libct/int: fix a data race

Fix for the following:

$ sudo go test -race -v ./libcontainer/integration/
=== RUN   TestUsernsCheckpoint
==================
WARNING: DATA RACE
Write at 0x00c00019e650 by goroutine 15:
  bytes.(*Buffer).ReadFrom()
      /usr/lib/golang/src/bytes/buffer.go:200 +0x48
  io.copyBuffer()
      /usr/lib/golang/src/io/io.go:395 +0x3fe
  io.Copy()
      /usr/lib/golang/src/io/io.go:368 +0x7a
  os/exec.(*Cmd).writerDescriptor.func1()
      /usr/lib/golang/src/os/exec/exec.go:311 +0x4a
  os/exec.(*Cmd).Start.func1()
      /usr/lib/golang/src/os/exec/exec.go:441 +0x38

Previous write at 0x00c00019e650 by goroutine 12:
  bytes.(*Buffer).ReadFrom()
      /usr/lib/golang/src/bytes/buffer.go:200 +0x48
  io.copyBuffer()
      /usr/lib/golang/src/io/io.go:395 +0x3fe
  io.Copy()
      /usr/lib/golang/src/io/io.go:368 +0x7a
  os/exec.(*Cmd).writerDescriptor.func1()
      /usr/lib/golang/src/os/exec/exec.go:311 +0x4a
  os/exec.(*Cmd).Start.func1()
      /usr/lib/golang/src/os/exec/exec.go:441 +0x38

Goroutine 15 (running) created at:
  os/exec.(*Cmd).Start()
      /usr/lib/golang/src/os/exec/exec.go:440 +0xaaa
  github.com/opencontainers/runc/libcontainer.(*linuxContainer).criuSwrk()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/container_linux.go:1490 +0x5f7
  github.com/opencontainers/runc/libcontainer.(*linuxContainer).Restore()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/container_linux.go:1398 +0x29ef
  github.com/opencontainers/runc/libcontainer/integration.testCheckpoint()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/integration/checkpoint_test.go:204 +0x1898
  github.com/opencontainers/runc/libcontainer/integration.TestUsernsCheckpoint()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/integration/checkpoint_test.go:48 +0x177
  testing.tRunner()
      /usr/lib/golang/src/testing/testing.go:1123 +0x202

Goroutine 12 (finished) created at:
  os/exec.(*Cmd).Start()
      /usr/lib/golang/src/os/exec/exec.go:440 +0xaaa
  github.com/opencontainers/runc/libcontainer.(*initProcess).start()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/process_linux.go:313 +0xda
  github.com/opencontainers/runc/libcontainer.(*linuxContainer).start()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/container_linux.go:366 +0xb0
  github.com/opencontainers/runc/libcontainer.(*linuxContainer).Start()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/container_linux.go:263 +0x27b
  github.com/opencontainers/runc/libcontainer.(*linuxContainer).Run()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/container_linux.go:273 +0x46
  github.com/opencontainers/runc/libcontainer/integration.testCheckpoint()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/integration/checkpoint_test.go:108 +0x941
  github.com/opencontainers/runc/libcontainer/integration.TestUsernsCheckpoint()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/integration/checkpoint_test.go:48 +0x177
  testing.tRunner()
      /usr/lib/golang/src/testing/testing.go:1123 +0x202
==================
==================
WARNING: DATA RACE
Read at 0x00c00019e630 by goroutine 15:
  bytes.(*Buffer).Len()
      /usr/lib/golang/src/bytes/buffer.go:73 +0x58
  bytes.(*Buffer).grow()
      /usr/lib/golang/src/bytes/buffer.go:118 +0x4f
  bytes.(*Buffer).ReadFrom()
      /usr/lib/golang/src/bytes/buffer.go:202 +0x7c
  io.copyBuffer()
      /usr/lib/golang/src/io/io.go:395 +0x3fe
  io.Copy()
      /usr/lib/golang/src/io/io.go:368 +0x7a
  os/exec.(*Cmd).writerDescriptor.func1()
      /usr/lib/golang/src/os/exec/exec.go:311 +0x4a
  os/exec.(*Cmd).Start.func1()
      /usr/lib/golang/src/os/exec/exec.go:441 +0x38

Previous write at 0x00c00019e630 by goroutine 12:
  bytes.(*Buffer).ReadFrom()
      /usr/lib/golang/src/bytes/buffer.go:209 +0x1c4
  io.copyBuffer()
      /usr/lib/golang/src/io/io.go:395 +0x3fe
  io.Copy()
      /usr/lib/golang/src/io/io.go:368 +0x7a
  os/exec.(*Cmd).writerDescriptor.func1()
      /usr/lib/golang/src/os/exec/exec.go:311 +0x4a
  os/exec.(*Cmd).Start.func1()
      /usr/lib/golang/src/os/exec/exec.go:441 +0x38

Goroutine 15 (running) created at:
  os/exec.(*Cmd).Start()
      /usr/lib/golang/src/os/exec/exec.go:440 +0xaaa
  github.com/opencontainers/runc/libcontainer.(*linuxContainer).criuSwrk()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/container_linux.go:1490 +0x5f7
  github.com/opencontainers/runc/libcontainer.(*linuxContainer).Restore()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/container_linux.go:1398 +0x29ef
  github.com/opencontainers/runc/libcontainer/integration.testCheckpoint()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/integration/checkpoint_test.go:204 +0x1898
  github.com/opencontainers/runc/libcontainer/integration.TestUsernsCheckpoint()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/integration/checkpoint_test.go:48 +0x177
  testing.tRunner()
      /usr/lib/golang/src/testing/testing.go:1123 +0x202

