Adjust imsgbuf_read to return an int with either 1, 0 or -1.

Handle EAGAIN by a simple return 1 (same for the fd check).
This way the caller will poll again and then retry later.

OK tb@

3 files changed, 34 insertions, 19 deletions

diff --git a/lib/libutil/imsg.c b/lib/libutil/imsg.c
index fbe001e4341..f9497a47ec0 100644
--- a/lib/libutil/imsg.c
+++ b/lib/libutil/imsg.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: imsg.c,v 1.30 2024/11/21 12:54:10 claudio Exp $	*/
+/*	$OpenBSD: imsg.c,v 1.31 2024/11/21 12:54:52 claudio Exp $	*/
 
 /*
  * Copyright (c) 2023 Claudio Jeker <claudio@openbsd.org>
@@ -49,7 +49,7 @@ imsgbuf_init(struct imsgbuf *imsgbuf, int fd)
 	TAILQ_INIT(&imsgbuf->fds);
 }
 
-ssize_t
+int
 imsgbuf_read(struct imsgbuf *imsgbuf)
 {
 	struct msghdr		 msg;
@@ -59,7 +59,7 @@ imsgbuf_read(struct imsgbuf *imsgbuf)
 		char	buf[CMSG_SPACE(sizeof(int) * 1)];
 	} cmsgbuf;
 	struct iovec		 iov;
-	ssize_t			 n = -1;
+	ssize_t			 n;
 	int			 fd;
 	struct imsg_fd		*ifd;
 
@@ -80,17 +80,25 @@ again:
 	if (getdtablecount() + imsg_fd_overhead +
 	    (int)((CMSG_SPACE(sizeof(int))-CMSG_SPACE(0))/sizeof(int))
 	    >= getdtablesize()) {
-		errno = EAGAIN;
 		free(ifd);
-		return (-1);
+		return (1);
 	}
 
 	if ((n = recvmsg(imsgbuf->fd, &msg, 0)) == -1) {
 		if (errno == EINTR)
 			goto again;
+		if (errno == EAGAIN) {
+			free(ifd);
+			return (1);
+		}
 		goto fail;
 	}
 
+	if (n == 0) {	/* connection closed */
+		free(ifd);
+		return (0);
+	}
+
 	imsgbuf->r.wpos += n;
 
 	for (cmsg = CMSG_FIRSTHDR(&msg); cmsg != NULL;
@@ -121,9 +129,12 @@ again:
 		/* we do not handle other ctl data level */
 	}
 
+	free(ifd);
+	return (1);
+
 fail:
 	free(ifd);
-	return (n);
+	return (-1);
 }
 
 int
diff --git a/lib/libutil/imsg.h b/lib/libutil/imsg.h
index 6e9ec6eaf47..56701b18c0c 100644
--- a/lib/libutil/imsg.h
+++ b/lib/libutil/imsg.h
@@ -1,4 +1,4 @@
-/*	$OpenBSD: imsg.h,v 1.13 2024/11/21 12:54:10 claudio Exp $	*/
+/*	$OpenBSD: imsg.h,v 1.14 2024/11/21 12:54:52 claudio Exp $	*/
 
 /*
  * Copyright (c) 2023 Claudio Jeker <claudio@openbsd.org>
@@ -132,7 +132,7 @@ int		 msgbuf_write(struct msgbuf *);
 
 /* imsg.c */
 void	 imsgbuf_init(struct imsgbuf *, int);
-ssize_t	 imsgbuf_read(struct imsgbuf *);
+int	 imsgbuf_read(struct imsgbuf *);
 int	 imsgbuf_write(struct imsgbuf *);
 int	 imsgbuf_flush(struct imsgbuf *);
 void	 imsgbuf_clear(struct imsgbuf *);
diff --git a/lib/libutil/imsg_init.3 b/lib/libutil/imsg_init.3
index 59a35e9941b..a2fa656b53a 100644
--- a/lib/libutil/imsg_init.3
+++ b/lib/libutil/imsg_init.3
@@ -1,4 +1,4 @@
-.\" $OpenBSD: imsg_init.3,v 1.38 2024/11/21 12:54:10 claudio Exp $
+.\" $OpenBSD: imsg_init.3,v 1.39 2024/11/21 12:54:52 claudio Exp $
 .\"
 .\" Copyright (c) 2023 Claudio Jeker <claudio@openbsd.org>
 .\" Copyright (c) 2010 Nicholas Marriott <nicm@openbsd.org>
@@ -87,7 +87,7 @@
 .Fn imsgbuf_init "struct imsgbuf *imsgbuf" "int fd"
 .Ft uint32_t
 .Fn imsgbuf_queuelen "struct imsgbuf *imsgbuf"
-.Ft ssize_t
+.Ft int
 .Fn imsgbuf_read "struct imsgbuf *imsgbuf"
 .Ft int
 .Fn imsgbuf_write "struct imsgbuf *imsgbuf"
@@ -127,13 +127,18 @@ routine reads pending data with
 .Xr recvmsg 2
 and queues it as individual messages on
 .Fa imsgbuf .
-It returns the number of bytes read on success, or \-1 on error.
-A return value of \-1 from
-.Fn imsgbuf_read
-invalidates
-.Fa imsgbuf ,
-and renders it suitable only for passing to
-.Fn imsgbuf_clear .
+It returns 1 on success, 0 if the connection is closed, or \-1 on error
+and the global variable
+.Va errno
+is set to indicate the error.
+The errors
+.Er EINTR
+and
+.Er EAGAIN
+are treated as follows.
+.Er EINTR
+will automatically retry the read operation while the other errors are
+ignored with a 1 return.
 .Pp
 .Fn imsgbuf_write
 writes out queued messages.
@@ -149,7 +154,6 @@ are treated as follows.
 .Er EINTR
 will automatically retry the write operation while the other errors are
 ignored with a 0 return.
-The caller will then retry the operation at a later stage.
 .Pp
 .Fn imsgbuf_flush
 calls
@@ -401,7 +405,7 @@ dispatch_imsg(struct imsgbuf *imsgbuf)
 	ssize_t         n;
 	int		idata;
 
-	if ((n = imsgbuf_read(imsgbuf)) == -1 && errno != EAGAIN) {
+	if ((n = imsgbuf_read(imsgbuf)) == -1) {
 		/* handle read error */
 	}
 	if (n == 0) {