diff --git a/batcher/batcher_test.go b/batcher/batcher_test.go
index 80a355d..15ca7d3 100644
--- a/batcher/batcher_test.go
+++ b/batcher/batcher_test.go
@@ -7,11 +7,14 @@ import (
 	"time"
 
 	"github.com/stretchr/testify/require"
+	"go.uber.org/goleak"
 
 	"gitlab.com/slon/shad-go/batcher/slow"
 )
 
 func TestSimple(t *testing.T) {
+	defer goleak.VerifyNone(t)
+
 	var value slow.Value
 	b := NewBatcher(&value)
 
@@ -25,6 +28,8 @@ func TestSimple(t *testing.T) {
 }
 
 func TestStaleRead(t *testing.T) {
+	defer goleak.VerifyNone(t)
+
 	const (
 		N = 100
 		K = 100
@@ -69,6 +74,8 @@ func TestStaleRead(t *testing.T) {
 }
 
 func TestSpeed(t *testing.T) {
+	defer goleak.VerifyNone(t)
+
 	const (
 		N = 100
 		K = 200
diff --git a/digitalclock/main_test.go b/digitalclock/main_test.go
index 83b83d8..b5b70cc 100644
--- a/digitalclock/main_test.go
+++ b/digitalclock/main_test.go
@@ -57,7 +57,7 @@ func startServer(t *testing.T) (port string, stop func()) {
 		<-done
 	}
 
-	if err = testtool.WaitForPort(time.Second*30, port); err != nil {
+	if err = testtool.WaitForPort(t, time.Second*30, port); err != nil {
 		stop()
 	}
 
diff --git a/fetchall/main_test.go b/fetchall/main_test.go
index 71c3855..2dca656 100644
--- a/fetchall/main_test.go
+++ b/fetchall/main_test.go
@@ -35,7 +35,7 @@ func TestMain(m *testing.M) {
 
 func TestFetchall_valid(t *testing.T) {
 	binary, err := binCache.GetBinary(fetchallImportPath)
-	require.Nil(t, err)
+	require.NoError(t, err)
 
 	type endpoint string
 
@@ -73,14 +73,14 @@ func TestFetchall_valid(t *testing.T) {
 			cmd.Stdout = nil
 			cmd.Stderr = os.Stderr
 
-			require.Nil(t, cmd.Run())
+			require.NoError(t, cmd.Run())
 		})
 	}
 }
 
 func TestFetchall_multipleURLs(t *testing.T) {
 	binary, err := binCache.GetBinary(fetchallImportPath)
-	require.Nil(t, err)
+	require.NoError(t, err)
 
 	var fooHit, barHit int32
 
@@ -101,7 +101,7 @@ func TestFetchall_multipleURLs(t *testing.T) {
 	cmd.Stdout = nil
 	cmd.Stderr = os.Stderr
 
-	require.Nil(t, cmd.Run())
+	require.NoError(t, cmd.Run())
 
 	require.Equal(t, int32(1), atomic.LoadInt32(&fooHit))
 	require.Equal(t, int32(1), atomic.LoadInt32(&barHit))
@@ -109,7 +109,7 @@ func TestFetchall_multipleURLs(t *testing.T) {
 
 func TestFetchall_malformed(t *testing.T) {
 	binary, err := binCache.GetBinary(fetchallImportPath)
-	require.Nil(t, err)
+	require.NoError(t, err)
 
 	hit := int32(0)
 	s := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
@@ -123,14 +123,14 @@ func TestFetchall_malformed(t *testing.T) {
 	cmd.Stderr = os.Stderr
 
 	err = cmd.Run()
-	require.Nil(t, err)
+	require.NoError(t, err)
 
 	require.True(t, atomic.LoadInt32(&hit) >= 2)
 }
 
 func TestFetchall_concurrency(t *testing.T) {
 	binary, err := binCache.GetBinary(fetchallImportPath)
-	require.Nil(t, err)
+	require.NoError(t, err)
 
 	var mu sync.Mutex
 	var callOrder []time.Duration
@@ -140,7 +140,7 @@ func TestFetchall_concurrency(t *testing.T) {
 		require.NotEmpty(t, s)
 
 		d, err := time.ParseDuration(s)
-		require.Nil(t, err)
+		require.NoError(t, err)
 
 		time.Sleep(d)
 
@@ -165,7 +165,7 @@ func TestFetchall_concurrency(t *testing.T) {
 	cmd.Stdout = nil
 	cmd.Stderr = os.Stderr
 
