
 	// Wait for the gio app to render.
-	// TODO(mvdan): synchronize with the app instead
-	time.Sleep(400 * time.Millisecond)
+	<-d.frameNotifs
 
 	return cleanups
 }
@@ -183,8 +200,7 @@ func (d *X11TestDriver) Click(x, y int) {
 	d.xdotool("mousemove", x, y)
 	d.xdotool("click", "1")
 
-	// TODO(mvdan): synchronize with the app instead
-	time.Sleep(200 * time.Millisecond)
+	<-d.frameNotifs
 }
 
 func TestX11(t *testing.T) {