[PATCH v2 3/3] sha1_file: remove recursion in unpack_entry

[Thomas Rast <trast@student.ethz.ch>]

Similar to the recursion in packed_object_info(), this leads to
problems on stack-space-constrained systems in the presence of long
delta chains.

We proceed in three phases:

1. Dig through the delta chain, saving each delta object's offsets and
   size on an ad-hoc stack.

2. Unpack the base object at the bottom.

3. Apply the deltas from the stack.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
---
 sha1_file.c | 231 +++++++++++++++++++++++++++++++++++++++---------------------
 1 file changed, 150 insertions(+), 81 deletions(-)

diff --git a/sha1_file.c b/sha1_file.c
index bd054d1..1b685b9 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -1864,68 +1864,6 @@ static void add_delta_base_cache(struct packed_git *p, off_t base_offset,
 static void *read_object(const unsigned char *sha1, enum object_type *type,
 			 unsigned long *size);
 
-static void *unpack_delta_entry(struct packed_git *p,
-				struct pack_window **w_curs,
-				off_t curpos,
-				unsigned long delta_size,
-				off_t obj_offset,
-				enum object_type *type,
-				unsigned long *sizep)
-{
-	void *delta_data, *result, *base;
-	unsigned long base_size;
-	off_t base_offset;
-
-	base_offset = get_delta_base(p, w_curs, &curpos, *type, obj_offset);
-	if (!base_offset) {
-		error("failed to validate delta base reference "
-		      "at offset %"PRIuMAX" from %s",
-		      (uintmax_t)curpos, p->pack_name);
-		return NULL;
-	}
-	unuse_pack(w_curs);
-	base = cache_or_unpack_entry(p, base_offset, &base_size, type, 0);
-	if (!base) {
-		/*
-		 * We're probably in deep shit, but let's try to fetch
-		 * the required base anyway from another pack or loose.
-		 * This is costly but should happen only in the presence
-		 * of a corrupted pack, and is better than failing outright.
-		 */
-		struct revindex_entry *revidx;
-		const unsigned char *base_sha1;
-		revidx = find_pack_revindex(p, base_offset);
-		if (!revidx)
-			return NULL;
-		base_sha1 = nth_packed_object_sha1(p, revidx->nr);
-		error("failed to read delta base object %s"
-		      " at offset %"PRIuMAX" from %s",
-		      sha1_to_hex(base_sha1), (uintmax_t)base_offset,
-		      p->pack_name);
-		mark_bad_packed_object(p, base_sha1);
-		base = read_object(base_sha1, type, &base_size);
-		if (!base)
-			return NULL;
-	}
-
-	delta_data = unpack_compressed_entry(p, w_curs, curpos, delta_size);
-	if (!delta_data) {
-		error("failed to unpack compressed delta "
-		      "at offset %"PRIuMAX" from %s",
-		      (uintmax_t)curpos, p->pack_name);
-		free(base);
-		return NULL;
-	}
-	result = patch_delta(base, base_size,
-			     delta_data, delta_size,
-			     sizep);
-	if (!result)
-		die("failed to apply delta");
-	free(delta_data);
-	add_delta_base_cache(p, base_offset, base, base_size, *type);
-	return result;
-}
-
 static void write_pack_access_log(struct packed_git *p, off_t obj_offset)
 {
 	static FILE *log_file;
@@ -1946,48 +1884,179 @@ static void write_pack_access_log(struct packed_git *p, off_t obj_offset)
 
 int do_check_packed_object_crc;
 
+#define UNPACK_ENTRY_STACK_PREALLOC 64
+struct unpack_entry_stack_ent {
+	off_t obj_offset;
+	off_t curpos;
+	unsigned long size;
+};
+
 void *unpack_entry(struct packed_git *p, off_t obj_offset,
-		   enum object_type *type, unsigned long *sizep)
+		   enum object_type *final_type, unsigned long *final_size)
 {
 	struct pack_window *w_curs = NULL;
 	off_t curpos = obj_offset;
-	void *data;
+	void *data = NULL;
+	unsigned long size;
+	enum object_type type;
+	struct unpack_entry_stack_ent small_delta_stack[UNPACK_ENTRY_STACK_PREALLOC];
+	struct unpack_entry_stack_ent *delta_stack = small_delta_stack;
+	int delta_stack_nr = 0, delta_stack_alloc = UNPACK_ENTRY_STACK_PREALLOC;
+	int base_from_cache = 0;
 
 	if (log_pack_access)
 		write_pack_access_log(p, obj_offset);
 
