dnsforward: imp code, docs

CHANGELOG.md

@@ -114,7 +114,7 @@ In this release, the schema version has changed from 10 to 12.
 
 ### Fixed
 
-- Panic while shutdown ([#3655]).
+- Occasional panic during shutdown ([#3655]).
 - Addition of IPs into only one as opposed to all matching ipsets on Linux
   ([#3638]).
 - Removal of temporary filter files ([#3567]).

internal/dnsforward/dns.go

@@ -8,7 +8,6 @@ import (
 	"github.com/AdguardTeam/AdGuardHome/internal/dhcpd"
 	"github.com/AdguardTeam/AdGuardHome/internal/filtering"
 	"github.com/AdguardTeam/dnsproxy/proxy"
-	"github.com/AdguardTeam/golibs/errors"
 	"github.com/AdguardTeam/golibs/log"
 	"github.com/AdguardTeam/golibs/netutil"
 	"github.com/AdguardTeam/golibs/stringutil"
@@ -545,7 +544,7 @@ func (s *Server) processUpstream(ctx *dnsContext) (rc resultCode) {
 
 	prx := s.proxy()
 	if prx == nil {
-		ctx.err = errors.Error("server is closed")
+		ctx.err = srvClosedErr
 
 		return resultCodeError
 	}

internal/dnsforward/dnsforward.go

@@ -551,13 +551,15 @@ func (s *Server) IsRunning() bool {
 	return s.isRunning
 }
 
-// proxy returns the underlying *proxy.Proxy from s to be accessed in
-// concurrent-safe methods.  It should prevent problems caused by unexpected
-// rewrites of the field inside s.  It returns nil when s is stopping or already
-// closed itself.
+// srvClosedErr is returned when the method can't complete without unacessible
+// data from the closing server.
+const srvClosedErr errors.Error = "server is closed"
+
+// proxy returns a pointer to the current DNS proxy instance.  If p is nil, the
+// server is closing.
 //
 // See https://github.com/AdguardTeam/AdGuardHome/issues/3655.
-func (s *Server) proxy() (prx *proxy.Proxy) {
+func (s *Server) proxy() (p *proxy.Proxy) {
 	s.serverLock.RLock()
 	defer s.serverLock.RUnlock()
 

internal/dnsforward/msg.go

@@ -251,7 +251,7 @@ func (s *Server) genBlockedHost(request *dns.Msg, newAddr string, d *proxy.DNSCo
 
 	prx := s.proxy()
 	if prx == nil {
-		log.Debug("dns server is closed")
+		log.Debug("dns: %s", srvClosedErr)
 
 		return s.genServerFailure(request)
 	}