net: BUG section for ReadFrom, ReadFromIP methods of IPConn on "ip4" network

Fixes #3944.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/13263043

src/pkg/net/ipraw_test.go

@@ -202,7 +202,7 @@ func TestPacketConnICMPEcho(t *testing.T) {
 			if _, _, err := c.ReadFrom(rb); err != nil {
 				t.Fatalf("PacketConn.ReadFrom failed: %v", err)
 			}
-			// TODO: fix issue 3944
+			// See BUG section.
 			//if net == "ip4" {
 			//	rb = ipv4Payload(rb)
 			//}

src/pkg/net/iprawsock_posix.go

@@ -11,6 +11,18 @@ import (
 	"time"
 )
 
+// BUG(mikio): On every POSIX platform, reads from the "ip4" network
+// using the ReadFrom or ReadFromIP method might not return a complete
+// IPv4 packet, including its header, even if there is space
+// available. This can occur even in cases where Read or ReadMsgIP
+// could return a complete packet. For this reason, it is recommended
+// that you do not uses these methods if it is important to receive a
+// full packet.
+//
+// The Go 1 compatibliity guidelines make it impossible for us to
+// change the behavior of these methods; use Read or ReadMsgIP
+// instead.
+
 func sockaddrToIP(sa syscall.Sockaddr) Addr {
 	switch sa := sa.(type) {
 	case *syscall.SockaddrInet4: