Subject: [PATCH v5 10/11] pack-objects: shrink delta_size field in struct object_entry

From: Nguyễn Thái Ngọc Duy <[email-masked]>
Date: Sat, 17 Mar 2018 15:10:32 +0100

Allowing a delta size of 64 bits is crazy. Shrink this field down to
31 bits with one overflow bit.

If we find an existing delta larger than 2GB, we do not cache
delta_size at all and will get the value from oe_size(), potentially
from disk if it's larger than 4GB.

Signed-off-by: Nguyễn Thái Ngọc Duy <[email-masked]>
---
 builtin/pack-objects.c | 24 ++++++++++++++----------
 pack-objects.h         | 23 ++++++++++++++++++++++-
 2 files changed, 36 insertions(+), 11 deletions(-)

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 887e12c556..fb2aba80bf 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -30,10 +30,12 @@
 #include "packfile.h"
 
 #define IN_PACK(obj) oe_in_pack(&to_pack, obj)
+#define DELTA_SIZE(obj) oe_delta_size(&to_pack, obj)
 #define DELTA(obj) oe_delta(&to_pack, obj)
 #define DELTA_CHILD(obj) oe_delta_child(&to_pack, obj)
 #define DELTA_SIBLING(obj) oe_delta_sibling(&to_pack, obj)
 #define SET_DELTA(obj, val) oe_set_delta(&to_pack, obj, val)
+#define SET_DELTA_SIZE(obj, val) oe_set_delta_size(&to_pack, obj, val)
 #define SET_DELTA_CHILD(obj, val) oe_set_delta_child(&to_pack, obj, val)
 #define SET_DELTA_SIBLING(obj, val) oe_set_delta_sibling(&to_pack, obj, val)
 
@@ -140,7 +142,7 @@ static void *get_delta(struct object_entry *entry)
 		    oid_to_hex(&DELTA(entry)->idx.oid));
 	delta_buf = diff_delta(base_buf, base_size,
 			       buf, size, &delta_size, 0);
-	if (!delta_buf || delta_size != entry->delta_size)
+	if (!delta_buf || delta_size != DELTA_SIZE(entry))
 		die("delta size changed");
 	free(buf);
 	free(base_buf);
@@ -291,14 +293,14 @@ static unsigned long write_no_reuse_object(struct hashfile *f, struct object_ent
 		FREE_AND_NULL(entry->delta_data);
 		entry->z_delta_size = 0;
 	} else if (entry->delta_data) {
-		size = entry->delta_size;
+		size = DELTA_SIZE(entry);
 		buf = entry->delta_data;
 		entry->delta_data = NULL;
 		type = (allow_ofs_delta && DELTA(entry)->idx.offset) ?
 			OBJ_OFS_DELTA : OBJ_REF_DELTA;
 	} else {
 		buf = get_delta(entry);
-		size = entry->delta_size;
+		size = DELTA_SIZE(entry);
 		type = (allow_ofs_delta && DELTA(entry)->idx.offset) ?
 			OBJ_OFS_DELTA : OBJ_REF_DELTA;
 	}
@@ -1509,7 +1511,7 @@ static void check_object(struct object_entry *entry)
 			 */
 			oe_set_type(entry, entry->in_pack_type);
 			SET_DELTA(entry, base_entry);
-			entry->delta_size = oe_size(entry);
+			SET_DELTA_SIZE(entry, oe_size(entry));
 			entry->delta_sibling_idx = base_entry->delta_child_idx;
 			SET_DELTA_CHILD(base_entry, entry);
 			unuse_pack(&w_curs);
@@ -1895,7 +1897,7 @@ static int try_delta(struct unpacked *trg, struct unpacked *src,
 		max_size = trg_size/2 - 20;
 		ref_depth = 1;
 	} else {
-		max_size = trg_entry->delta_size;
+		max_size = DELTA_SIZE(trg_entry);
 		ref_depth = trg->depth;
 	}
 	max_size = (uint64_t)max_size * (max_depth - src->depth) /
@@ -1966,10 +1968,12 @@ static int try_delta(struct unpacked *trg, struct unpacked *src,
 	delta_buf = create_delta(src->index, trg->data, trg_size, &delta_size, max_size);
 	if (!delta_buf)
 		return 0;
+	if (delta_size >= (1 << OE_DELTA_SIZE_BITS))
+		return 0;
 
