net: make WriteTo, WriteToUnix and WriteMsgUnix fail when connectionless-mode...

net: make WriteTo, WriteToUnix and WriteMsgUnix fail when connectionless-mode UnixConn is already connected

This CL tries to fill the gap between Linux and other Unix-like systems
in the same way UDPConn already did.

Fixes #7677.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/83330045

src/pkg/net/unix_test.go

@@ -151,6 +151,73 @@ func TestUnixAutobindClose(t *testing.T) {
 	ln.Close()
 }
 
+func TestUnixgramWrite(t *testing.T) {
+	addr := testUnixAddr()
+	laddr, err := ResolveUnixAddr("unixgram", addr)
+	if err != nil {
+		t.Fatalf("ResolveUnixAddr failed: %v", err)
+	}
+	c, err := ListenPacket("unixgram", addr)
+	if err != nil {
+		t.Fatalf("ListenPacket failed: %v", err)
+	}
+	defer os.Remove(addr)
+	defer c.Close()
+
+	testUnixgramWriteConn(t, laddr)
+	testUnixgramWritePacketConn(t, laddr)
+}
+
+func testUnixgramWriteConn(t *testing.T, raddr *UnixAddr) {
+	c, err := Dial("unixgram", raddr.String())
+	if err != nil {
+		t.Fatalf("Dial failed: %v", err)
+	}
+	defer c.Close()
+
+	if _, err := c.(*UnixConn).WriteToUnix([]byte("Connection-oriented mode socket"), raddr); err == nil {
+		t.Fatal("WriteToUnix should fail")
+	} else if err.(*OpError).Err != ErrWriteToConnected {
+		t.Fatalf("WriteToUnix should fail as ErrWriteToConnected: %v", err)
+	}
+	if _, err = c.(*UnixConn).WriteTo([]byte("Connection-oriented mode socket"), raddr); err == nil {
+		t.Fatal("WriteTo should fail")
+	} else if err.(*OpError).Err != ErrWriteToConnected {
+		t.Fatalf("WriteTo should fail as ErrWriteToConnected: %v", err)
+	}
+	if _, _, err = c.(*UnixConn).WriteMsgUnix([]byte("Connection-oriented mode socket"), nil, raddr); err == nil {
+		t.Fatal("WriteTo should fail")
+	} else if err.(*OpError).Err != ErrWriteToConnected {
+		t.Fatalf("WriteMsgUnix should fail as ErrWriteToConnected: %v", err)
+	}
+	if _, err := c.Write([]byte("Connection-oriented mode socket")); err != nil {
+		t.Fatalf("Write failed: %v", err)
+	}
+}
+
+func testUnixgramWritePacketConn(t *testing.T, raddr *UnixAddr) {
+	addr := testUnixAddr()
+	c, err := ListenPacket("unixgram", addr)
+	if err != nil {
+		t.Fatalf("ListenPacket failed: %v", err)
+	}
+	defer os.Remove(addr)
+	defer c.Close()
+
+	if _, err := c.(*UnixConn).WriteToUnix([]byte("Connectionless mode socket"), raddr); err != nil {
+		t.Fatalf("WriteToUnix failed: %v", err)
+	}
+	if _, err := c.WriteTo([]byte("Connectionless mode socket"), raddr); err != nil {
+		t.Fatalf("WriteTo failed: %v", err)
+	}
+	if _, _, err := c.(*UnixConn).WriteMsgUnix([]byte("Connectionless mode socket"), nil, raddr); err != nil {
+		t.Fatalf("WriteMsgUnix failed: %v", err)
+	}
+	if _, err := c.(*UnixConn).Write([]byte("Connectionless mode socket")); err == nil {
+		t.Fatal("Write should fail")
+	}
+}
+
 func TestUnixConnLocalAndRemoteNames(t *testing.T) {
 	for _, laddr := range []string{"", testUnixAddr()} {
 		laddr := laddr

src/pkg/net/unixsock_posix.go

@@ -171,6 +171,9 @@ func (c *UnixConn) WriteToUnix(b []byte, addr *UnixAddr) (n int, err error) {
 	if !c.ok() {
 		return 0, syscall.EINVAL
 	}
+	if c.fd.isConnected {
+		return 0, &OpError{Op: "write", Net: c.fd.net, Addr: addr, Err: ErrWriteToConnected}
+	}
 	if addr == nil {
 		return 0, &OpError{Op: "write", Net: c.fd.net, Addr: nil, Err: errMissingAddress}
 	}
@@ -200,6 +203,9 @@ func (c *UnixConn) WriteMsgUnix(b, oob []byte, addr *UnixAddr) (n, oobn int, err
 	if !c.ok() {
 		return 0, 0, syscall.EINVAL
 	}
+	if c.fd.sotype == syscall.SOCK_DGRAM && c.fd.isConnected {
+		return 0, 0, &OpError{Op: "write", Net: c.fd.net, Addr: addr, Err: ErrWriteToConnected}
+	}
 	if addr != nil {
 		if addr.Net != sotypeToNet(c.fd.sotype) {
 			return 0, 0, syscall.EAFNOSUPPORT