diff --git a/integration-tests/config.go.in b/integration-tests/config.go.in
index 3cc4766c..3d79297e 100644
--- a/integration-tests/config.go.in
+++ b/integration-tests/config.go.in
@@ -1,6 +1,6 @@
 package nghttp2
 
 const (
-	buildDir = "@top_builddir@"
+	buildDir  = "@top_builddir@"
 	sourceDir = "@top_srcdir@"
 )
diff --git a/integration-tests/nghttpx_http1_test.go b/integration-tests/nghttpx_http1_test.go
index 9479d585..a083f0e6 100644
--- a/integration-tests/nghttpx_http1_test.go
+++ b/integration-tests/nghttpx_http1_test.go
@@ -99,6 +99,8 @@ func TestH1H1MultipleRequestCL(t *testing.T) {
 		t.Fatalf("Error http.ReadResponse() = %v", err)
 	}
 
+	defer resp.Body.Close()
+
 	if got, want := resp.StatusCode, http.StatusBadRequest; got != want {
 		t.Errorf("status: %v; want %v", got, want)
 	}
@@ -196,7 +198,10 @@ func TestH1H1GracefulShutdown(t *testing.T) {
 		t.Errorf("status: %v; want %v", got, want)
 	}
 
-	st.cmd.Process.Signal(syscall.SIGQUIT)
+	if err := st.cmd.Process.Signal(syscall.SIGQUIT); err != nil {
+		t.Fatalf("Error st.cmd.Process.Signal() = %v", err)
+	}
+
 	time.Sleep(150 * time.Millisecond)
 
 	res, err = st.http1(requestParam{
@@ -264,6 +269,9 @@ func TestH1H1BadHost(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Error http.ReadResponse() = %v", err)
 	}
+
+	defer resp.Body.Close()
+
 	if got, want := resp.StatusCode, http.StatusBadRequest; got != want {
 		t.Errorf("status: %v; want %v", got, want)
 	}
@@ -287,6 +295,9 @@ func TestH1H1BadAuthority(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Error http.ReadResponse() = %v", err)
 	}
+
+	defer resp.Body.Close()
+
 	if got, want := resp.StatusCode, http.StatusBadRequest; got != want {
 		t.Errorf("status: %v; want %v", got, want)
 	}
@@ -310,6 +321,9 @@ func TestH1H1BadScheme(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Error http.ReadResponse() = %v", err)
 	}
+
+	defer resp.Body.Close()
+
 	if got, want := resp.StatusCode, http.StatusBadRequest; got != want {
 		t.Errorf("status: %v; want %v", got, want)
 	}
@@ -335,6 +349,8 @@ func TestH1H1HTTP10(t *testing.T) {
 		t.Fatalf("Error http.ReadResponse() = %v", err)
 	}
 
+	defer resp.Body.Close()
+
 	if got, want := resp.StatusCode, http.StatusOK; got != want {
 		t.Errorf("status: %v; want %v", got, want)
 	}
@@ -364,6 +380,8 @@ func TestH1H1HTTP10NoHostRewrite(t *testing.T) {
 		t.Fatalf("Error http.ReadResponse() = %v", err)
 	}
 
+	defer resp.Body.Close()
+
 	if got, want := resp.StatusCode, http.StatusOK; got != want {
 		t.Errorf("status: %v; want %v", got, want)
 	}
@@ -492,20 +510,19 @@ func TestH1H1HeaderFields(t *testing.T) {
 func TestH1H1Websocket(t *testing.T) {
 	opts := options{
 		handler: websocket.Handler(func(ws *websocket.Conn) {
-			io.Copy(ws, ws)
+			if _, err := io.Copy(ws, ws); err != nil {
+				t.Fatalf("Error io.Copy() = %v", err)
+			}
 		}).ServeHTTP,
 	}
 	st := newServerTester(t, opts)
 	defer st.Close()
 
 	content := []byte("hello world")
-	res, err := st.websocket(requestParam{
+	res := st.websocket(requestParam{
 		name: "TestH1H1Websocket",
 		body: content,
 	})
-	if err != nil {
-		t.Fatalf("Error st.websocket() = %v", err)
-	}
 	if got, want := res.body, content; !bytes.Equal(got, want) {
 		t.Errorf("echo: %q; want %q", got, want)
 	}
@@ -766,6 +783,8 @@ func TestH1H2NoHost(t *testing.T) {
 		t.Fatalf("Error http.ReadResponse() = %v", err)
 	}
 
+	defer resp.Body.Close()
+
 	if got, want := resp.StatusCode, http.StatusBadRequest; got != want {
 		t.Errorf("status: %v; want %v", got, want)
 	}
@@ -792,6 +811,8 @@ func TestH1H2HTTP10(t *testing.T) {
 		t.Fatalf("Error http.ReadResponse() = %v", err)
 	}
 
