net: allow LookupAddr to use getnameinfo when cgo is enabled

This change allows LookupAddr to use getnameinfo through cgo for working
together with various name services other than DNS.

Fixes #7855.

Change-Id: I5b3b4aefe3d1b904541c3350865734d8cbb1c1c4
Reviewed-on: https://go-review.googlesource.com/3420Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/cgo_resnew.go

+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build cgo,!netgo
+// +build darwin linux solaris
+
+package net
+
+/*
+#include <sys/types.h>
+#include <sys/socket.h>
+
+#include <netdb.h>
+*/
+import "C"
+
+import "unsafe"
+
+func cgoNameinfoPTR(b []byte, sa *C.struct_sockaddr, salen C.socklen_t) (int, error) {
+	gerrno, err := C.getnameinfo(sa, salen, (*C.char)(unsafe.Pointer(&b[0])), C.socklen_t(len(b)), nil, 0, C.NI_NAMEREQD)
+	return int(gerrno), err
+}

src/net/cgo_resold.go

+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build cgo,!netgo
+// +build freebsd dragonfly netbsd openbsd
+
+package net
+
+/*
+#include <sys/types.h>
+#include <sys/socket.h>
+
+#include <netdb.h>
+*/
+import "C"
+
+import "unsafe"
+
+func cgoNameinfoPTR(b []byte, sa *C.struct_sockaddr, salen C.socklen_t) (int, error) {
+	gerrno, err := C.getnameinfo(sa, salen, (*C.char)(unsafe.Pointer(&b[0])), C.size_t(len(b)), nil, 0, C.NI_NAMEREQD)
+	return int(gerrno), err
+}

src/net/cgo_socknew.go

+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build cgo,!netgo
+// +build android linux solaris
+
+package net
+
+/*
+#include <sys/types.h>
+#include <sys/socket.h>
+
+#include <netinet/in.h>
+*/
+import "C"
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+func cgoSockaddrInet4(ip IP) *C.struct_sockaddr {
+	sa := syscall.RawSockaddrInet4{Family: syscall.AF_INET}
+	copy(sa.Addr[:], ip)
+	return (*C.struct_sockaddr)(unsafe.Pointer(&sa))
+}
+
+func cgoSockaddrInet6(ip IP) *C.struct_sockaddr {
+	sa := syscall.RawSockaddrInet6{Family: syscall.AF_INET6}
+	copy(sa.Addr[:], ip)
+	return (*C.struct_sockaddr)(unsafe.Pointer(&sa))
+}

src/net/cgo_sockold.go

+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build cgo,!netgo
+// +build darwin dragonfly freebsd netbsd openbsd
+
+package net
+
+/*
+#include <sys/types.h>
+#include <sys/socket.h>
+
+#include <netinet/in.h>
+*/
+import "C"
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+func cgoSockaddrInet4(ip IP) *C.struct_sockaddr {
+	sa := syscall.RawSockaddrInet4{Len: syscall.SizeofSockaddrInet4, Family: syscall.AF_INET}
+	copy(sa.Addr[:], ip)
+	return (*C.struct_sockaddr)(unsafe.Pointer(&sa))
+}
+
+func cgoSockaddrInet6(ip IP) *C.struct_sockaddr {
+	sa := syscall.RawSockaddrInet6{Len: syscall.SizeofSockaddrInet6, Family: syscall.AF_INET6}
+	copy(sa.Addr[:], ip)
+	return (*C.struct_sockaddr)(unsafe.Pointer(&sa))
+}

src/net/cgo_stub.go

@@ -27,3 +27,7 @@ func cgoLookupIP(name string) (addrs []IPAddr, err error, completed bool) {
 func cgoLookupCNAME(name string) (cname string, err error, completed bool) {
 	return "", nil, false
 }
+
+func cgoLookupPTR(addr string) (ptrs []string, err error, completed bool) {
+	return nil, nil, false
+}

src/net/cgo_unix.go

@@ -169,6 +169,72 @@ func cgoLookupCNAME(name string) (cname string, err error, completed bool) {
 	return
 }
 