 	if (DELTA(trg_entry)) {
 		/* Prefer only shallower same-sized deltas. */
-		if (delta_size == trg_entry->delta_size &&
+		if (delta_size == DELTA_SIZE(trg_entry) &&
 		    src->depth + 1 >= trg->depth) {
 			free(delta_buf);
 			return 0;
@@ -1984,7 +1988,7 @@ static int try_delta(struct unpacked *trg, struct unpacked *src,
 	free(trg_entry->delta_data);
 	cache_lock();
 	if (trg_entry->delta_data) {
-		delta_cache_size -= trg_entry->delta_size;
+		delta_cache_size -= DELTA_SIZE(trg_entry);
 		trg_entry->delta_data = NULL;
 	}
 	if (delta_cacheable(src_size, trg_size, delta_size)) {
@@ -1997,7 +2001,7 @@ static int try_delta(struct unpacked *trg, struct unpacked *src,
 	}
 
 	SET_DELTA(trg_entry, src_entry);
-	trg_entry->delta_size = delta_size;
+	SET_DELTA_SIZE(trg_entry, delta_size);
 	trg->depth = src->depth + 1;
 
 	return 1;
@@ -2120,11 +2124,11 @@ static void find_deltas(struct object_entry **list, unsigned *list_size,
 		if (entry->delta_data && !pack_to_stdout) {
 			unsigned long size;
 
-			size = do_compress(&entry->delta_data, entry->delta_size);
+			size = do_compress(&entry->delta_data, DELTA_SIZE(entry));
 			if (size < (1 << OE_Z_DELTA_BITS)) {
 				entry->z_delta_size = size;
 				cache_lock();
-				delta_cache_size -= entry->delta_size;
+				delta_cache_size -= DELTA_SIZE(entry);
 				delta_cache_size += entry->z_delta_size;
 				cache_unlock();
 			} else {
diff --git a/pack-objects.h b/pack-objects.h
index 9a4ed7fdbe..2507b157d5 100644
--- a/pack-objects.h
+++ b/pack-objects.h
@@ -5,6 +5,7 @@
 #define OE_DEPTH_BITS		12
 #define OE_IN_PACK_BITS		14
 #define OE_Z_DELTA_BITS		16
+#define OE_DELTA_SIZE_BITS	31
 
 /*
  * State flags for depth-first search used for analyzing delta cycles.
@@ -81,7 +82,8 @@ struct object_entry {
 				     * uses the same base as me
 				     */
 	void *delta_data;	/* cached delta (uncompressed) */
-	unsigned long delta_size;	/* delta data size (uncompressed) */
+	uint32_t delta_size_:OE_DELTA_SIZE_BITS; /* delta data size (uncompressed) */
+	uint32_t delta_size_valid:1;
 	unsigned z_delta_size:OE_Z_DELTA_BITS;
 	unsigned type_:TYPE_BITS;
 	unsigned in_pack_type:TYPE_BITS; /* could be delta */
@@ -306,4 +308,23 @@ static inline void oe_set_size(struct object_entry *e,
 	e->size_valid = e->size_ == size;
 }
 
+static inline unsigned long oe_delta_size(struct packing_data *pack,
+					  const struct object_entry *e)
+{
+	if (e->delta_size_valid)
+		return e->delta_size_;
+	return oe_size(e);
+}
+
+static inline void oe_set_delta_size(struct packing_data *pack,
+				     struct object_entry *e,
+				     unsigned long size)
+{
+	e->delta_size_ = size;
+	e->delta_size_valid = e->delta_size_ == size;
+	if (!e->delta_size_valid && size != oe_size(e))
+		die("BUG: this can only happen in check_object() "
+		    "where delta size is the same as entry size");
+}
+
 #endif
-- 
2.17.0.rc0.347.gf9cf61673a