+	defer resp.Body.Close()
+
 	if got, want := resp.StatusCode, http.StatusOK; got != want {
 		t.Errorf("status: %v; want %v", got, want)
 	}
@@ -822,6 +843,8 @@ func TestH1H2HTTP10NoHostRewrite(t *testing.T) {
 		t.Fatalf("Error http.ReadResponse() = %v", err)
 	}
 
+	defer resp.Body.Close()
+
 	if got, want := resp.StatusCode, http.StatusOK; got != want {
 		t.Errorf("status: %v; want %v", got, want)
 	}
@@ -1310,6 +1333,8 @@ func TestH1ResponseBeforeRequestEnd(t *testing.T) {
 		t.Fatalf("Error http.ReadResponse() = %v", err)
 	}
 
+	defer resp.Body.Close()
+
 	if got, want := resp.StatusCode, http.StatusNotFound; got != want {
 		t.Errorf("status: %v; want %v", got, want)
 	}
@@ -1331,7 +1356,9 @@ func TestH1H1ChunkedEndsPrematurely(t *testing.T) {
 				return
 			}
 			defer conn.Close()
-			bufrw.WriteString("HTTP/1.1 200\r\nTransfer-Encoding: chunked\r\n\r\n")
+			if _, err := bufrw.WriteString("HTTP/1.1 200\r\nTransfer-Encoding: chunked\r\n\r\n"); err != nil {
+				t.Fatalf("Error bufrw.WriteString() = %v", err)
+			}
 			bufrw.Flush()
 		},
 	}
diff --git a/integration-tests/nghttpx_http2_test.go b/integration-tests/nghttpx_http2_test.go
index e410d0da..5324a18c 100644
--- a/integration-tests/nghttpx_http2_test.go
+++ b/integration-tests/nghttpx_http2_test.go
@@ -642,7 +642,9 @@ func TestH2H1BadResponseCL(t *testing.T) {
 		handler: func(w http.ResponseWriter, r *http.Request) {
 			// we set content-length: 1024, but only send 3 bytes.
 			w.Header().Add("Content-Length", "1024")
-			w.Write([]byte("foo"))
+			if _, err := w.Write([]byte("foo")); err != nil {
+				t.Fatalf("Error w.Write() = %v", err)
+			}
 		},
 	}
 	st := newServerTester(t, opts)
@@ -1273,7 +1275,7 @@ func TestH2H1Upgrade(t *testing.T) {
 // header field includes obfuscated address even if PROXY protocol
 // version 1 containing TCP4 entry is accepted.
 func TestH2H1ProxyProtocolV1ForwardedForObfuscated(t *testing.T) {
-	pattern := fmt.Sprintf(`^for=_[^;]+$`)
+	pattern := `^for=_[^;]+$`
 	validFwd := regexp.MustCompile(pattern)
 	opts := options{
 		args: []string{
@@ -1291,7 +1293,9 @@ func TestH2H1ProxyProtocolV1ForwardedForObfuscated(t *testing.T) {
 	st := newServerTester(t, opts)
 	defer st.Close()
 
-	st.conn.Write([]byte("PROXY TCP4 192.168.0.2 192.168.0.100 12345 8080\r\n"))
+	if _, err := st.conn.Write([]byte("PROXY TCP4 192.168.0.2 192.168.0.100 12345 8080\r\n")); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	res, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV1ForwardedForObfuscated",
@@ -1329,7 +1333,9 @@ func TestH2H1ProxyProtocolV1TCP4(t *testing.T) {
 	st := newServerTester(t, opts)
 	defer st.Close()
 
-	st.conn.Write([]byte("PROXY TCP4 192.168.0.2 192.168.0.100 12345 8080\r\n"))
+	if _, err := st.conn.Write([]byte("PROXY TCP4 192.168.0.2 192.168.0.100 12345 8080\r\n")); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	res, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV1TCP4",
@@ -1367,7 +1373,9 @@ func TestH2H1ProxyProtocolV1TCP6(t *testing.T) {
 	st := newServerTester(t, opts)
 	defer st.Close()
 
-	st.conn.Write([]byte("PROXY TCP6 2001:0db8:85a3:0000:0000:8a2e:0370:7334 ::1 12345 8080\r\n"))
+	if _, err := st.conn.Write([]byte("PROXY TCP6 2001:0db8:85a3:0000:0000:8a2e:0370:7334 ::1 12345 8080\r\n")); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	res, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV1TCP6",
@@ -1480,7 +1488,9 @@ func TestH2H1ProxyProtocolV1Unknown(t *testing.T) {
 	st := newServerTester(t, opts)
 	defer st.Close()
 
