Allow using the whole per thread log buffer

The logics were off by one in the available buffer space checks,
causing us to loose one word of per thread log buffer space, and
making reasoning about available space tricky. Fix this.

Refs 10749ffe
Refs #3745

bin/varnishd/cache/cache_shmlog.c

@@ -347,17 +347,17 @@ VSLbt(struct vsl_log *vsl, enum VSL_tag_e tag, txt t)
 	if (l > mlen - 1)
 		l = mlen - 1;
 
-	assert(vsl->wlp < vsl->wle);
+	assert(vsl->wlp <= vsl->wle);
 
 	/* Flush if necessary */
-	if (VSL_END(vsl->wlp, l + 1) >= vsl->wle)
+	if (VSL_END(vsl->wlp, l + 1) > vsl->wle)
 		VSL_Flush(vsl, 1);
-	assert(VSL_END(vsl->wlp, l + 1) < vsl->wle);
+	assert(VSL_END(vsl->wlp, l + 1) <= vsl->wle);
 	p = VSL_DATA(vsl->wlp);
 	memcpy(p, t.b, l);
 	p[l++] = '\0';		/* NUL-terminated */
 	vsl->wlp = vsl_hdr(tag, vsl->wlp, l, vsl->wid);
-	assert(vsl->wlp < vsl->wle);
+	assert(vsl->wlp <= vsl->wle);
 	vsl->wlr++;
 
 	if (DO_DEBUG(DBG_SYNCVSL))
@@ -531,16 +531,16 @@ VSLb_bin(struct vsl_log *vsl, enum VSL_tag_e tag, ssize_t len, const void *ptr)
 	/* Truncate */
 	len = vmin_t(ssize_t, len, mlen);
 
-	assert(vsl->wlp < vsl->wle);
+	assert(vsl->wlp <= vsl->wle);
 
 	/* Flush if necessary */
-	if (VSL_END(vsl->wlp, len) >= vsl->wle)
+	if (VSL_END(vsl->wlp, len) > vsl->wle)
 		VSL_Flush(vsl, 1);
-	assert(VSL_END(vsl->wlp, len) < vsl->wle);
+	assert(VSL_END(vsl->wlp, len) <= vsl->wle);
 	p = VSL_DATA(vsl->wlp);
 	memcpy(p, ptr, len);
 	vsl->wlp = vsl_hdr(tag, vsl->wlp, len, vsl->wid);
-	assert(vsl->wlp < vsl->wle);
+	assert(vsl->wlp <= vsl->wle);
 	vsl->wlr++;
 
 	if (DO_DEBUG(DBG_SYNCVSL))