Pull request: 2704 local addresses vol.2

Merge in DNS/adguard-home from 2704-local-addresses-vol.2 to master

Updates #2704.
Updates #2829.

Squashed commit of the following:

commit 507d038c2709de59246fc0b65c3c4ab8e38d1990
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Wed Mar 31 14:33:05 2021 +0300

    aghtest: fix file name

commit 8e19f99337bee1d88ad6595adb96f9bb23fa3c41
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Wed Mar 31 14:06:43 2021 +0300

    aghnet: rm redundant mutexes

commit 361fa418b33ed160ca20862be1c455ab9378c03f
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Wed Mar 31 13:45:30 2021 +0300

    all: fix names, docs

commit 14034f4f0230d7aaa3645054946ae5c278089a99
Merge: 35e265cc a72ce1cf
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Wed Mar 31 13:38:15 2021 +0300

    Merge branch 'master' into 2704-local-addresses-vol.2

commit 35e265cc8cd308ef1fda414b58c0217cb5f258e4
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Wed Mar 31 13:33:35 2021 +0300

    aghnet: imp naming

commit 7a7edac7208a40697d7bc50682b923a144e28e2b
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Mar 30 20:59:54 2021 +0300

    changelog: oops, nope yet

commit d26a5d2513daf662ac92053b5e235189a64cc022
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Mar 30 20:55:53 2021 +0300

    all: some renaming for the glory of semantics

commit 9937fa619452b0742616217b975e3ff048d58acb
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Mon Mar 29 15:34:42 2021 +0300

    all: log changes

commit d8d9e6dfeea8474466ee25f27021efdd3ddb1592
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Mar 26 18:32:23 2021 +0300

    all: imp localresolver, imp cutting off own addresses

commit 344140df449b85925f19b460fd7dc7c08e29c35a
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Mar 26 14:53:33 2021 +0300

    all: imp code quality

commit 1c5c0babec73b125044e23dd3aa75d8eefc19b28
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Thu Mar 25 20:44:08 2021 +0300

    all: fix go.mod

commit 0b9fb3c2369a752e893af8ddc45a86bb9fb27ce5
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Thu Mar 25 20:38:51 2021 +0300

    all: add error handling

commit a7a2e51f57fc6f8f74b95a264ad345cd2a9e026e
Merge: c13be634 27f4f052
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Thu Mar 25 19:48:36 2021 +0300

    Merge branch 'master' into 2704-local-addresses-vol.2

commit c13be634f47bcaed9320a732a51c0e4752d0dad0
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Thu Mar 25 18:52:28 2021 +0300

    all: cover rdns with tests, imp aghnet functionality

commit 48bed9025944530c613ee53e7961d6d5fbabf8be
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Wed Mar 24 20:18:07 2021 +0300

    home: make rdns great again

commit 1dbacfc8d5b6895807797998317fe3cc814617c1
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Wed Mar 24 16:07:52 2021 +0300

    all: imp external client restriction

commit 1208a319a7f4ffe7b7fa8956f245d7a19437c0a4
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Mon Mar 22 15:26:45 2021 +0300

    all: finish local ptr processor

commit c8827fc3db289e1a5d7a11d057743bab39957b02
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Mar 2 13:41:22 2021 +0300

    all: imp ipdetector, add local ptr processor

internal/aghnet/exchanger.go

@@ -0,0 +1,79 @@
+package aghnet
+
+import (
+	"time"
+
+	"github.com/AdguardTeam/AdGuardHome/internal/agherr"
+	"github.com/AdguardTeam/dnsproxy/upstream"
+	"github.com/miekg/dns"
+)
+
+// This package is not the best place for this functionality, but we put it here
+// since we need to use it in both rDNS (home) and dnsServer (dnsforward).
+
+// NoUpstreamsErr should be returned when there are no upstreams inside
+// Exchanger implementation.
+const NoUpstreamsErr agherr.Error = "no upstreams specified"
+
+// Exchanger represents an object able to resolve DNS messages.
+//
+// TODO(e.burkov): Maybe expand with method like ExchangeParallel to be able to
+// use user's upstream mode settings.  Also, think about Update method to
+// refresh the internal state.
+type Exchanger interface {
+	Exchange(req *dns.Msg) (resp *dns.Msg, err error)
+}
+
+// multiAddrExchanger is the default implementation of Exchanger interface.
+type multiAddrExchanger struct {
+	ups []upstream.Upstream
+}
+
+// NewMultiAddrExchanger creates an Exchanger instance from passed addresses.
+// It returns an error if any of addrs failed to become an upstream.
+func NewMultiAddrExchanger(addrs []string, timeout time.Duration) (e Exchanger, err error) {
+	defer agherr.Annotate("exchanger: %w", &err)
+
+	if len(addrs) == 0 {
+		return &multiAddrExchanger{}, nil
+	}
+
+	var ups []upstream.Upstream = make([]upstream.Upstream, 0, len(addrs))
+	for _, addr := range addrs {
+		var u upstream.Upstream
+		u, err = upstream.AddressToUpstream(addr, upstream.Options{Timeout: timeout})
+		if err != nil {
+			return nil, err
+		}
+
+		ups = append(ups, u)
+	}
+
+	return &multiAddrExchanger{ups: ups}, nil
+}
+
+// Exсhange performs a query to each resolver until first response.
+func (e *multiAddrExchanger) Exchange(req *dns.Msg) (resp *dns.Msg, err error) {
+	defer agherr.Annotate("exchanger: %w", &err)
+
+	// TODO(e.burkov): Maybe prohibit the initialization without upstreams.
+	if len(e.ups) == 0 {
+		return nil, NoUpstreamsErr
+	}
+
+	var errs []error
+	for _, u := range e.ups {
+		resp, err = u.Exchange(req)
+		if err != nil {
+			errs = append(errs, err)
+
+			continue
+		}
+
+		if resp != nil {
+			return resp, nil
+		}
+	}
+
+	return nil, agherr.Many("can't exchange", errs...)
+}