-	st.conn.Write([]byte("PROXY UNKNOWN 192.168.0.2 192.168.0.100 12345 8080\r\n"))
+	if _, err := st.conn.Write([]byte("PROXY UNKNOWN 192.168.0.2 192.168.0.100 12345 8080\r\n")); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	res, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV1Unknown",
@@ -1507,7 +1517,9 @@ func TestH2H1ProxyProtocolV1JustUnknown(t *testing.T) {
 	st := newServerTester(t, opts)
 	defer st.Close()
 
-	st.conn.Write([]byte("PROXY UNKNOWN\r\n"))
+	if _, err := st.conn.Write([]byte("PROXY UNKNOWN\r\n")); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	res, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV1JustUnknown",
@@ -1534,7 +1546,9 @@ func TestH2H1ProxyProtocolV1TooLongLine(t *testing.T) {
 	st := newServerTester(t, opts)
 	defer st.Close()
 
-	st.conn.Write([]byte("PROXY UNKNOWN ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff 65535 655350\r\n"))
+	if _, err := st.conn.Write([]byte("PROXY UNKNOWN ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff 65535 655350\r\n")); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	_, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV1TooLongLine",
@@ -1554,7 +1568,9 @@ func TestH2H1ProxyProtocolV1BadLineEnd(t *testing.T) {
 	st := newServerTester(t, opts)
 	defer st.Close()
 
-	st.conn.Write([]byte("PROXY TCP6 ::1 ::1 12345 8080\r \n"))
+	if _, err := st.conn.Write([]byte("PROXY TCP6 ::1 ::1 12345 8080\r \n")); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	_, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV1BadLineEnd",
@@ -1574,7 +1590,9 @@ func TestH2H1ProxyProtocolV1NoEnd(t *testing.T) {
 	st := newServerTester(t, opts)
 	defer st.Close()
 
-	st.conn.Write([]byte("PROXY TCP6 ::1 ::1 12345 8080"))
+	if _, err := st.conn.Write([]byte("PROXY TCP6 ::1 ::1 12345 8080")); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	_, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV1NoEnd",
@@ -1596,7 +1614,9 @@ func TestH2H1ProxyProtocolV1EmbeddedNULL(t *testing.T) {
 
 	b := []byte("PROXY TCP6 ::1*foo ::1 12345 8080\r\n")
 	b[14] = 0
-	st.conn.Write(b)
+	if _, err := st.conn.Write(b); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	_, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV1EmbeddedNULL",
@@ -1616,7 +1636,9 @@ func TestH2H1ProxyProtocolV1MissingSrcPort(t *testing.T) {
 	st := newServerTester(t, opts)
 	defer st.Close()
 
-	st.conn.Write([]byte("PROXY TCP6 ::1 ::1  8080\r\n"))
+	if _, err := st.conn.Write([]byte("PROXY TCP6 ::1 ::1  8080\r\n")); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	_, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV1MissingSrcPort",
@@ -1636,7 +1658,9 @@ func TestH2H1ProxyProtocolV1MissingDstPort(t *testing.T) {
 	st := newServerTester(t, opts)
 	defer st.Close()
 
-	st.conn.Write([]byte("PROXY TCP6 ::1 ::1 12345 \r\n"))
+	if _, err := st.conn.Write([]byte("PROXY TCP6 ::1 ::1 12345 \r\n")); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	_, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV1MissingDstPort",
@@ -1656,7 +1680,9 @@ func TestH2H1ProxyProtocolV1InvalidSrcPort(t *testing.T) {
 	st := newServerTester(t, opts)
 	defer st.Close()
 
-	st.conn.Write([]byte("PROXY TCP6 ::1 ::1 123x 8080\r\n"))
+	if _, err := st.conn.Write([]byte("PROXY TCP6 ::1 ::1 123x 8080\r\n")); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	_, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV1InvalidSrcPort",
@@ -1676,7 +1702,9 @@ func TestH2H1ProxyProtocolV1InvalidDstPort(t *testing.T) {
 	st := newServerTester(t, opts)
 	defer st.Close()
 
-	st.conn.Write([]byte("PROXY TCP6 ::1 ::1 123456 80x\r\n"))
+	if _, err := st.conn.Write([]byte("PROXY TCP6 ::1 ::1 123456 80x\r\n")); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	_, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV1InvalidDstPort",
@@ -1697,7 +1725,9 @@ func TestH2H1ProxyProtocolV1LeadingZeroPort(t *testing.T) {
 	st := newServerTester(t, opts)
 	defer st.Close()
 
-	st.conn.Write([]byte("PROXY TCP6 ::1 ::1 03000 8080\r\n"))
+	if _, err := st.conn.Write([]byte("PROXY TCP6 ::1 ::1 03000 8080\r\n")); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	_, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV1LeadingZeroPort",
@@ -1717,7 +1747,9 @@ func TestH2H1ProxyProtocolV1TooLargeSrcPort(t *testing.T) {
 	st := newServerTester(t, opts)
 	defer st.Close()
 
