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/aghnet/systemresolvers_others.go

@@ -0,0 +1,96 @@
+// +build !windows
+
+package aghnet
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net"
+	"sync"
+	"time"
+
+	"github.com/AdguardTeam/AdGuardHome/internal/agherr"
+)
+
+// defaultHostGen is the default method of generating host for Refresh.
+func defaultHostGen() (host string) {
+	// TODO(e.burkov): Use strings.Builder.
+	return fmt.Sprintf("test%d.org", time.Now().UnixNano())
+}
+
+// systemResolvers is a default implementation of SystemResolvers interface.
+type systemResolvers struct {
+	resolver    *net.Resolver
+	hostGenFunc HostGenFunc
+
+	// addrs is the map that contains cached local resolvers' addresses.
+	addrs     map[string]unit
+	addrsLock sync.RWMutex
+}
+
+func (sr *systemResolvers) Refresh() (err error) {
+	defer agherr.Annotate("systemResolvers: %w", &err)
+
+	_, err = sr.resolver.LookupHost(context.Background(), sr.hostGenFunc())
+	dnserr := &net.DNSError{}
+	if errors.As(err, &dnserr) && dnserr.Err == fakeDialErr.Error() {
+		return nil
+	}
+
+	return err
+}
+
+func newSystemResolvers(refreshIvl time.Duration, hostGenFunc HostGenFunc) (sr SystemResolvers) {
+	if hostGenFunc == nil {
+		hostGenFunc = defaultHostGen
+	}
+	s := &systemResolvers{
+		resolver: &net.Resolver{
+			PreferGo: true,
+		},
+		hostGenFunc: hostGenFunc,
+		addrs:       make(map[string]unit),
+	}
+	s.resolver.Dial = s.dialFunc
+
+	return s
+}
+
+// dialFunc gets the resolver's address and puts it into internal cache.
+func (sr *systemResolvers) dialFunc(_ context.Context, _, address string) (_ net.Conn, err error) {
+	// Just validate the passed address is a valid IP.
+	var host string
+	host, err = SplitHost(address)
+	if err != nil {
+		// TODO(e.burkov): Maybe use a structured badAddrPassedErr to
+		// allow unwrapping of the real error.
+		return nil, fmt.Errorf("%s: %w", err, badAddrPassedErr)
+	}
+
+	if net.ParseIP(host) == nil {
+		return nil, fmt.Errorf("parsing %q: %w", host, badAddrPassedErr)
+	}
+
+	sr.addrsLock.Lock()
+	defer sr.addrsLock.Unlock()
+
+	sr.addrs[address] = unit{}
+
+	return nil, fakeDialErr
+}
+
+func (sr *systemResolvers) Get() (rs []string) {
+	sr.addrsLock.RLock()
+	defer sr.addrsLock.RUnlock()
+
+	addrs := sr.addrs
+	rs = make([]string, len(addrs))
+	var i int
+	for addr := range addrs {
+		rs[i] = addr
+		i++
+	}
+
+	return rs
+}