net: ensure identical queries are not sent multiple times in builtin stub resolver

Prevents non-rooted queries with > ndots dots from being tried twice on error.
Fixes #8616.

Benchmark results on linux/amd64
benchmark                        old ns/op    new ns/op    delta
BenchmarkGoLookupIPNoSuchHost      8212394      4413293  -46.26%

benchmark                       old allocs   new allocs    delta
BenchmarkGoLookupIPNoSuchHost          216          108  -50.00%

benchmark                        old bytes    new bytes    delta
BenchmarkGoLookupIPNoSuchHost        17460         8726  -50.02%

LGTM=iant, mikioh.mikioh
R=golang-codereviews, iant, mikioh.mikioh
CC=golang-codereviews
https://golang.org/cl/137870043

src/pkg/net/dnsclient_unix.go

@@ -310,13 +310,10 @@ func lookup(name string, qtype uint16) (cname string, addrs []dnsRR, err error)
 		}
 		// Can try as ordinary name.
 		cname, addrs, err = tryOneName(cfg.dnsConfig, rname, qtype)
-		if err == nil {
+		if rooted || err == nil {
 			return
 		}
 	}
-	if rooted {
-		return
-	}
 
 	// Otherwise, try suffixes.
 	for i := 0; i < len(cfg.dnsConfig.search); i++ {
@@ -330,15 +327,15 @@ func lookup(name string, qtype uint16) (cname string, addrs []dnsRR, err error)
 		}
 	}
 
-	// Last ditch effort: try unsuffixed.
-	rname := name
-	if !rooted {
-		rname += "."
-	}
-	cname, addrs, err = tryOneName(cfg.dnsConfig, rname, qtype)
+	// Last ditch effort: try unsuffixed only if we haven't already,
+	// that is, name is not rooted and has less than ndots dots.
+	if count(name, '.') < cfg.dnsConfig.ndots {
+		cname, addrs, err = tryOneName(cfg.dnsConfig, name+".", qtype)
 		if err == nil {
 			return
 		}
+	}
+
 	if e, ok := err.(*DNSError); ok {
 		// Show original name passed to lookup, not suffixed one.
 		// In general we might have tried many suffixes; showing

src/pkg/net/dnsclient_unix_test.go

@@ -217,3 +217,9 @@ func TestReloadResolvConfChange(t *testing.T) {
 	r.SetConf("nameserver 8.8.4.4")
 	r.WantServers([]string{"[8.8.4.4]"})
 }
+
+func BenchmarkGoLookupIPNoSuchHost(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		goLookupIP("some.nonexistent")
+	}
+}