-	st.conn.Write([]byte("PROXY TCP6 ::1 ::1 65536 8080\r\n"))
+	if _, err := st.conn.Write([]byte("PROXY TCP6 ::1 ::1 65536 8080\r\n")); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	_, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV1TooLargeSrcPort",
@@ -1737,7 +1769,9 @@ func TestH2H1ProxyProtocolV1TooLargeDstPort(t *testing.T) {
 	st := newServerTester(t, opts)
 	defer st.Close()
 
-	st.conn.Write([]byte("PROXY TCP6 ::1 ::1 12345 65536\r\n"))
+	if _, err := st.conn.Write([]byte("PROXY TCP6 ::1 ::1 12345 65536\r\n")); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	_, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV1TooLargeDstPort",
@@ -1757,7 +1791,9 @@ func TestH2H1ProxyProtocolV1InvalidSrcAddr(t *testing.T) {
 	st := newServerTester(t, opts)
 	defer st.Close()
 
-	st.conn.Write([]byte("PROXY TCP6 192.168.0.1 ::1 12345 8080\r\n"))
+	if _, err := st.conn.Write([]byte("PROXY TCP6 192.168.0.1 ::1 12345 8080\r\n")); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	_, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV1InvalidSrcAddr",
@@ -1777,7 +1813,9 @@ func TestH2H1ProxyProtocolV1InvalidDstAddr(t *testing.T) {
 	st := newServerTester(t, opts)
 	defer st.Close()
 
-	st.conn.Write([]byte("PROXY TCP6 ::1 192.168.0.1 12345 8080\r\n"))
+	if _, err := st.conn.Write([]byte("PROXY TCP6 ::1 192.168.0.1 12345 8080\r\n")); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	_, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV1InvalidDstAddr",
@@ -1797,7 +1835,9 @@ func TestH2H1ProxyProtocolV1InvalidProtoFamily(t *testing.T) {
 	st := newServerTester(t, opts)
 	defer st.Close()
 
-	st.conn.Write([]byte("PROXY UNIX ::1 ::1 12345 8080\r\n"))
+	if _, err := st.conn.Write([]byte("PROXY UNIX ::1 ::1 12345 8080\r\n")); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	_, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV1InvalidProtoFamily",
@@ -1817,7 +1857,9 @@ func TestH2H1ProxyProtocolV1InvalidID(t *testing.T) {
 	st := newServerTester(t, opts)
 	defer st.Close()
 
-	st.conn.Write([]byte("PR0XY TCP6 ::1 ::1 12345 8080\r\n"))
+	if _, err := st.conn.Write([]byte("PR0XY TCP6 ::1 ::1 12345 8080\r\n")); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	_, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV1InvalidID",
@@ -1852,7 +1894,7 @@ func TestH2H1ProxyProtocolV2TCP4(t *testing.T) {
 	defer st.Close()
 
 	var b bytes.Buffer
-	writeProxyProtocolV2(&b, proxyProtocolV2{
+	if err := writeProxyProtocolV2(&b, proxyProtocolV2{
 		command: proxyProtocolV2CommandProxy,
 		sourceAddress: &net.TCPAddr{
 			IP:   net.ParseIP("192.168.0.2").To4(),
@@ -1863,8 +1905,13 @@ func TestH2H1ProxyProtocolV2TCP4(t *testing.T) {
 			Port: 8080,
 		},
 		additionalData: []byte("foobar"),
-	})
-	st.conn.Write(b.Bytes())
+	}); err != nil {
+		t.Fatalf("Error writeProxyProtocolV2() = %v", err)
+	}
+
+	if _, err := st.conn.Write(b.Bytes()); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	res, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV2TCP4",
@@ -1903,7 +1950,7 @@ func TestH2H1ProxyProtocolV2TCP6(t *testing.T) {
 	defer st.Close()
 