-	require.Nil(t, cmd.Run())
+	require.NoError(t, cmd.Run())
 
 	mu.Lock()
 	defer mu.Unlock()
diff --git a/go.mod b/go.mod
index 029fc61..78dbe0c 100644
--- a/go.mod
+++ b/go.mod
@@ -5,6 +5,7 @@ go 1.13
 require (
 	github.com/spf13/cobra v0.0.5
 	github.com/stretchr/testify v1.4.0
+	go.uber.org/goleak v1.0.0
 	golang.org/x/tools v0.0.0-20200125223703-d33eef8e6825
 	gopkg.in/yaml.v2 v2.2.8
 )
diff --git a/go.sum b/go.sum
index 4094bc3..25e688a 100644
--- a/go.sum
+++ b/go.sum
@@ -11,6 +11,9 @@ github.com/fsnotify/fsnotify v1.4.7/go.mod h1:jwhsz4b93w/PPRr/qN1Yymfu8t87LnFCMo
 github.com/hashicorp/hcl v1.0.0/go.mod h1:E5yfLk+7swimpb2L/Alb/PJmXilQ/rhwaUYs4T20WEQ=
 github.com/inconshreveable/mousetrap v1.0.0 h1:Z8tu5sraLXCXIcARxBp/8cbvlwVa7Z1NHg9XEKhtSvM=
 github.com/inconshreveable/mousetrap v1.0.0/go.mod h1:PxqpIevigyE2G7u3NXJIT2ANytuPF1OarO4DADm73n8=
+github.com/kr/pretty v0.1.0/go.mod h1:dAy3ld7l9f0ibDNOQOHHMYYIIbhfbHSm3C4ZsoJORNo=
+github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
+github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
 github.com/magiconair/properties v1.8.0/go.mod h1:PppfXfuXeibc/6YijjN8zIbojt8czPbwD3XqdrwzmxQ=
 github.com/mitchellh/go-homedir v1.1.0/go.mod h1:SfyaCUpYCn1Vlf4IUYiD9fPX4A5wJrkLzIz1N1q0pr0=
 github.com/mitchellh/mapstructure v1.1.2/go.mod h1:FVVH3fgwuzCH5S8UJGiWEs2h04kUh9fWfEaFds41c1Y=
@@ -33,10 +36,15 @@ github.com/stretchr/testify v1.4.0 h1:2E4SXV/wtOkTonXsotYi4li6zVWxYlZuYNCXe9XRJy
 github.com/stretchr/testify v1.4.0/go.mod h1:j7eGeouHqKxXV5pUuKE4zz7dFj8WfuZ+81PSLYec5m4=
 github.com/ugorji/go/codec v0.0.0-20181204163529-d75b2dcb6bc8/go.mod h1:VFNgLljTbGfSG7qAOspJ7OScBnGdDN/yBr0sguwnwf0=
 github.com/xordataexchange/crypt v0.0.3-0.20170626215501-b2862e3d0a77/go.mod h1:aYKd//L2LvnjZzWKhF00oedf4jCCReLcmhLdhm1A27Q=
+go.uber.org/goleak v1.0.0 h1:qsup4IcBdlmsnGfqyLl4Ntn3C2XCCuKAE7DwHpScyUo=
+go.uber.org/goleak v1.0.0/go.mod h1:8a7PlsEVH3e/a/GLqe5IIrQx6GzcnRmZEufDUTk4A7A=
 golang.org/x/crypto v0.0.0-20181203042331-505ab145d0a9/go.mod h1:6SG95UA2DQfeDnfUPMdvaQW0Q7yPrPDi9nlGo2tz2b4=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
 golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550/go.mod h1:yigFU9vqHzYiE8UmvKecakEJjdnWj3jj499lnFckfCI=
+golang.org/x/lint v0.0.0-20190930215403-16217165b5de h1:5hukYrvBGR8/eNkX5mdUezrA6JiaEZDtJb9Ei+1LlBs=
+golang.org/x/lint v0.0.0-20190930215403-16217165b5de/go.mod h1:6SW0HCj/g11FgYtHlgUYUwCkIfeOF89ocIRzGO/8vkc=
 golang.org/x/mod v0.1.1-0.20191105210325-c90efee705ee/go.mod h1:QqPTAvyqsEbceGzBzNggFXnrqF1CaUcvgkdR5Ot7KZg=
+golang.org/x/net v0.0.0-20190311183353-d8887717615a/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
 golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
 golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
 golang.org/x/sync v0.0.0-20190423024810-112230192c58 h1:8gQV6CLnAEikrhgkHFbMAEhagSSnXWGV915qUMm9mrU=
@@ -45,12 +53,16 @@ golang.org/x/sys v0.0.0-20181205085412-a5c9d58dba9a/go.mod h1:STP8DvDyc/dI5b8T5h
 golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+golang.org/x/tools v0.0.0-20190311212946-11955173bddd/go.mod h1:LCzVGOaR6xXOjkQ3onu1FJEFr0SW1gC7cKk1uF8kGRs=
+golang.org/x/tools v0.0.0-20191108193012-7d206e10da11/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
 golang.org/x/tools v0.0.0-20200125223703-d33eef8e6825 h1:aNQeSIHKi0RWpKA5NO0CqyLjx6Beh5l0LLUEnndEjz0=
 golang.org/x/tools v0.0.0-20200125223703-d33eef8e6825/go.mod h1:TB2adYChydJhpapKDTa4BR/hXlZSLoq2Wpct/0txZ28=
+golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898 h1:/atklqdjdhuosWIl6AIbOeHJjicWYPqR9bpxqxYG2pA=
 golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/yaml.v2 v2.2.2 h1:ZCJp+EgiOT7lHqUV2J862kp8Qj64Jo6az82+3Td9dZw=
 gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
 gopkg.in/yaml.v2 v2.2.8 h1:obN1ZagJSUGI0Ek/LBmuj4SNLPfIny3KsKFopxRdj10=
diff --git a/tools/testtool/bincache.go b/tools/testtool/bincache.go
index 24ae2ca..aa60c01 100644
--- a/tools/testtool/bincache.go
+++ b/tools/testtool/bincache.go
@@ -10,6 +10,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"runtime"
 	"sync"
 )
 
