Index: sys/kern/subr_sbuf.c
===================================================================
--- sys/kern/subr_sbuf.c
+++ sys/kern/subr_sbuf.c
@@ -70,6 +70,7 @@
 #define	SBUF_ISDYNAMIC(s)	((s)->s_flags & SBUF_DYNAMIC)
 #define	SBUF_ISDYNSTRUCT(s)	((s)->s_flags & SBUF_DYNSTRUCT)
 #define	SBUF_ISFINISHED(s)	((s)->s_flags & SBUF_FINISHED)
+#define	SBUF_HASTRAILINGNL(s)	((s)->s_flags & SBUF_TRAILINGNL)
 #define	SBUF_HASROOM(s)		((s)->s_len < (s)->s_size - 1)
 #define	SBUF_FREESPACE(s)	((s)->s_size - ((s)->s_len + 1))
 #define	SBUF_CANEXTEND(s)	((s)->s_flags & SBUF_AUTOEXTEND)
@@ -225,6 +226,7 @@
 	s->s_flags |= flags;
 	s->s_size = length;
 	s->s_buf = buf;
+	SBUF_SETFLAG(s, SBUF_TRAILINGNL);
 
 	/*
 	 * Allocate DYNAMIC, i.e., heap data buffer backing the sbuf, if no
@@ -384,6 +386,12 @@
 
 	KASSERT(s->s_len > 0, ("Shouldn't drain empty sbuf %p", s));
 	KASSERT(s->s_error == 0, ("Called %s with error on %p", __func__, s));
+
+	if (s->s_buf[s->s_len - 1] == '\n')
+		SBUF_SETFLAG(s, SBUF_TRAILINGNL);
+	else
+		SBUF_CLEARFLAG(s, SBUF_TRAILINGNL);
+
 	if (SBUF_DODRAINTOEOR(s) && s->s_rec_off == 0)
 		return (s->s_error = EDEADLK);
 	len = s->s_drain_func(s->s_drain_arg, s->s_buf,
@@ -716,6 +724,34 @@
 	return (0);
 }
 
+/*
+ * Append a trailing newline to a non-empty sbuf, if one is not already
+ * present.  Handles sbufs with drain functions correctly.
+ */
+int
+sbuf_nl_terminate(struct sbuf *s)
+{
+
+	assert_sbuf_integrity(s);
+	assert_sbuf_state(s, 0);
+
+	/*
+	 * We track NL-termination status of fully drained buffers with the
+	 * SBUF_TRAILINGNL flag in sbuf_drain().
+	 *
+	 * If the buffer isn't empty, we can just refer to it.
+	 */
+	if (__predict_false(s->s_len == 0)) {
+		if (__predict_true(!SBUF_HASTRAILINGNL(s)))
+			sbuf_put_byte(s, '\n');
+	} else if (__predict_true(s->s_buf[s->s_len - 1] != '\n'))
+		sbuf_put_byte(s, '\n');
+
+	if (s->s_error != 0)
+		return (-1);
+	return (0);
+}
+
 /*
  * Trim whitespace characters from end of an sbuf.
  */
Index: sys/sys/sbuf.h
===================================================================
--- sys/sys/sbuf.h
+++ sys/sys/sbuf.h
@@ -58,6 +58,7 @@
 #define	SBUF_FINISHED	0x00020000	/* set by sbuf_finish() */
 #define	SBUF_DYNSTRUCT	0x00080000	/* sbuf must be freed */
 #define	SBUF_INSECTION	0x00100000	/* set by sbuf_start_section() */
+#define	SBUF_TRAILINGNL	0x00200000	/* drained contents ended in \n */
 	int		 s_flags;	/* flags */
 	ssize_t		 s_sect_len;	/* current length of section */
 	ssize_t		 s_rec_off;	/* current record start offset */
@@ -103,6 +104,7 @@
 	__printflike(2, 3);
 int		 sbuf_vprintf(struct sbuf *, const char *, __va_list)
 	__printflike(2, 0);
+int		 sbuf_nl_terminate(struct sbuf *);
 int		 sbuf_putc(struct sbuf *, int);
 void		 sbuf_set_drain(struct sbuf *, sbuf_drain_func *, void *);
 int		 sbuf_trim(struct sbuf *);