net: fix case insensitivity lookup for local database such as /etc/hosts

The previous change for #12806 modified internal lookup tables and made
LookupAddr return forcibly lowercased host names by accident.

This change fixes the issue again without any behavioral change for
LookupAddr and adds missing test cases for lookupStaticHost and
lookupStaticAddr.

Updates #12806.
Fixes #13359.

Change-Id: Ifff4741cd79eb8b320b1b0f8c5e02b3a167c9fa8
Reviewed-on: https://go-review.googlesource.com/17217
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/hosts.go

@@ -27,11 +27,21 @@ func parseLiteralIP(addr string) string {
 	return ip.String() + "%" + zone
 }
 
-// Simple cache.
+// hosts contains known host entries.
 var hosts struct {
 	sync.Mutex
+
+	// Key for the list of literal IP addresses must be a host
+	// name. It would be part of DNS labels, a FQDN or an absolute
+	// FQDN.
+	// For now the key is converted to lower case for convenience.
 	byName map[string][]string
+
+	// Key for the list of host names must be a literal IP address
+	// including IPv6 address with zone identifier.
+	// We don't support old-classful IP address notation.
 	byAddr map[string][]string
+
 	expire time.Time
 	path   string
 }
@@ -60,11 +70,12 @@ func readHosts() {
 				continue
 			}
 			for i := 1; i < len(f); i++ {
+				name := f[i]
 				h := []byte(f[i])
 				lowerASCIIBytes(h)
-				lh := string(h)
-				hs[lh] = append(hs[lh], addr)
-				is[addr] = append(is[addr], lh)
+				key := string(h)
+				hs[key] = append(hs[key], addr)
+				is[addr] = append(is[addr], name)
 			}
 		}
 		// Update the data cache.

src/net/hosts_test.go

@@ -6,6 +6,7 @@ package net
 
 import (
 	"reflect"
+	"strings"
 	"testing"
 )
 
@@ -48,6 +49,13 @@ var lookupStaticHostTests = []struct {
 			{"localhost.localdomain", []string{"fe80::3%lo0"}},
 		},
 	},
+	{
+		"testdata/case-hosts", // see golang.org/issue/12806
+		[]staticHostEntry{
+			{"PreserveMe", []string{"127.0.0.1", "::1"}},
+			{"PreserveMe.local", []string{"127.0.0.1", "::1"}},
+		},
+	},
 }
 
 func TestLookupStaticHost(t *testing.T) {
@@ -56,9 +64,12 @@ func TestLookupStaticHost(t *testing.T) {
 	for _, tt := range lookupStaticHostTests {
 		testHookHostsPath = tt.name
 		for _, ent := range tt.ents {
-			addrs := lookupStaticHost(ent.in)
-			if !reflect.DeepEqual(addrs, ent.out) {
-				t.Errorf("%s, lookupStaticHost(%s) = %v; want %v", tt.name, ent.in, addrs, ent.out)
+			ins := []string{ent.in, strings.ToLower(ent.in), strings.ToUpper(ent.in)}
+			for _, in := range ins {
+				addrs := lookupStaticHost(in)
+				if !reflect.DeepEqual(addrs, ent.out) {
+					t.Errorf("%s, lookupStaticHost(%s) = %v; want %v", tt.name, in, addrs, ent.out)
+				}
 			}
 		}
 	}
@@ -74,7 +85,6 @@ var lookupStaticAddrTests = []struct {
 			{"255.255.255.255", []string{"broadcasthost"}},
 			{"127.0.0.2", []string{"odin"}},
 			{"127.0.0.3", []string{"odin"}},
-			{"127.0.0.4", []string{"bor"}},
 			{"::2", []string{"odin"}},
 			{"127.1.1.1", []string{"thor"}},
 			{"127.1.1.2", []string{"ullr", "ullrhost"}},
@@ -104,6 +114,13 @@ var lookupStaticAddrTests = []struct {
 			{"fe80::3%lo0", []string{"localhost", "localhost.localdomain"}},
 		},
 	},
+	{
+		"testdata/case-hosts", // see golang.org/issue/12806
+		[]staticHostEntry{
+			{"127.0.0.1", []string{"PreserveMe", "PreserveMe.local"}},
+			{"::1", []string{"PreserveMe", "PreserveMe.local"}},
+		},
+	},
 }
 
 func TestLookupStaticAddr(t *testing.T) {

src/net/testdata/case-hosts

+127.0.0.1	PreserveMe	PreserveMe.local
+::1		PreserveMe	PreserveMe.local

src/net/testdata/hosts

 255.255.255.255	broadcasthost
 127.0.0.2	odin
 127.0.0.3	odin  # inline comment 
-# case insensitivity
-127.0.0.4	Bor
 ::2             odin
 127.1.1.1	thor
 # aliases