 	var b bytes.Buffer
-	writeProxyProtocolV2(&b, proxyProtocolV2{
+	if err := writeProxyProtocolV2(&b, proxyProtocolV2{
 		command: proxyProtocolV2CommandProxy,
 		sourceAddress: &net.TCPAddr{
 			IP:   net.ParseIP("2001:0db8:85a3:0000:0000:8a2e:0370:7334"),
@@ -1914,8 +1961,13 @@ func TestH2H1ProxyProtocolV2TCP6(t *testing.T) {
 			Port: 8080,
 		},
 		additionalData: []byte("foobar"),
-	})
-	st.conn.Write(b.Bytes())
+	}); err != nil {
+		t.Fatalf("Error writeProxyProtocolV2() = %v", err)
+	}
+
+	if _, err := st.conn.Write(b.Bytes()); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	res, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV2TCP6",
@@ -1935,7 +1987,7 @@ func TestH2H1ProxyProtocolV2TCP6(t *testing.T) {
 // contains advertised src address.
 func TestH2H1ProxyProtocolV2TCP4TLS(t *testing.T) {
 	var v2Hdr bytes.Buffer
-	writeProxyProtocolV2(&v2Hdr, proxyProtocolV2{
+	if err := writeProxyProtocolV2(&v2Hdr, proxyProtocolV2{
 		command: proxyProtocolV2CommandProxy,
 		sourceAddress: &net.TCPAddr{
 			IP:   net.ParseIP("192.168.0.2").To4(),
@@ -1946,7 +1998,9 @@ func TestH2H1ProxyProtocolV2TCP4TLS(t *testing.T) {
 			Port: 8080,
 		},
 		additionalData: []byte("foobar"),
-	})
+	}); err != nil {
+		t.Fatalf("Error writeProxyProtocolV2() = %v", err)
+	}
 
 	opts := options{
 		args: []string{
@@ -1987,7 +2041,7 @@ func TestH2H1ProxyProtocolV2TCP4TLS(t *testing.T) {
 // contains advertised src address.
 func TestH2H1ProxyProtocolV2TCP6TLS(t *testing.T) {
 	var v2Hdr bytes.Buffer
-	writeProxyProtocolV2(&v2Hdr, proxyProtocolV2{
+	if err := writeProxyProtocolV2(&v2Hdr, proxyProtocolV2{
 		command: proxyProtocolV2CommandProxy,
 		sourceAddress: &net.TCPAddr{
 			IP:   net.ParseIP("2001:0db8:85a3:0000:0000:8a2e:0370:7334"),
@@ -1998,7 +2052,9 @@ func TestH2H1ProxyProtocolV2TCP6TLS(t *testing.T) {
 			Port: 8080,
 		},
 		additionalData: []byte("foobar"),
-	})
+	}); err != nil {
+		t.Fatalf("Error writeProxyProtocolV2() = %v", err)
+	}
 
 	opts := options{
 		args: []string{
@@ -2057,7 +2113,7 @@ func TestH2H1ProxyProtocolV2Local(t *testing.T) {
 	defer st.Close()
 
 	var b bytes.Buffer
-	writeProxyProtocolV2(&b, proxyProtocolV2{
+	if err := writeProxyProtocolV2(&b, proxyProtocolV2{
 		command: proxyProtocolV2CommandLocal,
 		sourceAddress: &net.TCPAddr{
 			IP:   net.ParseIP("192.168.0.2").To4(),
@@ -2068,8 +2124,13 @@ func TestH2H1ProxyProtocolV2Local(t *testing.T) {
 			Port: 8080,
 		},
 		additionalData: []byte("foobar"),
-	})
-	st.conn.Write(b.Bytes())
+	}); err != nil {
+		t.Fatalf("Error writeProxyProtocolV2() = %v", err)
+	}
+
+	if _, err := st.conn.Write(b.Bytes()); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	res, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV2Local",
@@ -2094,7 +2155,7 @@ func TestH2H1ProxyProtocolV2UnknownCmd(t *testing.T) {
 	defer st.Close()
 
 	var b bytes.Buffer
-	writeProxyProtocolV2(&b, proxyProtocolV2{
+	if err := writeProxyProtocolV2(&b, proxyProtocolV2{
 		command: 0xf,
 		sourceAddress: &net.TCPAddr{
 			IP:   net.ParseIP("192.168.0.2").To4(),
@@ -2105,8 +2166,13 @@ func TestH2H1ProxyProtocolV2UnknownCmd(t *testing.T) {
 			Port: 8080,
 		},
 		additionalData: []byte("foobar"),
-	})
-	st.conn.Write(b.Bytes())
+	}); err != nil {
+		t.Fatalf("Error writeProxyProtocolV2() = %v", err)
+	}
+
+	if _, err := st.conn.Write(b.Bytes()); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	_, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV2UnknownCmd",
@@ -2140,7 +2206,7 @@ func TestH2H1ProxyProtocolV2Unix(t *testing.T) {
 	defer st.Close()
 
 	var b bytes.Buffer
-	writeProxyProtocolV2(&b, proxyProtocolV2{
+	if err := writeProxyProtocolV2(&b, proxyProtocolV2{
 		command: proxyProtocolV2CommandProxy,
 		sourceAddress: &net.UnixAddr{
 			Name: "/foo",
@@ -2151,8 +2217,13 @@ func TestH2H1ProxyProtocolV2Unix(t *testing.T) {
 			Net:  "unix",
 		},
 		additionalData: []byte("foobar"),
-	})
-	st.conn.Write(b.Bytes())
+	}); err != nil {
+		t.Fatalf("Error writeProxyProtocolV2() = %v", err)
+	}
+
+	if _, err := st.conn.Write(b.Bytes()); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	res, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV2Unix",
@@ -2190,11 +2261,16 @@ func TestH2H1ProxyProtocolV2Unspec(t *testing.T) {
 	defer st.Close()
 
