Implement simple record boundary tracking in sbuf(9) to avoid record splitting

during drain operations. When an sbuf is configured to use this feature by way
of the SBUF_DRAINTOEOR sbuf_new() flag, top-level sections started with
sbuf_start_section() create a record boundary marker that is used to avoid
flushing partial records.

Reviewed by:	cem,imp,wblock
MFC after:	2 weeks
Sponsored by:	Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D8536

1 files changed, 10 insertions, 2 deletions

diff --git a/sys/kern/subr_sbuf.c b/sys/kern/subr_sbuf.c
index d7465f9dd758..80c95dc917d2 100644
--- a/sys/kern/subr_sbuf.c
+++ b/sys/kern/subr_sbuf.c
@@ -73,6 +73,8 @@ static MALLOC_DEFINE(M_SBUF, "sbuf", "string buffers");
 #define	SBUF_CANEXTEND(s)	((s)->s_flags & SBUF_AUTOEXTEND)
 #define	SBUF_ISSECTION(s)	((s)->s_flags & SBUF_INSECTION)
 #define	SBUF_NULINCLUDED(s)	((s)->s_flags & SBUF_INCLUDENUL)
+#define	SBUF_ISDRAINTOEOR(s)	((s)->s_flags & SBUF_DRAINTOEOR)
+#define	SBUF_DODRAINTOEOR(s)	(SBUF_ISSECTION(s) && SBUF_ISDRAINTOEOR(s))
 
 /*
  * Set / clear flags
@@ -308,6 +310,7 @@ sbuf_clear(struct sbuf *s)
 	SBUF_CLEARFLAG(s, SBUF_FINISHED);
 	s->s_error = 0;
 	s->s_len = 0;
+	s->s_rec_off = 0;
 	s->s_sect_len = 0;
 }
 
@@ -362,7 +365,10 @@ sbuf_drain(struct sbuf *s)
 
 	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));
-	len = s->s_drain_func(s->s_drain_arg, s->s_buf, s->s_len);
+	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,
+	    SBUF_DODRAINTOEOR(s) ? s->s_rec_off : s->s_len);
 	if (len < 0) {
 		s->s_error = -len;
 		return (s->s_error);
@@ -370,6 +376,7 @@ sbuf_drain(struct sbuf *s)
 	KASSERT(len > 0 && len <= s->s_len,
 	    ("Bad drain amount %d for sbuf %p", len, s));
 	s->s_len -= len;
+	s->s_rec_off -= len;
 	/*
 	 * Fast path for the expected case where all the data was
 	 * drained.
@@ -835,6 +842,7 @@ sbuf_start_section(struct sbuf *s, ssize_t *old_lenp)
 		    ("s_sect_len != 0 when starting a section"));
 		if (old_lenp != NULL)
 			*old_lenp = -1;
+		s->s_rec_off = s->s_len;
 		SBUF_SETFLAG(s, SBUF_INSECTION);
 	} else {
 		KASSERT(old_lenp != NULL,
@@ -865,7 +873,7 @@ sbuf_end_section(struct sbuf *s, ssize_t old_len, size_t pad, int c)
 	}
 	len = s->s_sect_len;
 	if (old_len == -1) {
-		s->s_sect_len = 0;
+		s->s_rec_off = s->s_sect_len = 0;
 		SBUF_CLEARFLAG(s, SBUF_INSECTION);
 	} else {
 		s->s_sect_len += old_len;