Add exec support to client

This also fixes a deadlock in the shim's reaper where execs would lockup
and/or miss a quick exiting exec process's exit status.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

container_test.go

@@ -3,9 +3,14 @@ package containerd
 import (
 	"bytes"
 	"context"
+	"syscall"
 	"testing"
 )
 
+func empty() IOCreation {
+	return BufferedIO(bytes.NewBuffer(nil), bytes.NewBuffer(nil), bytes.NewBuffer(nil))
+}
+
 func TestContainerList(t *testing.T) {
 	if testing.Short() {
 		t.Skip()
@@ -64,7 +69,6 @@ func TestNewContainer(t *testing.T) {
 func TestContainerStart(t *testing.T) {
 	if testing.Short() {
 		t.Skip()
-		return
 	}
 	client, err := New(address)
 	if err != nil {
@@ -133,7 +137,6 @@ func TestContainerStart(t *testing.T) {
 func TestContainerOutput(t *testing.T) {
 	if testing.Short() {
 		t.Skip()
-		return
 	}
 	client, err := New(address)
 	if err != nil {
@@ -201,3 +204,88 @@ func TestContainerOutput(t *testing.T) {
 		t.Errorf("expected output %q but received %q", expected, actual)
 	}
 }
+
+func TestContainerExec(t *testing.T) {
+	if testing.Short() {
+		t.Skip()
+	}
+	client, err := New(address)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer client.Close()
+
+	var (
+		ctx = context.Background()
+		id  = "ContainerExec"
+	)
+	image, err := client.GetImage(ctx, testImage)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	spec, err := GenerateSpec(WithImageConfig(ctx, image), WithProcessArgs("sleep", "100"))
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	container, err := client.NewContainer(ctx, id, spec, WithImage(image), WithNewRootFS(id, image))
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	defer container.Delete(ctx)
+
+	task, err := container.NewTask(ctx, empty())
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	defer task.Delete(ctx)
+
+	finished := make(chan struct{}, 1)
+	go func() {
+		if _, err := task.Wait(ctx); err != nil {
+			t.Error(err)
+		}
+		close(finished)
+	}()
+
+	// start an exec process without running the original container process info
+	processSpec := spec.Process
+	processSpec.Args = []string{
+		"sh", "-c",
+		"exit 6",
+	}
+
+	process, err := task.Exec(ctx, &processSpec, empty())
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	processStatusC := make(chan uint32, 1)
+	go func() {
+		status, err := process.Wait(ctx)
+		if err != nil {
+			t.Error(err)
+			return
+		}
+		processStatusC <- status
+	}()
+
+	if err := process.Start(ctx); err != nil {
+		t.Error(err)
+		return
+	}
+
+	// wait for the exec to return
+	status := <-processStatusC
+
+	if status != 6 {
+		t.Errorf("expected exec exit code 6 but received %d", status)
+	}
+	if err := task.Kill(ctx, syscall.SIGKILL); err != nil {
+		t.Error(err)
+	}
+	<-finished
+}