Pull request: 5089-windows-hosts-crash

Updates #5089.

Squashed commit of the following:

commit dd3ce76
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Nov 7 16:21:43 2022 +0300

    aghnet: fix comparison

commit 0d736fb
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Nov 7 14:13:05 2022 +0300

    aghnet: fix crash

CHANGELOG.md

@@ -17,6 +17,14 @@ and this project adheres to
 
 
 
+### Fixed
+
+- Crash on Windows when system hosts files are read ([#5089]).
+
+[#5089]: https://github.com/AdguardTeam/AdGuardHome/issues/5089
+
+
+
 <!--
 ## [v0.107.18] - 2022-11-16 (APPROX.)
 
@@ -1088,7 +1096,7 @@ See also the [v0.106.0 GitHub milestone][ms-v0.106.0].
 - Hostname uniqueness validation in the DHCP server ([#2952]).
 - Hostname generating for DHCP clients which don't provide their own ([#2723]).
 - New flag `--no-etc-hosts` to disable client domain name lookups in the
-  operating system's /etc/hosts files ([#1947]).
+  operating system's `/etc/hosts` files ([#1947]).
 - The ability to set up custom upstreams to resolve PTR queries for local
   addresses and to disable the automatic resolving of clients' addresses
   ([#2704]).

internal/aghnet/hostscontainer.go

@@ -139,6 +139,8 @@ type HostsRecord struct {
 func (rec *HostsRecord) equal(other *HostsRecord) (ok bool) {
 	if rec == nil {
 		return other == nil
+	} else if other == nil {
+		return false
 	}
 
 	return rec.Canonical == other.Canonical && rec.Aliases.Equal(other.Aliases)
@@ -478,7 +480,11 @@ func (hc *HostsContainer) refresh() (err error) {
 		return fmt.Errorf("refreshing : %w", err)
 	}
 
-	if maps.EqualFunc(hp.table, hc.last, (*HostsRecord).equal) {
+	// hc.last is nil on the first refresh, so let that one through.
+	//
+	// TODO(a.garipov): Once https://github.com/golang/go/issues/56621 is
+	// resolved, remove the first condition.
+	if hc.last != nil && maps.EqualFunc(hp.table, hc.last, (*HostsRecord).equal) {
 		log.Debug("%s: no changes detected", hostsContainerPref)
 
 		return nil