 	var b bytes.Buffer
-	writeProxyProtocolV2(&b, proxyProtocolV2{
+	if err := writeProxyProtocolV2(&b, proxyProtocolV2{
 		command:        proxyProtocolV2CommandProxy,
 		additionalData: []byte("foobar"),
-	})
-	st.conn.Write(b.Bytes())
+	}); err != nil {
+		t.Fatalf("Error writeProxyProtocolV2() = %v", err)
+	}
+
+	if _, err := st.conn.Write(b.Bytes()); err != nil {
+		t.Fatalf("Error st.conn.Write() = %v", err)
+	}
 
 	res, err := st.http2(requestParam{
 		name: "TestH2H1ProxyProtocolV2Unspec",
@@ -2342,7 +2418,9 @@ func TestH2H1Code204CL0(t *testing.T) {
 				return
 			}
 			defer conn.Close()
-			bufrw.WriteString("HTTP/1.1 204\r\nContent-Length: 0\r\n\r\n")
+			if _, err := bufrw.WriteString("HTTP/1.1 204\r\nContent-Length: 0\r\n\r\n"); err != nil {
+				t.Fatalf("Error bufrw.WriteString() = %v", err)
+			}
 			bufrw.Flush()
 		},
 	}
@@ -2381,7 +2459,9 @@ func TestH2H1Code204CLNonzero(t *testing.T) {
 				return
 			}
 			defer conn.Close()
-			bufrw.WriteString("HTTP/1.1 204\r\nContent-Length: 1\r\n\r\n")
+			if _, err := bufrw.WriteString("HTTP/1.1 204\r\nContent-Length: 1\r\n\r\n"); err != nil {
+				t.Fatalf("Error bufrw.WriteString() = %v", err)
+			}
 			bufrw.Flush()
 		},
 	}
@@ -2416,7 +2496,9 @@ func TestH2H1Code204TE(t *testing.T) {
 				return
 			}
 			defer conn.Close()
-			bufrw.WriteString("HTTP/1.1 204\r\nTransfer-Encoding: chunked\r\n\r\n")
+			if _, err := bufrw.WriteString("HTTP/1.1 204\r\nTransfer-Encoding: chunked\r\n\r\n"); err != nil {
+				t.Fatalf("Error bufrw.WriteString() = %v", err)
+			}
 			bufrw.Flush()
 		},
 	}
@@ -2522,7 +2604,10 @@ func TestH2H1GracefulShutdown(t *testing.T) {
 	}
 
 	// send SIGQUIT signal to nghttpx to perform graceful shutdown
-	st.cmd.Process.Signal(syscall.SIGQUIT)
+	if err := st.cmd.Process.Signal(syscall.SIGQUIT); err != nil {
+		t.Fatalf("Error st.cmd.Process.Signal() = %v", err)
+	}
+
 	time.Sleep(150 * time.Millisecond)
 
 	// after signal, finish request body
@@ -2546,7 +2631,7 @@ func TestH2H1GracefulShutdown(t *testing.T) {
 		}
 		switch f := fr.(type) {
 		case *http2.GoAwayFrame:
-			numGoAway += 1
+			numGoAway++
 			want := http2.ErrCodeNo
 			if got := f.ErrCode; got != want {
 				t.Fatalf("f.ErrCode(%v): %v; want %v", numGoAway, got, want)
@@ -3530,7 +3615,9 @@ func TestH2H1ChunkedEndsPrematurely(t *testing.T) {
 				return
 			}
 			defer conn.Close()
-			bufrw.WriteString("HTTP/1.1 200\r\nTransfer-Encoding: chunked\r\n\r\n")
+			if _, err := bufrw.WriteString("HTTP/1.1 200\r\nTransfer-Encoding: chunked\r\n\r\n"); err != nil {
+				t.Fatalf("Error bufrw.WriteString() = %v", err)
+			}
 			bufrw.Flush()
 		},
 	}
diff --git a/integration-tests/nghttpx_http3_test.go b/integration-tests/nghttpx_http3_test.go
index 4719381a..9ea85d74 100644
--- a/integration-tests/nghttpx_http3_test.go
+++ b/integration-tests/nghttpx_http3_test.go
@@ -180,7 +180,9 @@ func TestH3H1BadResponseCL(t *testing.T) {
 		handler: func(w http.ResponseWriter, r *http.Request) {
 			// we set content-length: 1024, but only send 3 bytes.
 			w.Header().Add("Content-Length", "1024")
-			w.Write([]byte("foo"))
+			if _, err := w.Write([]byte("foo")); err != nil {
+				t.Fatalf("Error w.Write() = %v", err)
+			}
 		},
 		quic: true,
 	}
@@ -372,7 +374,9 @@ func TestH3H1ChunkedEndsPrematurely(t *testing.T) {
 				return
 			}
 			defer conn.Close()
-			bufrw.WriteString("HTTP/1.1 200\r\nTransfer-Encoding: chunked\r\n\r\n")
+			if _, err := bufrw.WriteString("HTTP/1.1 200\r\nTransfer-Encoding: chunked\r\n\r\n"); err != nil {
+				t.Fatalf("Error bufrw.WriteString() = %v", err)
+			}
 			bufrw.Flush()
 		},
 		quic: true,
diff --git a/integration-tests/server_tester.go b/integration-tests/server_tester.go
index d6bebee4..f98a04a0 100644
--- a/integration-tests/server_tester.go
+++ b/integration-tests/server_tester.go
@@ -3,6 +3,7 @@ package nghttp2
 import (
 	"bufio"
 	"bytes"
+	"context"
 	"crypto/tls"
 	"encoding/binary"
 	"errors"
@@ -43,7 +44,6 @@ func pair(name, value string) hpack.HeaderField {
 }
 
