ws: Replace WS_Front() with WS_Reservation()

And add a companion function WS_ReservationSize(). This makes it explicit
that accessing the workspace front pointer is only valid for reservations,
and also informs you whether you are in the middle of a reservation, which
would be helpful for assertions.

bin/varnishd/cache/cache.h

@@ -797,12 +797,20 @@ void WS_Assert_Allocated(const struct ws *ws, const void *ptr, ssize_t len);
 void WS_VSB_new(struct vsb *, struct ws *);
 char *WS_VSB_finish(struct vsb *, struct ws *, size_t *);
 
-static inline char*
-WS_Front(const struct ws *ws)
+static inline void *
+WS_Reservation(const struct ws *ws)
+{
+
+	WS_Assert(ws);
+	return (ws->r != NULL ? ws->f : NULL);
+}
+
+static inline unsigned
+WS_ReservationSize(const struct ws *ws)
 {
 
 	AN(ws->r);
-	return ws->f;
+	return (ws->r - ws->f);
 }
 
 /* cache_rfc2616.c */

bin/varnishd/cache/cache_session.c

@@ -482,7 +482,7 @@ SES_Wait(struct sess *sp, const struct transport *xp)
 		return;
 	}
 
-	wp = (void*)WS_Front(sp->ws);
+	wp = WS_Reservation(sp->ws);
 	INIT_OBJ(wp, WAITED_MAGIC);
 	wp->fd = sp->fd;
 	wp->priv1 = sp;

bin/varnishd/cache/cache_vrt.c

@@ -554,7 +554,7 @@ VRT_UpperLowerStrands(VRT_CTX, VCL_STRANDS s, int up)
 	CHECK_OBJ_NOTNULL(ctx->ws, WS_MAGIC);
 	AN(s);
 	u = WS_ReserveAll(ctx->ws);
-	r = b = WS_Front(ctx->ws);
+	r = b = WS_Reservation(ctx->ws);
 	e = b + u;
 	for (i = 0; i < s->n; i++) {
 		if (s->p[i] == NULL || s->p[i][0] == '\0')

bin/varnishd/cache/cache_ws.c

@@ -377,7 +377,7 @@ WS_VSB_new(struct vsb *vsb, struct ws *ws)
 	if (WS_Overflowed(ws) || u < 2)
 		AN(VSB_init(vsb, bogus, sizeof bogus));
 	else
-		AN(VSB_init(vsb, WS_Front(ws), u));
+		AN(VSB_init(vsb, WS_Reservation(ws), u));
 }
 
 char *
@@ -389,7 +389,7 @@ WS_VSB_finish(struct vsb *vsb, struct ws *ws, size_t *szp)
 	WS_Assert(ws);
 	if (!VSB_finish(vsb)) {
 		p = VSB_data(vsb);
-		if (p == WS_Front(ws)) {
+		if (p == WS_Reservation(ws)) {
 			WS_Release(ws, VSB_len(vsb) + 1);
 			if (szp != NULL)
 				*szp = VSB_len(vsb);

include/vrt.h

@@ -57,6 +57,8 @@
  *	Added VCL_STRING VRT_BLOB_string(VRT_CTX, VCL_BLOB)
  *	[cache.h] WS_Reserve() removed
  *	[cache.h] WS_Printf() changed
+ *	[cache.h] WS_ReservationSize() added
+ *	[cache.h] WS_Front() replaced by WS_Reservation()
  * 11.0 (2020-03-16)
  *	Changed type of vsa_suckaddr_len from int to size_t
  *	New prefix_{ptr|len} fields in vrt_backend

lib/libvmod_blob/vmod_blob.c

@@ -341,7 +341,7 @@ vmod_decode(VRT_CTX, VCL_ENUM decs, VCL_INT length, VCL_STRANDS strings)
 	CHECK_OBJ_NOTNULL(ctx->ws, WS_MAGIC);
 
 	space = WS_ReserveAll(ctx->ws);
-	buf = WS_Front(ctx->ws);
+	buf = WS_Reservation(ctx->ws);
 
 	if (length <= 0)
 		length = -1;
@@ -378,7 +378,7 @@ encode(VRT_CTX, enum encoding enc, enum case_e kase, VCL_BLOB b)
 
 	CHECK_OBJ_NOTNULL(ctx->ws, WS_MAGIC);
 	space = WS_ReserveAll(ctx->ws);
-	buf = WS_Front(ctx->ws);
+	buf = WS_Reservation(ctx->ws);
 
 	len = func[enc].encode(enc, kase, buf, space, b->blob, b->len);