internal/uevent: handle state divergence

This requires the caller to arrange for a coldboot to happen, some time after this error is encountered, and to resume event processing.

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

internal/uevent/uevent.go

@@ -18,6 +18,13 @@ type (
 	Recoverable interface{ recoverable() }
 	// Nontrivial is satisfied by errors preferring a JSON encoding.
 	Nontrivial interface{ nontrivial() }
+
+	// NeedsColdboot is satisfied by errors indicating divergence of local state
+	// from the kernel, usually from lost uevent data.
+	NeedsColdboot interface {
+		Recoverable
+		coldboot()
+	}
 )
 
 // Conn represents a NETLINK_KOBJECT_UEVENT socket.
@@ -58,6 +65,18 @@ var (
 	ErrBadSocket = errors.New("unexpected socket address")
 )
 
+// ReceiveBufferError indicates one or more [Message] being lost due to the
+// socket receive buffer filling up. This is usually caused by epoll waking the
+// receiving program up too late.
+type ReceiveBufferError struct{ _ [0]*ReceiveBufferError }
+
+var _ NeedsColdboot = ReceiveBufferError{}
+
+func (ReceiveBufferError) recoverable()    {}
+func (ReceiveBufferError) coldboot()       {}
+func (ReceiveBufferError) Unwrap() error   { return syscall.ENOBUFS }
+func (e ReceiveBufferError) Error() string { return syscall.ENOBUFS.Error() }
+
 // BadPortError is returned by [Conn.Consume] upon receiving a message that did
 // not come from the kernel.
 type BadPortError syscall.SockaddrNetlink
@@ -84,6 +103,9 @@ func (c *Conn) Consume(ctx context.Context, events chan<- *Message) error {
 	for {
 		data, _, from, err := c.conn.Recvmsg(ctx, 0)
 		if err != nil {
+			if err == syscall.ENOBUFS {
+				return ReceiveBufferError{}
+			}
 			return err
 		}