@@ -107,5 +108,9 @@ func (c *ciBuildCache) GetBinary(importPath string) (string, error) {
 func RandomName() string {
 	var raw [8]byte
 	_, _ = rand.Read(raw[:])
-	return hex.EncodeToString(raw[:])
+	name := hex.EncodeToString(raw[:])
+	if runtime.GOOS == "windows" {
+		name += ".exe"
+	}
+	return name
 }
diff --git a/tools/testtool/commands/test_submission_test.go b/tools/testtool/commands/test_submission_test.go
index 6031b74..8a64296 100644
--- a/tools/testtool/commands/test_submission_test.go
+++ b/tools/testtool/commands/test_submission_test.go
@@ -29,11 +29,11 @@ func listDirs(dir string) ([]string, error) {
 
 func Test_testSubmission_correct(t *testing.T) {
 	testDirs, err := listDirs("../testdata/submissions/correct")
-	require.Nil(t, err)
+	require.NoError(t, err)
 
 	for _, dir := range testDirs {
 		absDir, err := filepath.Abs(dir)
-		require.Nil(t, err)
+		require.NoError(t, err)
 		problem := path.Base(absDir)
 		t.Run(problem, func(t *testing.T) {
 			studentRepo := path.Join(absDir, "student")
@@ -46,11 +46,11 @@ func Test_testSubmission_correct(t *testing.T) {
 
 func Test_testSubmission_incorrect(t *testing.T) {
 	testDirs, err := listDirs("../testdata/submissions/incorrect")
-	require.Nil(t, err)
+	require.NoError(t, err)
 
 	for _, dir := range testDirs {
 		absDir, err := filepath.Abs(dir)
-		require.Nil(t, err)
+		require.NoError(t, err)
 
 		problem := path.Base(absDir)
 		t.Run(problem, func(t *testing.T) {
diff --git a/tools/testtool/freeport.go b/tools/testtool/freeport.go
index b31ef87..84a9fde 100644
--- a/tools/testtool/freeport.go
+++ b/tools/testtool/freeport.go
@@ -3,7 +3,6 @@ package testtool
 import (
 	"fmt"
 	"net"
-	"os"
 	"strconv"
 	"time"
 )
@@ -26,10 +25,14 @@ func GetFreePort() (string, error) {
 	return strconv.Itoa(p), nil
 }
 
+type logger interface {
+	Logf(format string, args ...interface{})
+}
+
 // WaitForPort tries to connect to given local port with constant backoff.
 //
 // Returns error if port is not ready after timeout.
-func WaitForPort(timeout time.Duration, port string) error {
+func WaitForPort(l logger, timeout time.Duration, port string) error {
 	stopTimer := time.NewTimer(timeout)
 	defer stopTimer.Stop()
 
