Pull request: 2704 local resolvers vol.1

Merge in DNS/adguard-home from 2704-local-addresses-vol.1 to master

Updates #2704.
Updates #2829.
Updates #2846.

Squashed commit of the following:

commit 9a49b3d27edcb30da7f16a065226907833b1dc81
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Mon Mar 22 15:39:17 2021 +0300

    aghnet: imp docs and logging

commit 74f95a29c55b9e732276601b0ecc63fb7c3a9f9e
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Mar 19 20:56:51 2021 +0300

    all: fix friday evening mistakes

commit 0e2066bc5c16ed807fa601780b99e154502361a9
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Mar 19 20:51:15 2021 +0300

    all: upd testify, imp code quality

commit 8237c50b670c58361ccf7adec3ff2452b1196677
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Mar 19 20:19:29 2021 +0300

    aghnet: imp test naming

commit 14eb1e189339554c0a6d38e2ba7a93917774ebab
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Mar 19 19:41:43 2021 +0300

    aghnet: isolate windows-specific functionality

commit d461ac8b18c187999da3e3aba116571b7ebe6785
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Mar 19 14:50:05 2021 +0300

    aghnet: imp code quality

commit d0ee01cb1f8613de2085c0f2f2f396e46beb52a5
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Mar 19 11:59:10 2021 +0300

    all: mv funcs to agherr, mk system resolvers getter

internal/agherr/agherr.go

@@ -4,6 +4,8 @@ package agherr
 import (
 	"fmt"
 	"strings"
+
+	"github.com/AdguardTeam/golibs/log"
 )
 
 // Error is the constant error type.
@@ -95,6 +97,8 @@ type wrapper interface {
 //   }
 //
 // msg must contain the final ": %w" verb.
+//
+// TODO(a.garipov): Clearify the function usage.
 func Annotate(msg string, errPtr *error, args ...interface{}) {
 	if errPtr == nil {
 		return
@@ -107,3 +111,17 @@ func Annotate(msg string, errPtr *error, args ...interface{}) {
 		*errPtr = fmt.Errorf(msg, args...)
 	}
 }
+
+// LogPanic is a convinient helper function to log a panic in a goroutine.  It
+// should not be used where proper error handling is required.
+func LogPanic(prefix string) {
+	if v := recover(); v != nil {
+		if prefix != "" {
+			log.Error("%s: recovered from panic: %v", prefix, v)
+
+			return
+		}
+
+		log.Error("recovered from panic: %v", v)
+	}
+}

internal/agherr/agherr_test.go

@@ -1,10 +1,12 @@
 package agherr
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"testing"
 
+	"github.com/AdguardTeam/AdGuardHome/internal/aghtest"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -120,3 +122,39 @@ func TestAnnotate(t *testing.T) {
 		assert.Equal(t, wantMsg, err.Error())
 	})
 }
+
+func TestLogPanic(t *testing.T) {
+	buf := &bytes.Buffer{}
+	aghtest.ReplaceLogWriter(t, buf)
+
+	t.Run("prefix", func(t *testing.T) {
+		const (
+			panicMsg        = "spooky!"
+			prefix          = "packagename"
+			errWithNoPrefix = "[error] recovered from panic: spooky!"
+			errWithPrefix   = "[error] packagename: recovered from panic: spooky!"
+		)
+
+		panicFunc := func(prefix string) {
+			defer LogPanic(prefix)
+
+			panic(panicMsg)
+		}
+
+		panicFunc("")
+		assert.Contains(t, buf.String(), errWithNoPrefix)
+		buf.Reset()
+
+		panicFunc(prefix)
+		assert.Contains(t, buf.String(), errWithPrefix)
+		buf.Reset()
+	})
+
+	t.Run("don't_panic", func(t *testing.T) {
+		require.NotPanics(t, func() {
+			defer LogPanic("")
+		})
+
+		assert.Empty(t, buf.String())
+	})
+}