net: fix race in (*resolverConfig).tryUpdate

Fixes #14072.

Change-Id: Ie31caa06690ac621906fc5acd34da2efa4e2049f
Reviewed-on: https://go-review.googlesource.com/18860
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>

src/net/dnsclient_unix.go

@@ -229,7 +229,6 @@ type resolverConfig struct {
 	// time to recheck resolv.conf.
 	ch          chan struct{} // guards lastChecked and modTime
 	lastChecked time.Time     // last time resolv.conf was checked
-	modTime     time.Time     // time of resolv.conf modification
 
 	mu        sync.RWMutex // protects dnsConfig
 	dnsConfig *dnsConfig   // parsed resolv.conf structure used in lookups
@@ -239,16 +238,12 @@ var resolvConf resolverConfig
 
 // init initializes conf and is only called via conf.initOnce.
 func (conf *resolverConfig) init() {
-	// Set dnsConfig, modTime, and lastChecked so we don't parse
+	// Set dnsConfig and lastChecked so we don't parse
 	// resolv.conf twice the first time.
 	conf.dnsConfig = systemConf().resolv
 	if conf.dnsConfig == nil {
 		conf.dnsConfig = dnsReadConfig("/etc/resolv.conf")
 	}
-
-	if fi, err := os.Stat("/etc/resolv.conf"); err == nil {
-		conf.modTime = fi.ModTime()
-	}
 	conf.lastChecked = time.Now()
 
 	// Prepare ch so that only one update of resolverConfig may
@@ -274,18 +269,13 @@ func (conf *resolverConfig) tryUpdate(name string) {
 	}
 	conf.lastChecked = now
 
+	var mtime time.Time
 	if fi, err := os.Stat(name); err == nil {
-		if fi.ModTime().Equal(conf.modTime) {
-			return
+		mtime = fi.ModTime()
 	}
-		conf.modTime = fi.ModTime()
-	} else {
-		// If modTime wasn't set prior, assume nothing has changed.
-		if conf.modTime.IsZero() {
+	if mtime.Equal(conf.dnsConfig.mtime) {
 		return
 	}
-		conf.modTime = time.Time{}
-	}
 
 	dnsConf := dnsReadConfig(name)
 	conf.mu.Lock()

src/net/dnsconfig_unix.go

@@ -8,6 +8,8 @@
 
 package net
 
+import "time"
+
 var defaultNS = []string{"127.0.0.1", "::1"}
 
 type dnsConfig struct {
@@ -20,6 +22,7 @@ type dnsConfig struct {
 	unknownOpt bool      // anything unknown was encountered
 	lookup     []string  // OpenBSD top-level database "lookup" order
 	err        error     // any error that occurs during open of resolv.conf
+	mtime      time.Time // time of resolv.conf modification
 }
 
 // See resolv.conf(5) on a Linux machine.
@@ -38,6 +41,13 @@ func dnsReadConfig(filename string) *dnsConfig {
 		return conf
 	}
 	defer file.close()
+	if fi, err := file.file.Stat(); err == nil {
+		conf.mtime = fi.ModTime()
+	} else {
+		conf.servers = defaultNS
+		conf.err = err
+		return conf
+	}
 	for line, ok := file.readLine(); ok; line, ok = file.readLine() {
 		if len(line) > 0 && (line[0] == ';' || line[0] == '#') {
 			// comment.

src/net/dnsconfig_unix_test.go

@@ -10,6 +10,7 @@ import (
 	"os"
 	"reflect"
 	"testing"
+	"time"
 )
 
 var dnsReadConfigTests = []struct {
@@ -76,6 +77,7 @@ func TestDNSReadConfig(t *testing.T) {
 		if conf.err != nil {
 			t.Fatal(conf.err)
 		}
+		conf.mtime = time.Time{}
 		if !reflect.DeepEqual(conf, tt.want) {
 			t.Errorf("%s:\ngot: %+v\nwant: %+v", tt.name, conf, tt.want)
 		}