unix: test anyToSockaddr without creating actual socket

Test_anyToSockaddr on linux needs to create a socket with the given
domain, type and protocol in order to test anyToSockaddr. Depending on
kernel version, permission and other factors, a given combination might
not be available on the system that runs the test, as is e.g. the case
for AF_CAN/SOCK_DGRAM/CAN_J1939 on several builders after CL 272767.

The only reason to create the socket is to be able to get the socket
protocol in anyToSockaddr using GetsockoptInt(..., SO_PROTOCOL). Move
this implementation into a wrapper func which can be overriden in tests
to with a func unconditionally returning the protocol under test. This
makes the test less dependent on the system it runs on and should fix
the builders broken by CL 272767.

While at it also removed the unused SocketSpec type in
syscall_internal_bsd_test.go and remove an unnecessary error var
declaration.

Change-Id: Ie8754cf795fa96980b29ae43777f698cd86ae863
Reviewed-on: https://go-review.googlesource.com/c/sys/+/274046
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>

unix/syscall_internal_bsd_test.go

@@ -13,20 +13,12 @@ import (
 	"unsafe"
 )
 
-// as per socket(2)
-type SocketSpec struct {
-	domain   int
-	typ      int
-	protocol int
-}
-
 func Test_anyToSockaddr(t *testing.T) {
 	tests := []struct {
 		name string
 		rsa  *RawSockaddrAny
 		sa   Sockaddr
 		err  error
-		skt  SocketSpec
 	}{
 		{
 			name: "AF_UNIX zero length",
@@ -80,19 +72,6 @@ func Test_anyToSockaddr(t *testing.T) {
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			fd := int(0)
-			var err error
-			if tt.skt.domain != 0 {
-				fd, err = Socket(tt.skt.domain, tt.skt.typ, tt.skt.protocol)
-				// Some sockaddr types need specific kernel modules running: if these
-				// are not present we'll get EPROTONOSUPPORT back when trying to create
-				// the socket.  Skip the test in this situation.
-				if err == EPROTONOSUPPORT {
-					t.Skip("socket family/protocol not supported by kernel")
-				} else if err != nil {
-					t.Fatalf("socket(%v): %v", tt.skt, err)
-				}
-				defer Close(fd)
-			}
 			sa, err := anyToSockaddr(fd, tt.rsa)
 			if err != tt.err {
 				t.Fatalf("unexpected error: %v, want: %v", err, tt.err)