don't create an extra tab when starting a browser

Chrome already starts with a blank page, so use that for the first
target context instead of creating a new tab.

Add the first version of the Targets API, which is useful to test this
feature.

Fixes #291.

allocate.go

@@ -124,6 +124,10 @@ func (p *ExecAllocator) Allocate(ctx context.Context) (*Browser, error) {
 		p.wg.Done()
 	}()
 
+	// force the first page to be blank, instead of the welcome page
+	// TODO: why isn't --no-first-run enough?
+	args = append(args, "about:blank")
+
 	cmd = exec.CommandContext(ctx, p.execPath, args...)
 	stderr, err := cmd.StderrPipe()
 	if err != nil {

context.go

@@ -65,7 +65,8 @@ func Run(ctx context.Context, actions ...Action) error {
 	if c == nil || c.Allocator == nil {
 		return ErrInvalidContext
 	}
-	if c.Browser == nil {
+	first := c.Browser == nil
+	if first {
 		browser, err := c.Allocator.Allocate(ctx)
 		if err != nil {
 			return err
@@ -73,24 +74,47 @@ func Run(ctx context.Context, actions ...Action) error {
 		c.Browser = browser
 	}
 	if c.Target == nil {
-		if err := c.newSession(ctx); err != nil {
+		if err := c.newSession(ctx, first); err != nil {
 			return err
 		}
 	}
 	return Tasks(actions).Do(ctx, c.Target)
 }
 
-func (c *Context) newSession(ctx context.Context) error {
+func (c *Context) newSession(ctx context.Context, first bool) error {
-	targetID, err := target.CreateTarget("about:blank").Do(ctx, c.Browser)
+	var targetID target.ID
+	if first {
+		// If we just allocated this browser, and it has a single page
+		// that's blank and not attached, use it.
+		infos, err := target.GetTargets().Do(ctx, c.Browser)
 		if err != nil {
 			return err
 		}
+		pages := 0
+		for _, info := range infos {
+			if info.Type == "page" && info.URL == "about:blank" && !info.Attached {
+				targetID = info.TargetID
+				pages++
+			}
+		}
+		if pages > 1 {
+			// Multiple blank pages; just in case, don't use any.
+			targetID = ""
+		}
+	}
+
+	if targetID == "" {
+		var err error
+		targetID, err = target.CreateTarget("about:blank").Do(ctx, c.Browser)
+		if err != nil {
+			return err
+		}
+	}
 
 	sessionID, err := target.AttachToTarget(targetID).Do(ctx, c.Browser)
 	if err != nil {
 		return err
 	}
-
 	c.Target = c.Browser.newExecutorForTarget(ctx, sessionID)
 
 	// enable domains
@@ -111,3 +135,22 @@ func (c *Context) newSession(ctx context.Context) error {
 }
 
 type ContextOption func(*Context)
+
+// Targets lists all the targets in the browser attached to the given context.
+func Targets(ctx context.Context) ([]*target.Info, error) {
+	// Don't rely on Run, as that needs to be able to call Targets, and we
+	// don't want cyclic func calls.
+
+	c := FromContext(ctx)
+	if c == nil || c.Allocator == nil {
+		return nil, ErrInvalidContext
+	}
+	if c.Browser == nil {
+		browser, err := c.Allocator.Allocate(ctx)
+		if err != nil {
+			return nil, err
+		}
+		c.Browser = browser
+	}
+	return target.GetTargets().Do(ctx, c.Browser)
+}

context_test.go

@@ -0,0 +1,55 @@
+package chromedp
+
+import (
+	"context"
+	"testing"
+)
+
+func TestTargets(t *testing.T) {
+	t.Parallel()
+
+	// Start one browser with one tab.
+	ctx1, cancel := NewContext(context.Background())
+	defer cancel()
+	if err := Run(ctx1); err != nil {
+		t.Fatal(err)
+	}
+
+	{
+		infos, err := Targets(ctx1)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if want, got := 1, len(infos); want != got {
+			t.Fatalf("want %d targets, got %d", want, got)
+		}
+	}
+
+	// Start a second tab on the same browser
+	ctx2, cancel := NewContext(ctx1)
+	defer cancel()
+	if err := Run(ctx2); err != nil {
+		t.Fatal(err)
+	}
+
+	{
+		infos, err := Targets(ctx2)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if want, got := 2, len(infos); want != got {
+			t.Fatalf("want %d targets, got %d", want, got)
+		}
+	}
+
+	// The first context should also see both targets.
+	{
+		infos, err := Targets(ctx1)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if want, got := 2, len(infos); want != got {
+			t.Fatalf("want %d targets, got %d", want, got)
+		}
+	}
+}