httpd: convert proc.c to new imsg API (sync with relayd)

The following diff synchronizes httpd's proc.c with relayd's latest imsg
API chnages.

httpd: convert proc.c to new imsg API (sync with relayd proc.c)

Replace IMSG_SIZE_CHECK() + memcpy() with imsg_get_data(), which does
the length check and copy in one call. Use the imsg accessors
(imsg_get_*) instead of touching imsg.hdr directly and
imsgbuf_get()/imsgbuf_read() instead of imsg_get().

Rewrite proc_forward_imsg() to use imsg_forward() per target imsgbuf
instead of re-composing via proc_compose_imsg(); arm the write event
with imsg_event_add() after each forward.

proc_forward_imsg() never forwarded an fd, and imsg_forward() rewinds
the buffer internally, so multiple forwards per message keep working.

Drop the now-unused n parameter (all callers passed -1).

OK?

diff --git a/usr.sbin/httpd/control.c b/usr.sbin/httpd/control.c
index 6bbc9a4ce83..6496543bf52 100644
--- a/usr.sbin/httpd/control.c
+++ b/usr.sbin/httpd/control.c
@@ -269,7 +269,7 @@ control_dispatch_imsg(int fd, short event, void *arg)
 		case IMSG_CTL_SHUTDOWN:
 		case IMSG_CTL_RELOAD:
 		case IMSG_CTL_REOPEN:
-			proc_forward_imsg(env->sc_ps, &imsg, PROC_PARENT, -1);
+			proc_forward_imsg(env->sc_ps, &imsg, PROC_PARENT);
 			break;
 		case IMSG_CTL_NOTIFY:
 			if (c->flags & CTL_CONN_NOTIFY) {
@@ -288,8 +288,8 @@ control_dispatch_imsg(int fd, short event, void *arg)
 
 			memcpy(&verbose, imsg.data, sizeof(verbose));
 
-			proc_forward_imsg(env->sc_ps, &imsg, PROC_PARENT, -1);
-			proc_forward_imsg(env->sc_ps, &imsg, PROC_SERVER, -1);
+			proc_forward_imsg(env->sc_ps, &imsg, PROC_PARENT);
+			proc_forward_imsg(env->sc_ps, &imsg, PROC_SERVER);
 
 			memcpy(imsg.data, &verbose, sizeof(verbose));
 			control_imsg_forward(env->sc_ps, &imsg);
diff --git a/usr.sbin/httpd/httpd.h b/usr.sbin/httpd/httpd.h
index e4cac855b4b..d1f1be85d08 100644
--- a/usr.sbin/httpd/httpd.h
+++ b/usr.sbin/httpd/httpd.h
@@ -785,8 +785,8 @@ int	 proc_composev_imsg(struct privsep *, enum privsep_procid, int,
 	    u_int16_t, u_int32_t, int, const struct iovec *, int);
 int	 proc_composev(struct privsep *, enum privsep_procid,
 	    uint16_t, const struct iovec *, int);
-int	 proc_forward_imsg(struct privsep *, struct imsg *,
-	    enum privsep_procid, int);
+void	 proc_forward_imsg(struct privsep *, struct imsg *,
+	    enum privsep_procid);
 struct imsgbuf *
 	 proc_ibuf(struct privsep *, enum privsep_procid, int);
 struct imsgev *
diff --git a/usr.sbin/httpd/proc.c b/usr.sbin/httpd/proc.c
index e75950c1f49..700dcf8da5d 100644
--- a/usr.sbin/httpd/proc.c
+++ b/usr.sbin/httpd/proc.c
@@ -600,7 +600,7 @@ proc_dispatch(int fd, short event, void *arg)
 	struct imsgbuf		*ibuf;
 	struct imsg		 imsg;
 	ssize_t			 n;
-	int			 verbose;
+	int			 ver;
 	const char		*title;
 	struct privsep_fd	 pf;
 
