Pull request: 2846 cover aghnet vol.2

Merge in DNS/adguard-home from 2846-cover-aghnet-vol.2 to master

Updates #2846.
Closes #4408.

Squashed commit of the following:

commit 8d62b29d5b5be875cb71e518e479e321d853eb1a
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Mar 23 20:42:04 2022 +0300

    home: recover panic

commit 1d98109e910830bec712c7aecbbbcb8f659d823d
Merge: ac11d751 9ce2a0fb
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Mar 23 20:32:05 2022 +0300

    Merge branch 'master' into 2846-cover-aghnet-vol.2

commit ac11d751fb7951e3dd0940bf425a893223c32789
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Mar 23 20:29:41 2022 +0300

    aghnet: use iotest

commit 7c923df7bafd5d4b91c4b4a01e75ab161944f949
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Mar 23 20:17:19 2022 +0300

    aghnet: cover more

commit 3bfd4d587e4b887b5527d60c0eb6027da15c7e37
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Mar 23 14:13:59 2022 +0300

    aghnet: cover arpdb more

commit cd5cf7bbdecceeab6d3abee10a5572e1e907cc67
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Mar 23 13:05:35 2022 +0300

    all: rm arpdb initial refresh

commit 0fb8d9e44a4d130ca4e8fc2ea5d595ec08555302
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Mar 22 21:13:16 2022 +0300

    aghnet: cover arpdb

internal/aghnet/arpdb_test.go

@@ -6,6 +6,7 @@ import (
 	"strings"
 	"sync"
 	"testing"
+	"testing/iotest"
 
 	"github.com/AdguardTeam/golibs/errors"
 	"github.com/AdguardTeam/golibs/testutil"
@@ -13,6 +14,15 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func TestNewARPDB(t *testing.T) {
+	var a ARPDB
+	require.NotPanics(t, func() {
+		a = NewARPDB()
+	})
+
+	assert.NotNil(t, a)
+}
+
 // TestARPDB is the mock implementation of ARPDB to use in tests.
 type TestARPDB struct {
 	OnRefresh   func() (err error)
@@ -166,3 +176,46 @@ func TestCmdARPDB_arpa(t *testing.T) {
 		testutil.AssertErrorMsg(t, "cmd arpdb: running command: can't run", err)
 	})
 }
+
+func TestCmdARPDB_errors(t *testing.T) {
+	const errRead errors.Error = "can't read"
+
+	badReaderRunCmd := runCmdFunc(func() (r io.Reader, err error) {
+		return iotest.ErrReader(errRead), nil
+	})
+
+	a := &cmdARPDB{
+		runcmd: badReaderRunCmd,
+		parse:  parseArpA,
+		ns: &neighs{
+			mu: &sync.RWMutex{},
+			ns: make([]Neighbor, 0),
+		},
+	}
+
+	const wantErrMsg string = "cmd arpdb: scanning the output: " + string(errRead)
+
+	testutil.AssertErrorMsg(t, wantErrMsg, a.Refresh())
+}
+
+func TestEmptyARPDB(t *testing.T) {
+	a := EmptyARPDB{}
+
+	t.Run("refresh", func(t *testing.T) {
+		var err error
+		require.NotPanics(t, func() {
+			err = a.Refresh()
+		})
+
+		assert.NoError(t, err)
+	})
+
+	t.Run("neighbors", func(t *testing.T) {
+		var ns []Neighbor
+		require.NotPanics(t, func() {
+			ns = a.Neighbors()
+		})
+
+		assert.Empty(t, ns)
+	})
+}