+// These are roughly enough for the following:
+//
+// Source		Encoding			Maximum length of single name entry
+// Unicast DNS		ASCII or			<=253 + a NUL terminator
+//			Unicode in RFC 5892		252 * total number of labels + delimiters + a NUL terminator
+// Multicast DNS	UTF-8 in RFC 5198 or		<=253 + a NUL terminator
+//			the same as unicast DNS ASCII	<=253 + a NUL terminator
+// Local database	various				depends on implementation
+const (
+	nameinfoLen    = 64
+	maxNameinfoLen = 4096
+)
+
+func cgoLookupPTR(addr string) ([]string, error, bool) {
+	acquireThread()
+	defer releaseThread()
+
+	ip := ParseIP(addr)
+	if ip == nil {
+		return nil, &DNSError{Err: "invalid address", Name: addr}, true
+	}
+	sa, salen := cgoSockaddr(ip)
+	if sa == nil {
+		return nil, &DNSError{Err: "invalid address " + ip.String(), Name: addr}, true
+	}
+	var err error
+	var b []byte
+	var gerrno int
+	for l := nameinfoLen; l <= maxNameinfoLen; l *= 2 {
+		b = make([]byte, l)
+		gerrno, err = cgoNameinfoPTR(b, sa, salen)
+		if gerrno == 0 || gerrno != C.EAI_OVERFLOW {
+			break
+		}
+	}
+	if gerrno != 0 {
+		switch gerrno {
+		case C.EAI_SYSTEM:
+			if err == nil { // see golang.org/issue/6232
+				err = syscall.EMFILE
+			}
+		default:
+			err = addrinfoErrno(gerrno)
+		}
+		return nil, &DNSError{Err: err.Error(), Name: addr}, true
+	}
+
+	for i := 0; i < len(b); i++ {
+		if b[i] == 0 {
+			b = b[:i]
+			break
+		}
+	}
+	return []string{string(b)}, nil, true
+}
+
+func cgoSockaddr(ip IP) (*C.struct_sockaddr, C.socklen_t) {
+	if ip4 := ip.To4(); ip4 != nil {
+		return cgoSockaddrInet4(ip4), C.socklen_t(syscall.SizeofSockaddrInet4)
+	}
+	if ip6 := ip.To16(); ip6 != nil {
+		return cgoSockaddrInet6(ip6), C.socklen_t(syscall.SizeofSockaddrInet6)
+	}
+	return nil, 0
+}
+
 func copyIP(x IP) IP {
 	if len(x) < 16 {
 		return x.To16()

src/net/dnsclient_unix.go

@@ -479,3 +479,28 @@ func goLookupCNAME(name string) (cname string, err error) {
 	cname = rr[0].(*dnsRR_CNAME).Cname
 	return
 }
+
+// goLookupPTR is the native Go implementation of LookupAddr.
+// Used only if cgoLookupPTR refuses to handle the request (that is,
+// only if cgoLookupPTR is the stub in cgo_stub.go).
+// Normally we let cgo use the C library resolver instead of depending
+// on our lookup code, so that Go and C get the same answers.
+func goLookupPTR(addr string) ([]string, error) {
+	names := lookupStaticAddr(addr)
+	if len(names) > 0 {
+		return names, nil
+	}
+	arpa, err := reverseaddr(addr)
+	if err != nil {
+		return nil, err
+	}
+	_, rrs, err := lookup(arpa, dnsTypePTR)
+	if err != nil {
+		return nil, err
+	}
+	ptrs := make([]string, len(rrs))
+	for i, rr := range rrs {
+		ptrs[i] = rr.(*dnsRR_PTR).Ptr
+	}
+	return ptrs, nil
+}

src/net/lookup_test.go

@@ -174,7 +174,7 @@ func TestLookupGmailTXT(t *testing.T) {
 	}
 }
 
-var lookupGooglePublicDNSAddrs = []struct {
+var lookupGooglePublicDNSAddrTests = []struct {
 	addr, name string
 }{
 	{"8.8.8.8", ".google.com"},
@@ -191,7 +191,7 @@ func TestLookupGooglePublicDNSAddr(t *testing.T) {
 		t.Skip("both IPv4 and IPv6 are required")
 	}
 
-	for _, tt := range lookupGooglePublicDNSAddrs {
+	for _, tt := range lookupGooglePublicDNSAddrTests {
 		names, err := LookupAddr(tt.addr)
 		if err != nil {
 			t.Fatal(err)

src/net/lookup_unix.go

@@ -148,21 +148,9 @@ func lookupTXT(name string) ([]string, error) {
 }
 
 func lookupAddr(addr string) ([]string, error) {
-	names := lookupStaticAddr(addr)
-	if len(names) > 0 {
-		return names, nil
-	}
-	arpa, err := reverseaddr(addr)
-	if err != nil {
-		return nil, err
-	}
-	_, rrs, err := lookup(arpa, dnsTypePTR)
-	if err != nil {
-		return nil, err
-	}
-	ptrs := make([]string, len(rrs))
-	for i, rr := range rrs {
-		ptrs[i] = rr.(*dnsRR_PTR).Ptr
+	ptrs, err, ok := cgoLookupPTR(addr)
+	if !ok {
+		ptrs, err = goLookupPTR(addr)
 	}
-	return ptrs, nil
+	return ptrs, err
 }