@@ -608,9 +608,11 @@ proc_dispatch(int fd, short event, void *arg)
 	ibuf = &iev->ibuf;
 
 	if (event & EV_READ) {
-		if ((n = imsgbuf_read(ibuf)) == -1)
+		switch (imsgbuf_read(ibuf)) {
+		case -1:
 			fatal("%s: imsgbuf_read", __func__);
-		if (n == 0) {
+			break;
+		case 0:
 			/* this pipe is dead, so remove the event handler */
 			event_del(&iev->ev);
 			event_loopexit(NULL);
@@ -631,15 +633,16 @@ proc_dispatch(int fd, short event, void *arg)
 	}
 
 	for (;;) {
-		if ((n = imsg_get(ibuf, &imsg)) == -1)
-			fatal("%s: imsg_get", __func__);
+		if ((n = imsgbuf_get(ibuf, &imsg)) == -1)
+			fatal("%s: imsgbuf_get", __func__);
 		if (n == 0)
 			break;
 
 #if DEBUG > 1
 		log_debug("%s: %s %d got imsg %d peerid %d from %s %d",
 		    __func__, title, ps->ps_instance + 1,
-		    imsg.hdr.type, imsg.hdr.peerid, p->p_title, imsg.hdr.pid);
+		    imsg_get_type(&imsg), imsg_get_id(&imsg),
+		    p->p_title, imsg_get_pid(&imsg));
 #endif
 
 		/*
@@ -654,11 +657,12 @@ proc_dispatch(int fd, short event, void *arg)
 		/*
 		 * Generic message handling
 		 */
-		switch (imsg.hdr.type) {
+		switch (imsg_get_type(&imsg)) {
 		case IMSG_CTL_VERBOSE:
-			IMSG_SIZE_CHECK(&imsg, &verbose);
-			memcpy(&verbose, imsg.data, sizeof(verbose));
-			log_setverbose(verbose);
+			if (imsg_get_data(&imsg, &ver, sizeof(ver)) == -1)
+			       fatalx("%s: imsg_get_data", __func__);
+
+			log_setverbose(ver);
 			break;
 		case IMSG_CTL_PROCFD:
 			if (p->p_id != PROC_PARENT) {
@@ -666,17 +670,19 @@ proc_dispatch(int fd, short event, void *arg)
 				    "IMSG_CTL_PROCFD from %s",
 				    __func__, p->p_title);
 			}
-			IMSG_SIZE_CHECK(&imsg, &pf);
-			memcpy(&pf, imsg.data, sizeof(pf));
+
+			if (imsg_get_data(&imsg, &pf, sizeof(pf)) == -1)
+				fatalx("%s: imsg_get_data", __func__);
+
 			proc_accept(ps, imsg_get_fd(&imsg), pf.pf_procid,
-			    pf.pf_instance);
+				    pf.pf_instance);
 			break;
 		default:
 			fatalx("%s: %s %d got invalid imsg %d peerid %d "
 			    "from %s %d",
 			    __func__, title, ps->ps_instance + 1,
-			    imsg.hdr.type, imsg.hdr.peerid,
-			    p->p_title, imsg.hdr.pid);
+			    imsg_get_type(&imsg), imsg_get_id(&imsg),
+			    p->p_title, imsg_get_pid(&imsg));
 		}
 		imsg_free(&imsg);
 	}
@@ -794,12 +800,18 @@ proc_composev(struct privsep *ps, enum privsep_procid id,
 	return (proc_composev_imsg(ps, id, -1, type, -1, -1, iov, iovcnt));
 }
 
-int
+void
 proc_forward_imsg(struct privsep *ps, struct imsg *imsg,
-    enum privsep_procid id, int n)
+    enum privsep_procid id)
 {
-	return (proc_compose_imsg(ps, id, n, imsg->hdr.type,
-	    imsg->hdr.peerid, -1, imsg->data, IMSG_DATA_SIZE(imsg)));
+	int	 m, n = -1;
+
+	proc_range(ps, id, &n, &m);
+	for (; n < m; n++) {
+		if (imsg_forward(&ps->ps_ievs[id][n].ibuf, imsg) == -1)
+			fatal("%s: imsg_forward", __func__);
+		imsg_event_add(&ps->ps_ievs[id][n]);
+	}
 }
 
 struct imsgbuf *