Pull request: all: imp cyclo in new code

Updates #2646,

Squashed commit of the following:

commit af6a6fa2b7229bc0f1c7c9083b0391a6bec7ae70
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon May 31 20:00:36 2021 +0300

    all: imp code, docs

commit 1cd4781b13e635a9e1bccb758104c1b76c78d34e
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon May 31 18:51:23 2021 +0300

    all: imp cyclo in new code

internal/aghnet/etchostscontainer_test.go

@@ -0,0 +1,130 @@
+package aghnet
+
+import (
+	"net"
+	"os"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/AdguardTeam/AdGuardHome/internal/aghtest"
+	"github.com/miekg/dns"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestMain(m *testing.M) {
+	aghtest.DiscardLogOutput(m)
+}
+
+func prepareTestFile(t *testing.T) (f *os.File) {
+	t.Helper()
+
+	dir := t.TempDir()
+
+	f, err := os.CreateTemp(dir, "")
+	require.NoError(t, err)
+	require.NotNil(t, f)
+
+	t.Cleanup(func() {
+		assert.NoError(t, f.Close())
+	})
+
+	return f
+}
+
+func assertWriting(t *testing.T, f *os.File, strs ...string) {
+	t.Helper()
+
+	for _, str := range strs {
+		n, err := f.WriteString(str)
+		require.NoError(t, err)
+		assert.Equal(t, n, len(str))
+	}
+}
+
+func TestEtcHostsContainerResolution(t *testing.T) {
+	ehc := &EtcHostsContainer{}
+
+	f := prepareTestFile(t)
+
+	assertWriting(t, f,
+		"  127.0.0.1   host  localhost # comment \n",
+		"  ::1   localhost#comment  \n",
+	)
+	ehc.Init(f.Name())
+
+	t.Run("existing_host", func(t *testing.T) {
+		ips := ehc.Process("localhost", dns.TypeA)
+		require.Len(t, ips, 1)
+		assert.Equal(t, net.IPv4(127, 0, 0, 1), ips[0])
+	})
+
+	t.Run("unknown_host", func(t *testing.T) {
+		ips := ehc.Process("newhost", dns.TypeA)
+		assert.Nil(t, ips)
+
+		// Comment.
+		ips = ehc.Process("comment", dns.TypeA)
+		assert.Nil(t, ips)
+	})
+
+	t.Run("hosts_file", func(t *testing.T) {
+		names, ok := ehc.List()["127.0.0.1"]
+		require.True(t, ok)
+		assert.Equal(t, []string{"host", "localhost"}, names)
+	})
+
+	t.Run("ptr", func(t *testing.T) {
+		testCases := []struct {
+			wantIP   string
+			wantHost string
+			wantLen  int
+		}{
+			{wantIP: "127.0.0.1", wantHost: "host", wantLen: 2},
+			{wantIP: "::1", wantHost: "localhost", wantLen: 1},
+		}
+
+		for _, tc := range testCases {
+			a, err := dns.ReverseAddr(tc.wantIP)
+			require.NoError(t, err)
+
+			a = strings.TrimSuffix(a, ".")
+			hosts := ehc.ProcessReverse(a, dns.TypePTR)
+			require.Len(t, hosts, tc.wantLen)
+			assert.Equal(t, tc.wantHost, hosts[0])
+		}
+	})
+}
+
+func TestEtcHostsContainerFSNotify(t *testing.T) {
+	ehc := &EtcHostsContainer{}
+
+	f := prepareTestFile(t)
+
+	assertWriting(t, f, "  127.0.0.1   host  localhost  \n")
+	ehc.Init(f.Name())
+
+	t.Run("unknown_host", func(t *testing.T) {
+		ips := ehc.Process("newhost", dns.TypeA)
+		assert.Nil(t, ips)
+	})
+
+	// Start monitoring for changes.
+	ehc.Start()
+	t.Cleanup(ehc.Close)
+
+	assertWriting(t, f, "127.0.0.2   newhost\n")
+	require.NoError(t, f.Sync())
+
+	// Wait until fsnotify has triggerred and processed the
+	// file-modification event.
+	time.Sleep(50 * time.Millisecond)
+
+	t.Run("notified", func(t *testing.T) {
+		ips := ehc.Process("newhost", dns.TypeA)
+		assert.NotNil(t, ips)
+		require.Len(t, ips, 1)
+		assert.True(t, net.IP{127, 0, 0, 2}.Equal(ips[0]))
+	})
+}