aubuf: move low latency start to read function

The write functions should simply write data to aubuf. The overrun check should
be used only to bound the buffer to the given max size.

On the first read the latency can be controlled by dropping old data.

src/aubuf/aubuf.c

@@ -215,7 +215,6 @@ int aubuf_append_auframe(struct aubuf *ab, struct mbuf *mb,
 			 const struct auframe *af)
 {
 	struct frame *f;
-	size_t max_sz;
 	size_t sz;
 
 	if (!ab || !mb)
@@ -239,8 +238,7 @@ int aubuf_append_auframe(struct aubuf *ab, struct mbuf *mb,
 	list_insert_sorted(&ab->afl, frame_less_equal, NULL, &f->le, f);
 	ab->cur_sz += sz;
 
-	max_sz = ab->started ? ab->max_sz : ab->wish_sz;
-	if (ab->max_sz && ab->cur_sz > max_sz) {
+	if (ab->max_sz && ab->cur_sz > ab->max_sz) {
 #if AUBUF_DEBUG
 		if (ab->started) {
 			++ab->stats.or;
@@ -358,6 +356,16 @@ void aubuf_read_auframe(struct aubuf *ab, struct auframe *af)
 			ab->fill_sz = ab->wish_sz;
 	}
 
+	/* on first read drop old frames */
+	while (!ab->started && ab->wish_sz && ab->cur_sz > ab->wish_sz) {
+		struct frame *f = list_ledata(ab->afl.head);
+		if (f) {
+			ab->cur_sz -= mbuf_get_left(f->mb);
+			mem_deref(f);
+		}
+	}
+
+	ab->started = true;
 	read_auframe(ab, af);
 	if (as == AJB_HIGH) {
 #if AUBUF_DEBUG
@@ -376,7 +384,6 @@ void aubuf_read_auframe(struct aubuf *ab, struct auframe *af)
 			ab->fill_sz = 0;
 	}
 
-	ab->started = true;
 	mtx_unlock(ab->lock);
 }