internal/netlink: wrap netpoll via context

This removes netpoll boilerplate for the most common use case.

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

internal/netlink/netlink.go

@@ -2,10 +2,12 @@
 package netlink
 
 import (
+	"context"
 	"fmt"
 	"os"
 	"sync"
 	"syscall"
+	"time"
 	"unsafe"
 )
 
@@ -37,6 +39,10 @@ type conn struct {
 	pos int
 	// A page holding incoming and outgoing messages.
 	buf []byte
+	// An instant some time after conn was established, but before the first
+	// I/O operation on f through raw. This serves as a cached deadline to
+	// cancel blocking I/O.
+	t time.Time
 }
 
 // dial returns the address of a newly connected conn of specified family.
@@ -65,6 +71,7 @@ func dial(family int) (*conn, error) {
 
 	c.pos = syscall.NLMSG_HDRLEN
 	c.buf = make([]byte, os.Getpagesize())
+	c.t = time.Now().UTC()
 	return &c, nil
 }
 
@@ -79,35 +86,79 @@ func (c *conn) Close() error {
 
 // recvfrom wraps recv(2) with nonblocking behaviour via the runtime network poller.
 func (c *conn) recvfrom(
+	ctx context.Context,
 	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
+	if err = c.f.SetReadDeadline(time.Time{}); err != nil {
+		return
+	}
+
+	done := make(chan error, 1)
+	go func() {
+		done <- c.raw.Read(func(fd uintptr) (done bool) {
+			n, from, err = syscall.Recvfrom(int(fd), p, flags)
+			return err != syscall.EWOULDBLOCK
+		})
+	}()
+
+	select {
+	case rcErr := <-done:
+		if err != nil {
+			err = os.NewSyscallError("recvfrom", err)
+		} else {
+			err = rcErr
+		}
+
+	case <-ctx.Done():
+		cancelErr := c.f.SetReadDeadline(c.t)
+		<-done
+		if cancelErr != nil {
+			err = cancelErr
+		} else {
+			err = ctx.Err()
+		}
+		return
 	}
 	return
 }
 
 // sendto wraps send(2) with nonblocking behaviour via the runtime network poller.
 func (c *conn) sendto(
+	ctx context.Context,
 	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
+	if err = c.f.SetWriteDeadline(time.Time{}); err != nil {
+		return
+	}
+
+	done := make(chan error, 1)
+	go func() {
+		done <- c.raw.Write(func(fd uintptr) (done bool) {
+			err = syscall.Sendto(int(fd), p, flags, to)
+			return err != syscall.EWOULDBLOCK
+		})
+	}()
+
+	select {
+	case rcErr := <-done:
+		if err != nil {
+			err = os.NewSyscallError("sendto", err)
+		} else {
+			err = rcErr
+		}
+
+	case <-ctx.Done():
+		cancelErr := c.f.SetWriteDeadline(c.t)
+		<-done
+		if cancelErr != nil {
+			err = cancelErr
+		} else {
+			err = ctx.Err()
+		}
+		return
 	}
 	return
 }
@@ -192,10 +243,10 @@ type HandlerFunc func(resp []syscall.NetlinkMessage) error
 
 // receive receives from a socket with specified flags until a non-nil error is
 // returned by f. An error of type [Complete] is returned as nil.
-func (c *conn) receive(f HandlerFunc, flags int) error {
+func (c *conn) receive(ctx context.Context, f HandlerFunc, flags int) error {
 	for {
 		buf := c.buf
-		if n, _, err := c.recvfrom(buf, flags); err != nil {
+		if n, _, err := c.recvfrom(ctx, buf, flags); err != nil {
 			return err
 		} else if n < syscall.NLMSG_HDRLEN {
 			return syscall.EBADE
@@ -224,17 +275,17 @@ func (c *conn) receive(f HandlerFunc, flags int) error {
 }
 
 // Roundtrip sends the pending message and handles the reply.
-func (c *conn) Roundtrip(f HandlerFunc) error {
+func (c *conn) Roundtrip(ctx context.Context, f HandlerFunc) error {
 	if c.buf == nil {
 		return syscall.EINVAL
 	}
 	defer func() { c.seq++ }()
 
-	if err := c.sendto(c.pending(), 0, &syscall.SockaddrNetlink{
+	if err := c.sendto(ctx, c.pending(), 0, &syscall.SockaddrNetlink{
 		Family: syscall.AF_NETLINK,
 	}); err != nil {
 		return err
 	}
 
-	return c.receive(f, 0)
+	return c.receive(ctx, f, 0)
 }

internal/netlink/rtnl.go

@@ -1,6 +1,7 @@
 package netlink
 
 import (
+	"context"
 	"syscall"
 	"unsafe"
 )
@@ -72,6 +73,7 @@ func (c *RouteConn) writeIfAddrmsg(
 
 // SendIfAddrmsg sends an ifaddrmsg structure to rtnetlink.
 func (c *RouteConn) SendIfAddrmsg(
+	ctx context.Context,
 	typ, flags uint16,
 	msg *syscall.IfAddrmsg,
 	attrs ...RtAttrMsg[InAddr],
@@ -79,7 +81,7 @@ func (c *RouteConn) SendIfAddrmsg(
 	if !c.writeIfAddrmsg(typ, flags, msg, attrs...) {
 		return syscall.ENOMEM
 	}
-	return c.Roundtrip(rtnlConsume)
+	return c.Roundtrip(ctx, rtnlConsume)
 }
 
 // writeNewaddrLo writes a RTM_NEWADDR message for the loopback address.
@@ -104,11 +106,11 @@ func (c *RouteConn) writeNewaddrLo(lo uint32) bool {
 }
 
 // SendNewaddrLo sends a RTM_NEWADDR message for the loopback address to the kernel.
-func (c *RouteConn) SendNewaddrLo(lo uint32) error {
+func (c *RouteConn) SendNewaddrLo(ctx context.Context, lo uint32) error {
 	if !c.writeNewaddrLo(lo) {
 		return syscall.ENOMEM
 	}
-	return c.Roundtrip(rtnlConsume)
+	return c.Roundtrip(ctx, rtnlConsume)
 }
 
 // writeIfInfomsg writes an ifinfomsg structure to conn.
@@ -122,11 +124,12 @@ func (c *RouteConn) writeIfInfomsg(
 
 // SendIfInfomsg sends an ifinfomsg structure to rtnetlink.
 func (c *RouteConn) SendIfInfomsg(
+	ctx context.Context,
 	typ, flags uint16,
 	msg *syscall.IfInfomsg,
 ) error {
 	if !c.writeIfInfomsg(typ, flags, msg) {
 		return syscall.ENOMEM
 	}
-	return c.Roundtrip(rtnlConsume)
+	return c.Roundtrip(ctx, rtnlConsume)
 }