syscall: handle empty address in ReadFrom better

Handle return values from recvfrom correctly when the
kernel decides to not return an address.

Fixes #4636.
Fixes #4352.

R=rsc, mikioh.mikioh, dave
CC=golang-dev
https://golang.org/cl/7058062

src/pkg/net/unix_test.go

+// Copyright 2013 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 !plan9,!windows
+
+package net
+
+import (
+	"bytes"
+	"io/ioutil"
+	"os"
+	"syscall"
+	"testing"
+	"time"
+)
+
+// testUnixAddr uses ioutil.TempFile to get a name that is unique.
+func testUnixAddr() string {
+	f, err := ioutil.TempFile("", "nettest")
+	if err != nil {
+		panic(err)
+	}
+	addr := f.Name()
+	f.Close()
+	os.Remove(addr)
+	return addr
+}
+
+func TestReadUnixgramWithUnnamedSocket(t *testing.T) {
+	addr := testUnixAddr()
+	la, err := ResolveUnixAddr("unixgram", addr)
+	if err != nil {
+		t.Fatalf("ResolveUnixAddr failed: %v", err)
+	}
+	c, err := ListenUnixgram("unixgram", la)
+	if err != nil {
+		t.Fatalf("ListenUnixgram failed: %v", err)
+	}
+	defer func() {
+		c.Close()
+		os.Remove(addr)
+	}()
+
+	off := make(chan bool)
+	data := [5]byte{1, 2, 3, 4, 5}
+
+	go func() {
+		defer func() { off <- true }()
+		s, err := syscall.Socket(syscall.AF_UNIX, syscall.SOCK_DGRAM, 0)
+		if err != nil {
+			t.Errorf("syscall.Socket failed: %v", err)
+			return
+		}
+		defer syscall.Close(s)
+		rsa := &syscall.SockaddrUnix{Name: addr}
+		if err := syscall.Sendto(s, data[:], 0, rsa); err != nil {
+			t.Errorf("syscall.Sendto failed: %v", err)
+			return
+		}
+	}()
+
+	<-off
+	b := make([]byte, 64)
+	c.SetReadDeadline(time.Now().Add(100 * time.Millisecond))
+	n, from, err := c.ReadFrom(b)
+	if err != nil {
+		t.Errorf("UnixConn.ReadFrom failed: %v", err)
+		return
+	}
+	if from != nil {
+		t.Errorf("neighbor address is %v", from)
+	}
+	if !bytes.Equal(b[:n], data[:]) {
+		t.Errorf("got %v, want %v", b[:n], data[:])
+		return
+	}
+}
+
+func TestReadUnixgramWithZeroBytesBuffer(t *testing.T) {
+	// issue 4352: Recvfrom failed with "address family not
+	// supported by protocol family" if zero-length buffer provided
+
+	addr := testUnixAddr()
+	la, err := ResolveUnixAddr("unixgram", addr)
+	if err != nil {
+		t.Fatalf("ResolveUnixAddr failed: %v", err)
+	}
+	c, err := ListenUnixgram("unixgram", la)
+	if err != nil {
+		t.Fatalf("ListenUnixgram failed: %v", err)
+	}
+	defer func() {
+		c.Close()
+		os.Remove(addr)
+	}()
+
+	off := make(chan bool)
+	go func() {
+		defer func() { off <- true }()
+		c, err := DialUnix("unixgram", nil, la)
+		if err != nil {
+			t.Errorf("DialUnix failed: %v", err)
+			return
+		}
+		defer c.Close()
+		if _, err := c.Write([]byte{1, 2, 3, 4, 5}); err != nil {
+			t.Errorf("UnixConn.Write failed: %v", err)
+			return
+		}
+	}()
+
+	<-off
+	c.SetReadDeadline(time.Now().Add(100 * time.Millisecond))
+	var peer Addr
+	if _, peer, err = c.ReadFrom(nil); err != nil {
+		t.Errorf("UnixConn.ReadFrom failed: %v", err)
+		return
+	}
+	if peer != nil {
+		t.Errorf("peer adddress is %v", peer)
+	}
+}

src/pkg/net/unixsock_posix.go

@@ -124,18 +124,20 @@ func (c *UnixConn) ReadFromUnix(b []byte) (n int, addr *UnixAddr, err error) {
 	n, sa, err := c.fd.ReadFrom(b)
 	switch sa := sa.(type) {
 	case *syscall.SockaddrUnix:
-		addr = &UnixAddr{sa.Name, sotypeToNet(c.fd.sotype)}
+		if sa.Name != "" {
+			addr = &UnixAddr{sa.Name, sotypeToNet(c.fd.sotype)}
+		}
 	}
 	return
 }
 
 // ReadFrom implements the PacketConn ReadFrom method.
-func (c *UnixConn) ReadFrom(b []byte) (n int, addr Addr, err error) {
+func (c *UnixConn) ReadFrom(b []byte) (int, Addr, error) {
 	if !c.ok() {
 		return 0, nil, syscall.EINVAL
 	}
-	n, uaddr, err := c.ReadFromUnix(b)
-	return n, uaddr.toAddr(), err
+	n, addr, err := c.ReadFromUnix(b)
+	return n, addr.toAddr(), err
 }
 
 // ReadMsgUnix reads a packet from c, copying the payload into b and
@@ -149,7 +151,9 @@ func (c *UnixConn) ReadMsgUnix(b, oob []byte) (n, oobn, flags int, addr *UnixAdd
 	n, oobn, flags, sa, err := c.fd.ReadMsg(b, oob)
 	switch sa := sa.(type) {
 	case *syscall.SockaddrUnix:
-		addr = &UnixAddr{sa.Name, sotypeToNet(c.fd.sotype)}
+		if sa.Name != "" {
+			addr = &UnixAddr{sa.Name, sotypeToNet(c.fd.sotype)}
+		}
 	}
 	return
 }

src/pkg/syscall/syscall_bsd.go

@@ -450,7 +450,9 @@ func Recvfrom(fd int, p []byte, flags int) (n int, from Sockaddr, err error) {
 	if n, err = recvfrom(fd, p, flags, &rsa, &len); err != nil {
 		return
 	}
-	from, err = anyToSockaddr(&rsa)
+	if rsa.Addr.Family != AF_UNSPEC {
+		from, err = anyToSockaddr(&rsa)
+	}
 	return
 }
 

src/pkg/syscall/syscall_linux.go

@@ -574,7 +574,9 @@ func Recvfrom(fd int, p []byte, flags int) (n int, from Sockaddr, err error) {
 	if n, err = recvfrom(fd, p, flags, &rsa, &len); err != nil {
 		return
 	}
-	from, err = anyToSockaddr(&rsa)
+	if rsa.Addr.Family != AF_UNSPEC {
+		from, err = anyToSockaddr(&rsa)
+	}
 	return
 }