@@ -39,10 +42,10 @@ func WaitForPort(timeout time.Duration, port string) error {
 	for {
 		select {
 		case <-stopTimer.C:
-			return fmt.Errorf("timeout")
+			return fmt.Errorf("no server started listening on port %s after timeout %s", port, timeout)
 		case <-t.C:
 			if err := portIsReady(port); err != nil {
-				_, _ = fmt.Fprintf(os.Stderr, "waiting for port: %s\n", err)
+				l.Logf("waiting for port: %s\n", err)
 				break
 			}
 			return nil
diff --git a/tools/testtool/freeport_test.go b/tools/testtool/freeport_test.go
index f9cbbbb..2189fb7 100644
--- a/tools/testtool/freeport_test.go
+++ b/tools/testtool/freeport_test.go
@@ -26,12 +26,14 @@ func TestWaitForPort(t *testing.T) {
 	_, port, err := net.SplitHostPort(u.Host)
 	require.Nil(t, err)
 
-	require.NoError(t, WaitForPort(time.Second, port))
+	require.NoError(t, WaitForPort(t, time.Second, port))
 }
 
 func TestWaitForPort_timeout(t *testing.T) {
 	p, err := GetFreePort()
 	require.NoError(t, err)
 
-	require.Error(t, WaitForPort(time.Second, p))
+	err = WaitForPort(t, time.Second, p)
+	require.Error(t, err)
+	t.Log(err.Error())
 }
diff --git a/urlfetch/main_test.go b/urlfetch/main_test.go
index 99b441d..dc5b763 100644
--- a/urlfetch/main_test.go
+++ b/urlfetch/main_test.go
@@ -33,7 +33,7 @@ func TestMain(m *testing.M) {
 
 func TestUrlFetch_valid(t *testing.T) {
 	binary, err := binCache.GetBinary(urlfetchImportPath)
-	require.Nil(t, err)
+	require.NoError(t, err)
 
 	type endpoint string
 	type response string
@@ -91,7 +91,7 @@ func TestUrlFetch_valid(t *testing.T) {
 			cmd.Stderr = os.Stderr
 
 			data, err := cmd.Output()
-			require.Nil(t, err)
+			require.NoError(t, err)
 
 			for _, r := range tc.expected {
 				require.True(t, bytes.Contains(data, []byte(r)),
@@ -104,7 +104,7 @@ func TestUrlFetch_valid(t *testing.T) {
 
 func TestUrlFetch_malformed(t *testing.T) {
 	binary, err := binCache.GetBinary(urlfetchImportPath)
-	require.Nil(t, err)
+	require.NoError(t, err)
 
 	s := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		_, _ = w.Write([]byte("success"))
@@ -141,7 +141,7 @@ func TestUrlFetch_malformed(t *testing.T) {
 
 func TestUrlFetch_order(t *testing.T) {
 	binary, err := binCache.GetBinary(urlfetchImportPath)
-	require.Nil(t, err)
+	require.NoError(t, err)
 
 	mux := http.NewServeMux()
 
@@ -175,7 +175,7 @@ func TestUrlFetch_order(t *testing.T) {
 	cmd.Stdout = nil
 	cmd.Stderr = os.Stderr
 
-	require.Nil(t, cmd.Run())
+	require.NoError(t, cmd.Run())
 
 	mu.Lock()
 	defer mu.Unlock()
diff --git a/wordcount/main_test.go b/wordcount/main_test.go
index e2413dd..179c227 100644
--- a/wordcount/main_test.go
+++ b/wordcount/main_test.go
@@ -33,7 +33,7 @@ func TestMain(m *testing.M) {
 
 func TestWordCount(t *testing.T) {
 	binary, err := binCache.GetBinary(wordcountImportPath)
-	require.Nil(t, err)
+	require.NoError(t, err)
 
 	type counts map[string]int64
 	type files []string
@@ -77,7 +77,7 @@ b`,
 		t.Run(tc.name, func(t *testing.T) {
 			// Create temp directory.
 			testDir, err := ioutil.TempDir("", "wordcount-testdata-")
-			require.Nil(t, err)
+			require.NoError(t, err)
 			defer func() { _ = os.RemoveAll(testDir) }()
 
 			// Create test files in temp directory.
@@ -85,7 +85,7 @@ b`,
 			for _, f := range tc.files {
 				file := path.Join(testDir, testtool.RandomName())
 				err = ioutil.WriteFile(file, []byte(f), 0644)
-				require.Nil(t, err)
+				require.NoError(t, err)
 				files = append(files, file)
 			}
 
@@ -94,11 +94,11 @@ b`,
 			cmd.Stderr = os.Stderr
 
 			output, err := cmd.Output()
-			require.Nil(t, err)
+			require.NoError(t, err)
 
 			// Parse output and compare with an expected one.
 			counts, err := parseStdout(output)
-			require.Nil(t, err)
+			require.NoError(t, err)
 
 			require.True(t, reflect.DeepEqual(tc.expected, counts),
 				fmt.Sprintf("expected: %v; got: %v", tc.expected, counts))