net: detect changes to /etc/resolv.conf.

Implement the changes as suggested by rsc.
Fixes #6670.

LGTM=josharian, iant
R=golang-codereviews, iant, josharian, mikioh.mikioh, alex, gobot
CC=golang-codereviews, rsc
https://golang.org/cl/83690045

src/pkg/net/dnsclient_unix_test.go

@@ -7,7 +7,13 @@
 package net
 
 import (
+	"io"
+	"io/ioutil"
+	"os"
+	"path"
+	"reflect"
 	"testing"
+	"time"
 )
 
 func TestTCPLookup(t *testing.T) {
@@ -25,3 +31,129 @@ func TestTCPLookup(t *testing.T) {
 		t.Fatalf("exchange failed: %v", err)
 	}
 }
+
+type resolvConfTest struct {
+	*testing.T
+	dir     string
+	path    string
+	started bool
+	quitc   chan chan struct{}
+}
+
+func newResolvConfTest(t *testing.T) *resolvConfTest {
+	dir, err := ioutil.TempDir("", "resolvConfTest")
+	if err != nil {
+		t.Fatalf("could not create temp dir: %v", err)
+	}
+
+	// Disable the default loadConfig
+	onceLoadConfig.Do(func() {})
+
+	r := &resolvConfTest{
+		T:     t,
+		dir:   dir,
+		path:  path.Join(dir, "resolv.conf"),
+		quitc: make(chan chan struct{}),
+	}
+
+	return r
+}
+
+func (r *resolvConfTest) Start() {
+	loadConfig(r.path, 100*time.Millisecond, r.quitc)
+	r.started = true
+}
+
+func (r *resolvConfTest) SetConf(s string) {
+	// Make sure the file mtime will be different once we're done here,
+	// even on systems with coarse (1s) mtime resolution.
+	time.Sleep(time.Second)
+
+	f, err := os.OpenFile(r.path, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0600)
+	if err != nil {
+		r.Fatalf("failed to create temp file %s: %v", r.path, err)
+	}
+	if _, err := io.WriteString(f, s); err != nil {
+		f.Close()
+		r.Fatalf("failed to write temp file: %v", err)
+	}
+	f.Close()
+
+	if r.started {
+		cfg.ch <- struct{}{} // fill buffer
+		cfg.ch <- struct{}{} // wait for reload to begin
+		cfg.ch <- struct{}{} // wait for reload to complete
+	}
+}
+
+func (r *resolvConfTest) WantServers(want []string) {
+	cfg.mu.RLock()
+	defer cfg.mu.RUnlock()
+	if got := cfg.dnsConfig.servers; !reflect.DeepEqual(got, want) {
+		r.Fatalf("Unexpected dns server loaded, got %v want %v", got, want)
+	}
+}
+
+func (r *resolvConfTest) Close() {
+	resp := make(chan struct{})
+	r.quitc <- resp
+	<-resp
+	if err := os.RemoveAll(r.dir); err != nil {
+		r.Logf("failed to remove temp dir %s: %v", r.dir, err)
+	}
+}
+
+func TestReloadResolvConfFail(t *testing.T) {
+	if testing.Short() || !*testExternal {
+		t.Skip("skipping test to avoid external network")
+	}
+
+	r := newResolvConfTest(t)
+	defer r.Close()
+
+	// resolv.conf.tmp does not exist yet
+	r.Start()
+	if _, err := goLookupIP("golang.org"); err == nil {
+		t.Fatal("goLookupIP(missing) succeeded")
+	}
+
+	r.SetConf("nameserver 8.8.8.8")
+	if _, err := goLookupIP("golang.org"); err != nil {
+		t.Fatalf("goLookupIP(missing; good) failed: %v", err)
+	}
+
+	// Using a bad resolv.conf while we had a good
+	// one before should not update the config
+	r.SetConf("")
+	if _, err := goLookupIP("golang.org"); err != nil {
+		t.Fatalf("goLookupIP(missing; good; bad) failed: %v", err)
+	}
+}
+
+func TestReloadResolvConfChange(t *testing.T) {
+	if testing.Short() || !*testExternal {
+		t.Skip("skipping test to avoid external network")
+	}
+
+	r := newResolvConfTest(t)
+	defer r.Close()
+
+	r.SetConf("nameserver 8.8.8.8")
+	r.Start()
+
+	if _, err := goLookupIP("golang.org"); err != nil {
+		t.Fatalf("goLookupIP(good) failed: %v", err)
+	}
+	r.WantServers([]string{"[8.8.8.8]"})
+
+	// Using a bad resolv.conf when we had a good one
+	// before should not update the config
+	r.SetConf("")
+	if _, err := goLookupIP("golang.org"); err != nil {
+		t.Fatalf("goLookupIP(good; bad) failed: %v", err)
+	}
+
+	// A new good config should get picked up
+	r.SetConf("nameserver 8.8.4.4")
+	r.WantServers([]string{"[8.8.4.4]"})
+}