net: LookupAddr("127.0.0.1") is "localhost" not "localhost."

Fixes #13564.

Change-Id: I30c827ef4a112fee21b8493a67d0227109e35072
Reviewed-on: https://go-review.googlesource.com/18384Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/dnsclient.go

@@ -165,8 +165,18 @@ func isDomainName(s string) bool {
 // trailing dot to match pure Go reverse resolver and all other lookup
 // routines.
 // See golang.org/issue/12189.
+// But we don't want to add dots for local names from /etc/hosts.
+// It's hard to tell so we settle on the heuristic that names without dots
+// (like "localhost" or "myhost") do not get trailing dots, but any other
+// names do.
 func absDomainName(b []byte) string {
-	if len(b) > 0 && b[len(b)-1] != '.' {
+	hasDots := false
+	for _, x := range b {
+		if x == '.' {
+			hasDots = true
+		}
+	}
+	if hasDots && b[len(b)-1] != '.' {
 		b = append(b, '.')
 	}
 	return string(b)

src/net/lookup_test.go

@@ -436,9 +436,15 @@ func TestLookupDotsWithLocalSource(t *testing.T) {
 			t.Errorf("#%d: %v", i, err)
 			continue
 		}
+		mode := "netgo"
+		if i == 1 {
+			mode = "netcgo"
+		}
 		for _, name := range names {
-			if !strings.HasSuffix(name, ".") {
-				t.Errorf("#%d: got %s; want name ending with trailing dot", i, name)
+			if strings.Index(name, ".") == len(name)-1 { // "localhost" not "localhost."
+				t.Errorf("%s: got %s; want %s", mode, name, name[:len(name)-1])
+			} else if strings.Contains(name, ".") && !strings.HasSuffix(name, ".") { // "localhost.localdomain." not "localhost.localdomain"
+				t.Errorf("%s: got %s; want name ending with trailing dot", mode, name)
 			}
 		}
 	}