pack-objects: only throw away data during memory pressure

If pack-objects hit the memory limit, it deletes objects from the delta
window.

This patch make it only delete the data, which is recomputed, if needed again.

Signed-off-by: Martin Koegler <mkoegler@auto.tuwien.ac.at>
Acked-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-pack-objects.c

@@ -1464,7 +1464,7 @@ static unsigned int check_delta_limit(struct object_entry *me, unsigned int n)
 	return m;
 }
 
-static unsigned long free_unpacked(struct unpacked *n)
+static unsigned long free_unpacked_data(struct unpacked *n)
 {
 	unsigned long freed_mem = sizeof_delta_index(n->index);
 	free_delta_index(n->index);
@@ -1474,6 +1474,12 @@ static unsigned long free_unpacked(struct unpacked *n)
 		free(n->data);
 		n->data = NULL;
 	}
+	return freed_mem;
+}
+
+static unsigned long free_unpacked(struct unpacked *n)
+{
+	unsigned long freed_mem = free_unpacked_data(n);
 	n->entry = NULL;
 	n->depth = 0;
 	return freed_mem;
@@ -1514,7 +1520,7 @@ static void find_deltas(struct object_entry **list, unsigned *list_size,
 		       mem_usage > window_memory_limit &&
 		       count > 1) {
 			uint32_t tail = (idx + window - count) % window;
-			mem_usage -= free_unpacked(array + tail);
+			mem_usage -= free_unpacked_data(array + tail);
 			count--;
 		}
 
@@ -1547,6 +1553,9 @@ static void find_deltas(struct object_entry **list, unsigned *list_size,
 			if (!m->entry)
 				break;
 			ret = try_delta(n, m, max_depth, &mem_usage);
+			if (window_memory_limit &&
+			    mem_usage > window_memory_limit)
+				mem_usage -= free_unpacked_data(m);
 			if (ret < 0)
 				break;
 			else if (ret > 0)