reuse same code for setting pipes in run/exec

This also moves `exec -i` test to _unix_test.go because it seems to need a
pty to reliably reproduce the behavior.

Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com>

integration-cli/docker_cli_exec_unix_test.go

@@ -0,0 +1,47 @@
+// +build !windows,!test_no_exec
+
+package main
+
+import (
+	"bytes"
+	"io"
+	"os/exec"
+	"strings"
+	"time"
+
+	"github.com/go-check/check"
+	"github.com/kr/pty"
+)
+
+// regression test for #12546
+func (s *DockerSuite) TestExecInteractiveStdinClose(c *check.C) {
+	out, _, err := runCommandWithOutput(exec.Command(dockerBinary, "run", "-itd", "busybox", "/bin/cat"))
+	if err != nil {
+		c.Fatal(err)
+	}
+	contId := strings.TrimSpace(out)
+
+	cmd := exec.Command(dockerBinary, "exec", "-i", contId, "echo", "-n", "hello")
+	p, err := pty.Start(cmd)
+	if err != nil {
+		c.Fatal(err)
+	}
+
+	b := bytes.NewBuffer(nil)
+	go io.Copy(b, p)
+
+	ch := make(chan error)
+	go func() { ch <- cmd.Wait() }()
+
+	select {
+	case err := <-ch:
+		if err != nil {
+			c.Errorf("cmd finished with error %v", err)
+		}
+		if output := b.String(); strings.TrimSpace(output) != "hello" {
+			c.Fatalf("Unexpected output %s", output)
+		}
+	case <-time.After(1 * time.Second):
+		c.Fatal("timed out running docker exec")
+	}
+}