net: ensure dnsConfig search list is rooted

Avoids some extra work and string concatenation at query time.

benchmark                                      old allocs     new allocs     delta
BenchmarkGoLookupIP-32                         154            150            -2.60%
BenchmarkGoLookupIPNoSuchHost-32               446            442            -0.90%
BenchmarkGoLookupIPWithBrokenNameServer-32     564            568            +0.71%

benchmark                                      old bytes     new bytes     delta
BenchmarkGoLookupIP-32                         10824         10704         -1.11%
BenchmarkGoLookupIPNoSuchHost-32               43140         42992         -0.34%
BenchmarkGoLookupIPWithBrokenNameServer-32     46616         46680         +0.14%

BenchmarkGoLookupIPWithBrokenNameServer's regression appears to be
because it's actually only performing 1 LookupIP call, so the extra
work done parsing the DNS config file doesn't amortize as well as for
BenchmarkGoLookupIP or BenchmarkGoLOokupIPNoSuchHost, which perform
2000+ LookupIP calls per run.

Update #15473.

Change-Id: I98c8072f2f39e2f2ccd6c55e9e9bd309f5ad68f8
Reviewed-on: https://go-review.googlesource.com/22571
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/dnsclient_unix.go

@@ -361,23 +361,23 @@ func (conf *dnsConfig) nameList(name string) []string {
 	if rooted {
 		return []string{name}
 	}
+
+	hasNdots := count(name, '.') >= conf.ndots
+	name += "."
+
 	// Build list of search choices.
 	names := make([]string, 0, 1+len(conf.search))
 	// If name has enough dots, try unsuffixed first.
-	if count(name, '.') >= conf.ndots {
-		names = append(names, name+".")
+	if hasNdots {
+		names = append(names, name)
 	}
 	// Try suffixes.
 	for _, suffix := range conf.search {
-		suffixed := name + "." + suffix
-		if suffixed[len(suffixed)-1] != '.' {
-			suffixed += "."
-		}
-		names = append(names, suffixed)
+		names = append(names, name+suffix)
 	}
 	// Try unsuffixed, if not tried first above.
-	if count(name, '.') < conf.ndots {
-		names = append(names, name+".")
+	if !hasNdots {
+		names = append(names, name)
 	}
 	return names
 }

src/net/dnsconfig_unix.go

@@ -20,7 +20,7 @@ var (
 
 type dnsConfig struct {
 	servers    []string      // server addresses (in host:port form) to use
-	search     []string      // suffixes to append to local name
+	search     []string      // rooted suffixes to append to local name
 	ndots      int           // number of dots in name to trigger absolute lookup
 	timeout    time.Duration // wait before giving up on a query, including retries
 	attempts   int           // lost packets before giving up on server
@@ -78,13 +78,13 @@ func dnsReadConfig(filename string) *dnsConfig {
 
 		case "domain": // set search path to just this domain
 			if len(f) > 1 {
-				conf.search = []string{f[1]}
+				conf.search = []string{ensureRooted(f[1])}
 			}
 
 		case "search": // set search path to given servers
 			conf.search = make([]string, len(f)-1)
 			for i := 0; i < len(conf.search); i++ {
-				conf.search[i] = f[i+1]
+				conf.search[i] = ensureRooted(f[i+1])
 			}
 
 		case "options": // magic options
@@ -141,7 +141,7 @@ func dnsDefaultSearch() []string {
 		return nil
 	}
 	if i := byteIndex(hn, '.'); i >= 0 && i < len(hn)-1 {
-		return []string{hn[i+1:]}
+		return []string{ensureRooted(hn[i+1:])}
 	}
 	return nil
 }
@@ -149,3 +149,10 @@ func dnsDefaultSearch() []string {
 func hasPrefix(s, prefix string) bool {
 	return len(s) >= len(prefix) && s[:len(prefix)] == prefix
 }
+
+func ensureRooted(s string) string {
+	if len(s) > 0 && s[len(s)-1] == '.' {
+		return s
+	}
+	return s + "."
+}

src/net/dnsconfig_unix_test.go

@@ -22,7 +22,7 @@ var dnsReadConfigTests = []struct {
 		name: "testdata/resolv.conf",
 		want: &dnsConfig{
 			servers:    []string{"8.8.8.8:53", "[2001:4860:4860::8888]:53", "[fe80::1%lo0]:53"},
-			search:     []string{"localdomain"},
+			search:     []string{"localdomain."},
 			ndots:      5,
 			timeout:    10 * time.Second,
 			attempts:   3,
@@ -34,7 +34,7 @@ var dnsReadConfigTests = []struct {
 		name: "testdata/domain-resolv.conf",
 		want: &dnsConfig{
 			servers:  []string{"8.8.8.8:53"},
-			search:   []string{"localdomain"},
+			search:   []string{"localdomain."},
 			ndots:    1,
 			timeout:  5 * time.Second,
 			attempts: 2,
@@ -44,7 +44,7 @@ var dnsReadConfigTests = []struct {
 		name: "testdata/search-resolv.conf",
 		want: &dnsConfig{
 			servers:  []string{"8.8.8.8:53"},
-			search:   []string{"test", "invalid"},
+			search:   []string{"test.", "invalid."},
 			ndots:    1,
 			timeout:  5 * time.Second,
 			attempts: 2,
@@ -57,7 +57,7 @@ var dnsReadConfigTests = []struct {
 			ndots:    1,
 			timeout:  5 * time.Second,
 			attempts: 2,
-			search:   []string{"domain.local"},
+			search:   []string{"domain.local."},
 		},
 	},
 	{
@@ -105,7 +105,7 @@ func TestDNSReadMissingFile(t *testing.T) {
 		ndots:    1,
 		timeout:  5 * time.Second,
 		attempts: 2,
-		search:   []string{"domain.local"},
+		search:   []string{"domain.local."},
 	}
 	if !reflect.DeepEqual(conf, want) {
 		t.Errorf("missing resolv.conf:\ngot: %+v\nwant: %+v", conf, want)
@@ -119,11 +119,11 @@ var dnsDefaultSearchTests = []struct {
 }{
 	{
 		name: "host.long.domain.local",
-		want: []string{"long.domain.local"},
+		want: []string{"long.domain.local."},
 	},
 	{
 		name: "host.local",
-		want: []string{"local"},
+		want: []string{"local."},
 	},
 	{
 		name: "host",