commit 854dbb7fdcc85ca187e4d91a5a85d392242bb1cb
author Alex A Skinner <alex@lx.lc> Sat Aug 30 07:50:50 2014 +0900
committer Mikio Hara <mikioh.mikioh@gmail.com> Sat Aug 30 07:50:50 2014 +0900
tree d0d5c8e58cbe1121b31b2bb3e9261e7d86ab11aa
parent 858c57f5bd2170abe71cbbf505966cd648935d00

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

diff --git a/src/pkg/net/dnsclient_unix.go b/src/pkg/net/dnsclient_unix.go
index eb4b590..dc5eed9 100644
--- a/src/pkg/net/dnsclient_unix.go
+++ b/src/pkg/net/dnsclient_unix.go
@@ -310,13 +310,10 @@
 		}
 		// 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 @@
 		}
 	}
 
-	// Last ditch effort: try unsuffixed.
-	rname := name
-	if !rooted {
-		rname += "."
+	// 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
+		}
 	}
-	cname, addrs, err = tryOneName(cfg.dnsConfig, rname, 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

diff --git a/src/pkg/net/dnsclient_unix_test.go b/src/pkg/net/dnsclient_unix_test.go
index 39d82d99..204c8b7 100644
--- a/src/pkg/net/dnsclient_unix_test.go
+++ b/src/pkg/net/dnsclient_unix_test.go
@@ -217,3 +217,9 @@
 	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")
+	}
+}