Pull request 1938: AG-24132-rdns-ttl

Squashed commit of the following:

commit ba1e7b12cf7c0dc3ffab508d59c149f6c0930548
Merge: 8a94433ec ed86af582
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Tue Jul 25 13:43:25 2023 +0300

    Merge branch 'master' into AG-24132-rdns-ttl

commit 8a94433ec119d2158c166dd0222f57917908f3ad
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Mon Jul 24 19:30:21 2023 +0300

    all: imp docs

commit 4c1a3676b7be7ac4295c4e28550ddb6eb79a35d4
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Mon Jul 24 13:13:34 2023 +0300

    all: add rdns ttl

internal/rdns/rdns.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/AdguardTeam/golibs/errors"
 	"github.com/AdguardTeam/golibs/log"
+	"github.com/AdguardTeam/golibs/mathutil"
 	"github.com/bluele/gcache"
 )
 
@@ -32,7 +33,7 @@ func (Empty) Process(_ netip.Addr) (host string, changed bool) {
 type Exchanger interface {
 	// Exchange tries to resolve the ip in a suitable way, i.e. either as local
 	// or as external.
-	Exchange(ip netip.Addr) (host string, err error)
+	Exchange(ip netip.Addr) (host string, ttl time.Duration, err error)
 }
 
 // Config is the configuration structure for Default.
@@ -82,13 +83,16 @@ func (r *Default) Process(ip netip.Addr) (host string, changed bool) {
 		return fromCache, false
 	}
 
-	host, err := r.exchanger.Exchange(ip)
+	host, ttl, err := r.exchanger.Exchange(ip)
 	if err != nil {
 		log.Debug("rdns: resolving %q: %s", ip, err)
 	}
 
+	// TODO(s.chzhen):  Use built-in function max in Go 1.21.
+	ttl = mathutil.Max(ttl, r.cacheTTL)
+
 	item := &cacheItem{
-		expiry: time.Now().Add(r.cacheTTL),
+		expiry: time.Now().Add(ttl),
 		host:   host,
 	}
 

internal/rdns/rdns_test.go

@@ -55,18 +55,18 @@ func TestDefault_Process(t *testing.T) {
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
 			hit := 0
-			onExchange := func(ip netip.Addr) (host string, err error) {
+			onExchange := func(ip netip.Addr) (host string, ttl time.Duration, err error) {
 				hit++
 
 				switch ip {
 				case ip1:
-					return revAddr1, nil
+					return revAddr1, 0, nil
 				case ip2:
-					return revAddr2, nil
+					return revAddr2, 0, nil
 				case localIP:
-					return localRevAddr1, nil
+					return localRevAddr1, 0, nil
 				default:
-					return "", nil
+					return "", 0, nil
 				}
 			}
 			exchanger := &aghtest.Exchanger{