index-pack: eliminate unlimited recursion in get_base_data()

Revese the order of delta applying so that by the time a delta is
applied, its base is either non-delta or already inflated.
get_base_data() is still recursive, but because base's data is always
ready, the inner get_base_data() call never has any chance to call
itself again.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index 38ff03a5cdab4eb6107b471daa156dfd57753be7..dd1c5c961db087d1a9f649463d41c0ccbd7a5199 100644 (file)
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -515,14 +515,52 @@ static int is_delta_type(enum object_type type)
        return (type == OBJ_REF_DELTA || type == OBJ_OFS_DELTA);
 }
 
+/*
+ * This function is part of find_unresolved_deltas(). There are two
+ * walkers going in the opposite ways.
+ *
+ * The first one in find_unresolved_deltas() traverses down from
+ * parent node to children, deflating nodes along the way. However,
+ * memory for deflated nodes is limited by delta_base_cache_limit, so
+ * at some point parent node's deflated content may be freed.
+ *
+ * The second walker is this function, which goes from current node up
+ * to top parent if necessary to deflate the node. In normal
+ * situation, its parent node would be already deflated, so it just
+ * needs to apply delta.
+ *
+ * In the worst case scenario, parent node is no longer deflated because
+ * we're running out of delta_base_cache_limit; we need to re-deflate
+ * parents, possibly up to the top base.
+ *
+ * All deflated objects here are subject to be freed if we exceed
+ * delta_base_cache_limit, just like in find_unresolved_deltas(), we
+ * just need to make sure the last node is not freed.
+ */
 static void *get_base_data(struct base_data *c)
 {
        if (!c->data) {
                struct object_entry *obj = c->obj;
+               struct base_data **delta = NULL;
+               int delta_nr = 0, delta_alloc = 0;
 
-               if (is_delta_type(obj->type)) {
-                       void *base = get_base_data(c->base);
-                       void *raw = get_data_from_pack(obj);
+               while (is_delta_type(c->obj->type) && !c->data) {
+                       ALLOC_GROW(delta, delta_nr + 1, delta_alloc);
+                       delta[delta_nr++] = c;
+                       c = c->base;
+               }
+               if (!delta_nr) {
+                       c->data = get_data_from_pack(obj);
+                       c->size = obj->size;
+                       base_cache_used += c->size;
+                       prune_base_data(c);
+               }
+               for (; delta_nr > 0; delta_nr--) {
+                       void *base, *raw;
+                       c = delta[delta_nr - 1];
+                       obj = c->obj;
+                       base = get_base_data(c->base);
+                       raw = get_data_from_pack(obj);
                        c->data = patch_delta(
                                base, c->base->size,
                                raw, obj->size,
@@ -530,13 +568,10 @@ static void *get_base_data(struct base_data *c)
                        free(raw);
                        if (!c->data)
                                bad_object(obj->idx.offset, "failed to apply delta");
-               } else {
-                       c->data = get_data_from_pack(obj);
-                       c->size = obj->size;
+                       base_cache_used += c->size;
+                       prune_base_data(c);
                }
-
-               base_cache_used += c->size;
-               prune_base_data(c);
+               free(delta);
        }
        return c->data;
 }