testutil/daemon: fail gracefully if DEST is unset

If neither of the DOCKER_INTEGRATION_DAEMON_DEST or DEST environment
variables are set, integration tests panic with a nil-dereference panic
in os.(*File).Name(...). This is a very unhelpful behaviour for someone
trying to run integration tests interactively. Fix up the logic to avoid
dereferencing nil os.File pointers and instead fail the test immediately
with an actionable error message.

Signed-off-by: Cory Snider <csnider@mirantis.com>

Author: corhere

testutil/daemon/daemon.go

@@ -216,10 +216,9 @@ func New(t testing.TB, ops ...Option) *Daemon {
 	if dest == "" {
 		dest = os.Getenv("DEST")
 	}
+	assert.Assert(t, dest != "", "Please set the DOCKER_INTEGRATION_DAEMON_DEST or the DEST environment variable")
 	dest = filepath.Join(dest, t.Name())
 
-	assert.Check(t, dest != "", "Please set the DOCKER_INTEGRATION_DAEMON_DEST or the DEST environment variable")
-
 	if os.Getenv("DOCKER_ROOTLESS") != "" {
 		if os.Getenv("DOCKER_REMAP_ROOT") != "" {
 			t.Skip("DOCKER_ROOTLESS doesn't support DOCKER_REMAP_ROOT currently")
@@ -412,6 +411,9 @@ func (d *Daemon) ScanLogs(ctx context.Context, match func(s string) bool) (bool,
 
 // TailLogs tails N lines from the daemon logs
 func (d *Daemon) TailLogs(n int) ([][]byte, error) {
+	if d.logFile == nil {
+		return nil, errors.New("d.logFile is nil")
+	}
 	logF, err := os.Open(d.logFile.Name())
 	if err != nil {
 		return nil, errors.Wrap(err, "error opening daemon log file after failed start")