 type serverTester struct {
-	args          []string  // command-line arguments
 	cmd           *exec.Cmd // test frontend server process, which is test subject
 	url           string    // test frontend server URL
 	t             *testing.T
@@ -246,16 +246,11 @@ func newServerTester(t *testing.T, opts options) *serverTester {
 			tlsConn := tls.Client(conn, tlsConfig)
 			err = tlsConn.Handshake()
 			if err == nil {
-				cs := tlsConn.ConnectionState()
-				if !cs.NegotiatedProtocolIsMutual {
-					st.Close()
-					st.t.Fatalf("Error negotiated next protocol is not mutual")
-				}
 				conn = tlsConn
 			}
 		}
 		if err != nil {
-			retry += 1
+			retry++
 			if retry >= 100 {
 				st.Close()
 				st.t.Fatalf("Error server is not responding too long; server command-line arguments may be invalid")
@@ -282,16 +277,22 @@ func (st *serverTester) Close() {
 	if st.cmd != nil {
 		done := make(chan struct{})
 		go func() {
-			st.cmd.Wait()
+			if err := st.cmd.Wait(); err != nil {
+				st.t.Errorf("Error st.cmd.Wait() = %v", err)
+			}
 			close(done)
 		}()
 
-		st.cmd.Process.Signal(syscall.SIGQUIT)
+		if err := st.cmd.Process.Signal(syscall.SIGQUIT); err != nil {
+			st.t.Errorf("Error st.cmd.Process.Signal() = %v", err)
+		}
 
 		select {
 		case <-done:
 		case <-time.After(10 * time.Second):
-			st.cmd.Process.Kill()
+			if err := st.cmd.Process.Kill(); err != nil {
+				st.t.Errorf("Error st.cmd.Process.Kill() = %v", err)
+			}
 			<-done
 		}
 	}
@@ -354,7 +355,7 @@ func (cbr *chunkedBodyReader) Read(p []byte) (n int, err error) {
 	return cbr.body.Read(p)
 }
 
-func (st *serverTester) websocket(rp requestParam) (*serverResponse, error) {
+func (st *serverTester) websocket(rp requestParam) *serverResponse {
 	urlstring := st.url + "/echo"
 
 	config, err := websocket.NewConfig(urlstring, st.url)
@@ -386,7 +387,7 @@ func (st *serverTester) websocket(rp requestParam) (*serverResponse, error) {
 		body: msg[:n],
 	}
 
-	return res, nil
+	return res
 }
 
 func (st *serverTester) http3(rp requestParam) (*serverResponse, error) {
@@ -425,7 +426,10 @@ func (st *serverTester) http3(rp requestParam) (*serverResponse, error) {
 		reqURL = u.String() + rp.path
 	}
 
-	req, err := http.NewRequest(method, reqURL, body)
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	req, err := http.NewRequestWithContext(ctx, method, reqURL, body)
 	if err != nil {
 		return nil, err
 	}
@@ -492,7 +496,10 @@ func (st *serverTester) http1(rp requestParam) (*serverResponse, error) {
 		reqURL = u.String() + rp.path
 	}
 
