Pull request: all: fix client upstreams, imp code

Updates #3186.

Squashed commit of the following:

commit a8dd0e2cda3039839d069fe71a5bd0f9635ec064
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri May 28 12:54:07 2021 +0300

    all: imp code, names

commit 98f86c21ae23b665095075feb4a59dcfcc622bc7
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu May 27 21:11:37 2021 +0300

    all: fix client upstreams, imp code

internal/aghnet/addr.go

@@ -10,6 +10,19 @@ import (
 	"golang.org/x/net/idna"
 )
 
+// IPFromAddr returns an IP address from addr.  If addr is neither
+// a *net.TCPAddr nor a *net.UDPAddr, it returns nil.
+func IPFromAddr(addr net.Addr) (ip net.IP) {
+	switch addr := addr.(type) {
+	case *net.TCPAddr:
+		return addr.IP
+	case *net.UDPAddr:
+		return addr.IP
+	}
+
+	return nil
+}
+
 // IsValidHostOuterRune returns true if r is a valid initial or final rune for
 // a hostname label.
 func IsValidHostOuterRune(r rune) (ok bool) {

internal/aghnet/addr_test.go

@@ -9,6 +9,14 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func TestIPFromAddr(t *testing.T) {
+	ip := net.IP{1, 2, 3, 4}
+	assert.Equal(t, net.IP(nil), IPFromAddr(nil))
+	assert.Equal(t, net.IP(nil), IPFromAddr(struct{ net.Addr }{}))
+	assert.Equal(t, ip, IPFromAddr(&net.TCPAddr{IP: ip}))
+	assert.Equal(t, ip, IPFromAddr(&net.UDPAddr{IP: ip}))
+}
+
 func TestValidateHardwareAddress(t *testing.T) {
 	testCases := []struct {
 		name       string