Goroutine 12 (finished) created at:
  os/exec.(*Cmd).Start()
      /usr/lib/golang/src/os/exec/exec.go:440 +0xaaa
  github.com/opencontainers/runc/libcontainer.(*initProcess).start()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/process_linux.go:313 +0xda
  github.com/opencontainers/runc/libcontainer.(*linuxContainer).start()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/container_linux.go:366 +0xb0
  github.com/opencontainers/runc/libcontainer.(*linuxContainer).Start()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/container_linux.go:263 +0x27b
  github.com/opencontainers/runc/libcontainer.(*linuxContainer).Run()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/container_linux.go:273 +0x46
  github.com/opencontainers/runc/libcontainer/integration.testCheckpoint()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/integration/checkpoint_test.go:108 +0x941
  github.com/opencontainers/runc/libcontainer/integration.TestUsernsCheckpoint()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/integration/checkpoint_test.go:48 +0x177
  testing.tRunner()
      /usr/lib/golang/src/testing/testing.go:1123 +0x202
==================
==================
WARNING: DATA RACE
Read at 0x00c00019e648 by goroutine 15:
  bytes.(*Buffer).Len()
      /usr/lib/golang/src/bytes/buffer.go:73 +0x7e
  bytes.(*Buffer).grow()
      /usr/lib/golang/src/bytes/buffer.go:118 +0x4f
  bytes.(*Buffer).ReadFrom()
      /usr/lib/golang/src/bytes/buffer.go:202 +0x7c
  io.copyBuffer()
      /usr/lib/golang/src/io/io.go:395 +0x3fe
  io.Copy()
      /usr/lib/golang/src/io/io.go:368 +0x7a
  os/exec.(*Cmd).writerDescriptor.func1()
      /usr/lib/golang/src/os/exec/exec.go:311 +0x4a
  os/exec.(*Cmd).Start.func1()
      /usr/lib/golang/src/os/exec/exec.go:441 +0x38

Previous write at 0x00c00019e648 by goroutine 12:
  bytes.(*Buffer).grow()
      /usr/lib/golang/src/bytes/buffer.go:147 +0x2d7
  bytes.(*Buffer).ReadFrom()
      /usr/lib/golang/src/bytes/buffer.go:202 +0x7c
  io.copyBuffer()
      /usr/lib/golang/src/io/io.go:395 +0x3fe
  io.Copy()
      /usr/lib/golang/src/io/io.go:368 +0x7a
  os/exec.(*Cmd).writerDescriptor.func1()
      /usr/lib/golang/src/os/exec/exec.go:311 +0x4a
  os/exec.(*Cmd).Start.func1()
      /usr/lib/golang/src/os/exec/exec.go:441 +0x38

Goroutine 15 (running) created at:
  os/exec.(*Cmd).Start()
      /usr/lib/golang/src/os/exec/exec.go:440 +0xaaa
  github.com/opencontainers/runc/libcontainer.(*linuxContainer).criuSwrk()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/container_linux.go:1490 +0x5f7
  github.com/opencontainers/runc/libcontainer.(*linuxContainer).Restore()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/container_linux.go:1398 +0x29ef
  github.com/opencontainers/runc/libcontainer/integration.testCheckpoint()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/integration/checkpoint_test.go:204 +0x1898
  github.com/opencontainers/runc/libcontainer/integration.TestUsernsCheckpoint()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/integration/checkpoint_test.go:48 +0x177
  testing.tRunner()
      /usr/lib/golang/src/testing/testing.go:1123 +0x202

Goroutine 12 (finished) created at:
  os/exec.(*Cmd).Start()
      /usr/lib/golang/src/os/exec/exec.go:440 +0xaaa
  github.com/opencontainers/runc/libcontainer.(*initProcess).start()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/process_linux.go:313 +0xda
  github.com/opencontainers/runc/libcontainer.(*linuxContainer).start()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/container_linux.go:366 +0xb0
  github.com/opencontainers/runc/libcontainer.(*linuxContainer).Start()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/container_linux.go:263 +0x27b
  github.com/opencontainers/runc/libcontainer.(*linuxContainer).Run()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/container_linux.go:273 +0x46
  github.com/opencontainers/runc/libcontainer/integration.testCheckpoint()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/integration/checkpoint_test.go:108 +0x941
  github.com/opencontainers/runc/libcontainer/integration.TestUsernsCheckpoint()
      /home/kir/go/src/github.com/opencontainers/runc/libcontainer/integration/checkpoint_test.go:48 +0x177
  testing.tRunner()
      /usr/lib/golang/src/testing/testing.go:1123 +0x202
==================
    testing.go:1038: race detected during execution of test
--- FAIL: TestUsernsCheckpoint (0.62s)

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/integration/checkpoint_test.go

@@ -194,10 +194,11 @@ func testCheckpoint(t *testing.T, userns bool) {
 		t.Fatal(err)
 	}
 
+	var restoreStdout bytes.Buffer
 	restoreProcessConfig := &libcontainer.Process{
 		Cwd:    "/",
 		Stdin:  restoreStdinR,
-		Stdout: &stdout,
+		Stdout: &restoreStdout,
 		Init:   true,
 	}
 
@@ -242,7 +243,7 @@ func testCheckpoint(t *testing.T, userns bool) {
 		t.Fatal(s.String(), pid)
 	}
 
-	output := stdout.String()
+	output := restoreStdout.String()
 	if !strings.Contains(output, "Hello!") {
 		t.Fatal("Did not restore the pipe correctly:", output)
 	}