-	req, err := http.NewRequest(method, reqURL, body)
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	req, err := http.NewRequestWithContext(ctx, method, reqURL, body)
 	if err != nil {
 		return nil, err
 	}
@@ -750,7 +757,9 @@ func cloneHeader(h http.Header) http.Header {
 }
 
 func noopHandler(w http.ResponseWriter, r *http.Request) {
-	io.ReadAll(r.Body)
+	if _, err := io.ReadAll(r.Body); err != nil {
+		http.Error(w, fmt.Sprintf("Error io.ReadAll() = %v", err), http.StatusInternalServerError)
+	}
 }
 
 type APIResponse struct {
@@ -790,9 +799,13 @@ const (
 	proxyProtocolV2ProtocolDgram  proxyProtocolV2Protocol = 0x2
 )
 
-func writeProxyProtocolV2(w io.Writer, hdr proxyProtocolV2) {
-	w.Write([]byte{0x0D, 0x0A, 0x0D, 0x0A, 0x00, 0x0D, 0x0A, 0x51, 0x55, 0x49, 0x54, 0x0A})
-	w.Write([]byte{byte(0x20 | hdr.command)})
+func writeProxyProtocolV2(w io.Writer, hdr proxyProtocolV2) error {
+	if _, err := w.Write([]byte{0x0D, 0x0A, 0x0D, 0x0A, 0x00, 0x0D, 0x0A, 0x51, 0x55, 0x49, 0x54, 0x0A}); err != nil {
+		return err
+	}
+	if _, err := w.Write([]byte{byte(0x20 | hdr.command)}); err != nil {
+		return err
+	}
 
 	switch srcAddr := hdr.sourceAddress.(type) {
 	case *net.TCPAddr:
@@ -807,13 +820,25 @@ func writeProxyProtocolV2(w io.Writer, hdr proxyProtocolV2) {
 			fam = byte(proxyProtocolV2FamilyInet6 << 4)
 		}
 		fam |= byte(proxyProtocolV2ProtocolStream)
-		w.Write([]byte{fam})
+		if _, err := w.Write([]byte{fam}); err != nil {
+			return err
+		}
 		length := uint16(len(srcAddr.IP)*2 + 4 + len(hdr.additionalData))
-		binary.Write(w, binary.BigEndian, length)
-		w.Write(srcAddr.IP)
-		w.Write(dstAddr.IP)
-		binary.Write(w, binary.BigEndian, uint16(srcAddr.Port))
-		binary.Write(w, binary.BigEndian, uint16(dstAddr.Port))
+		if err := binary.Write(w, binary.BigEndian, length); err != nil {
+			return err
+		}
+		if _, err := w.Write(srcAddr.IP); err != nil {
+			return err
+		}
+		if _, err := w.Write(dstAddr.IP); err != nil {
+			return err
+		}
+		if err := binary.Write(w, binary.BigEndian, uint16(srcAddr.Port)); err != nil {
+			return err
+		}
+		if err := binary.Write(w, binary.BigEndian, uint16(dstAddr.Port)); err != nil {
+			return err
+		}
 	case *net.UnixAddr:
 		dstAddr := hdr.destinationAddress.(*net.UnixAddr)
 		if len(srcAddr.Name) > 108 {
@@ -831,20 +856,40 @@ func writeProxyProtocolV2(w io.Writer, hdr proxyProtocolV2) {
 		default:
 			fam |= byte(proxyProtocolV2ProtocolUnspec)
 		}
-		w.Write([]byte{fam})
+		if _, err := w.Write([]byte{fam}); err != nil {
+			return err
+		}
 		length := uint16(216 + len(hdr.additionalData))
-		binary.Write(w, binary.BigEndian, length)
+		if err := binary.Write(w, binary.BigEndian, length); err != nil {
+			return err
+		}
 		zeros := make([]byte, 108)
-		w.Write([]byte(srcAddr.Name))
-		w.Write(zeros[:108-len(srcAddr.Name)])
-		w.Write([]byte(dstAddr.Name))
-		w.Write(zeros[:108-len(dstAddr.Name)])
+		if _, err := w.Write([]byte(srcAddr.Name)); err != nil {
+			return err
+		}
+		if _, err := w.Write(zeros[:108-len(srcAddr.Name)]); err != nil {
+			return err
+		}
+		if _, err := w.Write([]byte(dstAddr.Name)); err != nil {
+			return err
+		}
+		if _, err := w.Write(zeros[:108-len(dstAddr.Name)]); err != nil {
+			return err
+		}
 	default:
 		fam := byte(proxyProtocolV2FamilyUnspec<<4) | byte(proxyProtocolV2ProtocolUnspec)
-		w.Write([]byte{fam})
+		if _, err := w.Write([]byte{fam}); err != nil {
+			return err
+		}
 		length := uint16(len(hdr.additionalData))
-		binary.Write(w, binary.BigEndian, length)
+		if err := binary.Write(w, binary.BigEndian, length); err != nil {
+			return err
+		}
 	}
 
-	w.Write(hdr.additionalData)
+	if _, err := w.Write(hdr.additionalData); err != nil {
+		return err
+	}
+
+	return nil
 }