Add FifoIO to expose fifos directly to client

This allows clients an easier way to interact with the fifos for a
container without having to use the built in copyIO functions when
opening fifos.

It's nothing that clients could not have already coded but since we use
this type of functionality in the tests it makes sense to add an
implementation here.

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

container_linux_test.go

@@ -3,7 +3,12 @@
 package containerd
 
 import (
+	"bytes"
 	"context"
+	"fmt"
+	"io"
+	"runtime"
+	"sync"
 	"syscall"
 	"testing"
 	"time"
@@ -256,3 +261,122 @@ func TestDaemonRestart(t *testing.T) {
 
 	<-statusC
 }
+
+func TestContainerAttach(t *testing.T) {
+	t.Parallel()
+
+	if runtime.GOOS == "windows" {
+		// On windows, closing the write side of the pipe closes the read
+		// side, sending an EOF to it and preventing reopening it.
+		// Hence this test will always fails on windows
+		t.Skip("invalid logic on windows")
+	}
+
+	client, err := newClient(t, address)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer client.Close()
+
+	var (
+		image       Image
+		ctx, cancel = testContext()
+		id          = t.Name()
+	)
+	defer cancel()
+
+	if runtime.GOOS != "windows" {
+		image, err = client.GetImage(ctx, testImage)
+		if err != nil {
+			t.Error(err)
+			return
+		}
+	}
+
+	spec, err := generateSpec(withImageConfig(ctx, image), withCat())
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	container, err := client.NewContainer(ctx, id, WithSpec(spec), withNewSnapshot(id, image))
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	defer container.Delete(ctx, WithSnapshotCleanup)
+
+	expected := "hello" + newLine
+
+	direct, err := NewDirectIO(ctx, false)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	defer direct.Delete()
+	var (
+		wg  sync.WaitGroup
+		buf = bytes.NewBuffer(nil)
+	)
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		io.Copy(buf, direct.Stdout)
+	}()
+
+	task, err := container.NewTask(ctx, direct.IOCreate)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	defer task.Delete(ctx)
+
+	status, err := task.Wait(ctx)
+	if err != nil {
+		t.Error(err)
+	}
+
+	if err := task.Start(ctx); err != nil {
+		t.Error(err)
+		return
+	}
+
+	if _, err := fmt.Fprint(direct.Stdin, expected); err != nil {
+		t.Error(err)
+	}
+
+	// load the container and re-load the task
+	if container, err = client.LoadContainer(ctx, id); err != nil {
+		t.Error(err)
+		return
+	}
+
+	if task, err = container.Task(ctx, direct.IOAttach); err != nil {
+		t.Error(err)
+		return
+	}
+
+	if _, err := fmt.Fprint(direct.Stdin, expected); err != nil {
+		t.Error(err)
+	}
+
+	direct.Stdin.Close()
+
+	if err := task.CloseIO(ctx, WithStdinCloser); err != nil {
+		t.Error(err)
+	}
+
+	<-status
+
+	wg.Wait()
+	if _, err := task.Delete(ctx); err != nil {
+		t.Error(err)
+	}
+
+	output := buf.String()
+
+	// we wrote the same thing after attach
+	expected = expected + expected
+	if output != expected {
+		t.Errorf("expected output %q but received %q", expected, output)
+	}
+}