Change the way fds passed over a socket are retreived on the receiving side.

Currently the receiver fetches an imsg via imsg_get() and if he expects
an fd, he then calls imsg_get_fd() to fetch the next fd queued on the
imsgbuf from which the imsg came.
This changes hides the fd queueing mechanism to the API user.  When closing
an imsg with an fd, the message is flagged so that the receiving end knows
it must dequeue the fd in imsg_get() and return it with the imsg structure.
This way there is no (less) possible screw up from imsg_get_fd() not being
called directly after imsg_get() by the user. The retreived imsg is
self-contained.
ok pyr@, "I like that" henning@

src/usr.sbin/ntpd/imsg.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: imsg.c,v 1.15 2009/06/07 00:40:46 eric Exp $	*/
+/*	$OpenBSD: imsg.c,v 1.16 2009/06/07 05:56:25 eric Exp $	*/
 
 /*
  * Copyright (c) 2003, 2004 Henning Brauer <henning@openbsd.org>
@@ -28,6 +28,8 @@
 
 #include "imsg.h"
 
+int	 imsg_get_fd(struct imsgbuf *);
+
 void
 imsg_init(struct imsgbuf *ibuf, int fd)
 {
@@ -113,9 +115,14 @@ imsg_get(struct imsgbuf *ibuf, struct imsg *imsg)
 	imsg->hdr.pid = ntohl(imsg->hdr.pid);
 	datalen = imsg->hdr.len - IMSG_HEADER_SIZE;
 	ibuf->r.rptr = ibuf->r.buf + IMSG_HEADER_SIZE;
-	if ((imsg->data = malloc(datalen)) == NULL) {
+	if ((imsg->data = malloc(datalen)) == NULL)
 		return (-1);
-	}
+
+	if (imsg->hdr.flags & IMSGF_HASFD)
+		imsg->fd = imsg_get_fd(ibuf);
+	else
+		imsg->fd = -1;
+
 	memcpy(imsg->data, ibuf->r.rptr, datalen);
 
 	if (imsg->hdr.len < av) {
@@ -216,6 +223,11 @@ imsg_close(struct imsgbuf *ibuf, struct buf *msg)
 	struct imsg_hdr	*hdr;
 
 	hdr = (struct imsg_hdr *)msg->buf;
+
+	hdr->flags &= ~IMSGF_HASFD;
+	if (msg->fd != -1)
+		hdr->flags |= IMSGF_HASFD;
+
 	hdr->type = htonl(hdr->type);
 	hdr->len = htons(msg->wpos);
 	hdr->flags = htons(hdr->flags);

src/usr.sbin/ntpd/imsg.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: imsg.h,v 1.2 2009/06/06 22:11:25 eric Exp $	*/
+/*	$OpenBSD: imsg.h,v 1.3 2009/06/07 05:56:25 eric Exp $	*/
 
 /*
  * Copyright (c) 2006, 2007 Pierre-Yves Ritschard <pyr@openbsd.org>
@@ -59,6 +59,8 @@ struct imsgbuf {
 	pid_t			 pid;
 };
 
+#define IMSGF_HASFD	1
+
 struct imsg_hdr {
 	u_int32_t	 type;
 	u_int16_t	 len;
@@ -69,6 +71,7 @@ struct imsg_hdr {
 
 struct imsg {
 	struct imsg_hdr	 hdr;
+	int		 fd;
 	void		*data;
 };
 
@@ -101,6 +104,5 @@ struct buf *imsg_create(struct imsgbuf *, u_int32_t, u_int32_t, pid_t,
 int	 imsg_add(struct buf *, void *, u_int16_t);
 void	 imsg_close(struct imsgbuf *, struct buf *);
 void	 imsg_free(struct imsg *);
-int	 imsg_get_fd(struct imsgbuf *);
 int	 imsg_flush(struct imsgbuf *);
 void	 imsg_clear(struct imsgbuf *);