internal/netlink: nonblocking socket I/O

This enables use with blocking calls like when used with NETLINK_KOBJECT_UEVENT.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/netlink/netlink.go

@@ -24,7 +24,9 @@ func getpid() uint32 {
 // A conn represents resources associated to a netlink socket.
 type conn struct {
 	// AF_NETLINK socket.
-	fd int
+	f *os.File
+	// For using runtime polling via f.
+	raw syscall.RawConn
 	// Kernel module or netlink group to communicate with.
 	family int
 	// Message sequence number.
@@ -42,7 +44,7 @@ func dial(family int) (*conn, error) {
 	var c conn
 	if fd, err := syscall.Socket(
 		syscall.AF_NETLINK,
-		syscall.SOCK_RAW|syscall.SOCK_CLOEXEC,
+		syscall.SOCK_RAW|syscall.SOCK_NONBLOCK|syscall.SOCK_CLOEXEC,
 		family,
 	); err != nil {
 		return nil, os.NewSyscallError("socket", err)
@@ -53,8 +55,14 @@ func dial(family int) (*conn, error) {
 		_ = syscall.Close(fd)
 		return nil, os.NewSyscallError("bind", err)
 	} else {
-		c.fd, c.family = fd, family
+		c.family = family
+		c.f = os.NewFile(uintptr(fd), "netlink")
+		if c.raw, err = c.f.SyscallConn(); err != nil {
+			_ = c.f.Close()
+			return nil, err
+		}
 	}
+
 	c.pos = syscall.NLMSG_HDRLEN
 	c.buf = make([]byte, os.Getpagesize())
 	return &c, nil
@@ -66,7 +74,42 @@ func (c *conn) Close() error {
 		return syscall.EINVAL
 	}
 	c.buf = nil
-	return syscall.Close(c.fd)
+	return c.f.Close()
+}
+
+// recvfrom wraps recv(2) with nonblocking behaviour via the runtime network poller.
+func (c *conn) recvfrom(
+	p []byte,
+	flags int,
+) (n int, from syscall.Sockaddr, err error) {
+	rcErr := c.raw.Read(func(fd uintptr) (done bool) {
+		n, from, err = syscall.Recvfrom(int(fd), p, flags)
+		return err != syscall.EWOULDBLOCK
+	})
+	if err != nil {
+		err = os.NewSyscallError("recvfrom", err)
+	} else {
+		err = rcErr
+	}
+	return
+}
+
+// sendto wraps send(2) with nonblocking behaviour via the runtime network poller.
+func (c *conn) sendto(
+	p []byte,
+	flags int,
+	to syscall.Sockaddr,
+) (err error) {
+	rcErr := c.raw.Write(func(fd uintptr) (done bool) {
+		err = syscall.Sendto(int(fd), p, flags, to)
+		return err != syscall.EWOULDBLOCK
+	})
+	if err != nil {
+		err = os.NewSyscallError("sendto", err)
+	} else {
+		err = rcErr
+	}
+	return
 }
 
 // Msg is type constraint for types sent over the wire via netlink.
@@ -152,8 +195,8 @@ type HandlerFunc func(resp []syscall.NetlinkMessage) error
 func (c *conn) receive(f HandlerFunc, flags int) error {
 	for {
 		buf := c.buf
-		if n, _, err := syscall.Recvfrom(c.fd, buf, flags); err != nil {
+		if n, _, err := c.recvfrom(buf, flags); err != nil {
-			return os.NewSyscallError("recvfrom", err)
+			return err
 		} else if n < syscall.NLMSG_HDRLEN {
 			return syscall.EBADE
 		} else {
@@ -187,10 +230,10 @@ func (c *conn) Roundtrip(f HandlerFunc) error {
 	}
 	defer func() { c.seq++ }()
 
-	if err := syscall.Sendto(c.fd, c.pending(), 0, &syscall.SockaddrNetlink{
+	if err := c.sendto(c.pending(), 0, &syscall.SockaddrNetlink{
 		Family: syscall.AF_NETLINK,
 	}); err != nil {
-		return os.NewSyscallError("sendto", err)
+		return err
 	}
 
 	return c.receive(f, 0)