conn: unbreak boundif on android

Another thing never tested ever.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

conn/boundif_android.go

@@ -0,0 +1,34 @@
+/* SPDX-License-Identifier: MIT
+ *
+ * Copyright (C) 2017-2020 WireGuard LLC. All Rights Reserved.
+ */
+
+package conn
+
+func (bind *nativeBind) PeekLookAtSocketFd4() (fd int, err error) {
+	sysconn, err := bind.ipv4.SyscallConn()
+	if err != nil {
+		return -1, err
+	}
+	err = sysconn.Control(func(f uintptr) {
+		fd = int(f)
+	})
+	if err != nil {
+		return -1, err
+	}
+	return
+}
+
+func (bind *nativeBind) PeekLookAtSocketFd6() (fd int, err error) {
+	sysconn, err := bind.ipv6.SyscallConn()
+	if err != nil {
+		return -1, err
+	}
+	err = sysconn.Control(func(f uintptr) {
+		fd = int(f)
+	})
+	if err != nil {
+		return -1, err
+	}
+	return
+}

conn/conn.go

@@ -57,6 +57,13 @@ type BindSocketToInterface interface {
 	BindSocketToInterface6(interfaceIndex uint32, blackhole bool) error
 }
 
+// PeekLookAtSocketFd is implemented by Bind objects that support having their
+// file descriptor peeked at.
+type PeekLookAtSocketFd interface {
+	PeekLookAtSocketFd4() (fd int, err error)
+	PeekLookAtSocketFd6() (fd int, err error)
+}
+
 // An Endpoint maintains the source/destination caching for a peer.
 //
 //	dst : the remote address of a peer ("endpoint" in uapi terminology)