-	if (do_check_packed_object_crc && p->index_version > 1) {
-		struct revindex_entry *revidx = find_pack_revindex(p, obj_offset);
-		unsigned long len = revidx[1].offset - obj_offset;
-		if (check_pack_crc(p, &w_curs, obj_offset, len, revidx->nr)) {
-			const unsigned char *sha1 =
-				nth_packed_object_sha1(p, revidx->nr);
-			error("bad packed object CRC for %s",
-			      sha1_to_hex(sha1));
-			mark_bad_packed_object(p, sha1);
-			unuse_pack(&w_curs);
-			return NULL;
+	/* PHASE 1: drill down to the innermost base object */
+	for (;;) {
+		off_t base_offset;
+		int i;
+		struct delta_base_cache_entry *ent;
+
+		if (do_check_packed_object_crc && p->index_version > 1) {
+			struct revindex_entry *revidx = find_pack_revindex(p, obj_offset);
+			unsigned long len = revidx[1].offset - obj_offset;
+			if (check_pack_crc(p, &w_curs, obj_offset, len, revidx->nr)) {
+				const unsigned char *sha1 =
+					nth_packed_object_sha1(p, revidx->nr);
+				error("bad packed object CRC for %s",
+				      sha1_to_hex(sha1));
+				mark_bad_packed_object(p, sha1);
+				unuse_pack(&w_curs);
+				return NULL;
+			}
+		}
+
+		ent = get_delta_base_cache_entry(p, curpos);
+		if (cmp_delta_base_cache_entry(ent, p, curpos)) {
+			type = ent->type;
+			data = ent->data;
+			size = ent->size;
+			clear_delta_base_cache_entry(ent);
+			base_from_cache = 1;
+			break;
+		}
+
+		type = unpack_object_header(p, &w_curs, &curpos, &size);
+		if (type != OBJ_OFS_DELTA && type != OBJ_REF_DELTA)
+			break;
+
+		base_offset = get_delta_base(p, &w_curs, &curpos, type, obj_offset);
+		if (!base_offset) {
+			error("failed to validate delta base reference "
+			      "at offset %"PRIuMAX" from %s",
+			      (uintmax_t)curpos, p->pack_name);
+			/* bail to phase 2, in hopes of recovery */
+			data = NULL;
+			break;
 		}
+
+		/* push object, proceed to base */
+		if (delta_stack_nr >= delta_stack_alloc
+		    && delta_stack == small_delta_stack) {
+			delta_stack_alloc = alloc_nr(delta_stack_nr);
+			delta_stack = xmalloc(sizeof(*delta_stack)*delta_stack_alloc);
+			memcpy(delta_stack, small_delta_stack,
+			       sizeof(*delta_stack)*delta_stack_nr);
+		} else {
+			ALLOC_GROW(delta_stack, delta_stack_nr+1, delta_stack_alloc);
+		}
+		i = delta_stack_nr++;
+		delta_stack[i].obj_offset = obj_offset;
+		delta_stack[i].curpos = curpos;
+		delta_stack[i].size = size;
+
+		curpos = obj_offset = base_offset;
 	}
 
-	*type = unpack_object_header(p, &w_curs, &curpos, sizep);
-	switch (*type) {
+	/* PHASE 2: handle the base */
+	switch (type) {
 	case OBJ_OFS_DELTA:
 	case OBJ_REF_DELTA:
-		data = unpack_delta_entry(p, &w_curs, curpos, *sizep,
-					  obj_offset, type, sizep);
+		if (data)
+			die("BUG in unpack_entry: left loop at a valid delta");
 		break;
 	case OBJ_COMMIT:
 	case OBJ_TREE:
 	case OBJ_BLOB:
 	case OBJ_TAG:
-		data = unpack_compressed_entry(p, &w_curs, curpos, *sizep);
+		if (!base_from_cache)
+			data = unpack_compressed_entry(p, &w_curs, curpos, size);
 		break;
 	default:
 		data = NULL;
 		error("unknown object type %i at offset %"PRIuMAX" in %s",
-		      *type, (uintmax_t)obj_offset, p->pack_name);
+		      type, (uintmax_t)obj_offset, p->pack_name);
 	}
+
+	/* PHASE 3: apply deltas in order */
+
+	/* invariants:
+	 *   'data' holds the base data, or NULL if there was corruption
+	 */
+	while (delta_stack_nr) {
+		void *delta_data;
+		void *base = data;
+		unsigned long delta_size, base_size = size;
+		int i;
+
+		data = NULL;
+
+		if (base)
+			add_delta_base_cache(p, obj_offset, base, base_size, type);
+
+		if (!base) {
+			/*
+			 * We're probably in deep shit, but let's try to fetch
+			 * the required base anyway from another pack or loose.
+			 * This is costly but should happen only in the presence
+			 * of a corrupted pack, and is better than failing outright.
+			 */
+			struct revindex_entry *revidx;
+			const unsigned char *base_sha1;
+			revidx = find_pack_revindex(p, obj_offset);
+			if (revidx) {
+				base_sha1 = nth_packed_object_sha1(p, revidx->nr);
+				error("failed to read delta base object %s"
+				      " at offset %"PRIuMAX" from %s",
+				      sha1_to_hex(base_sha1), (uintmax_t)obj_offset,
+				      p->pack_name);
+				mark_bad_packed_object(p, base_sha1);
+				base = read_object(base_sha1, &type, &base_size);
+			}
+		}
+
+		i = --delta_stack_nr;
+		obj_offset = delta_stack[i].obj_offset;
+		curpos = delta_stack[i].curpos;
+		delta_size = delta_stack[i].size;
+
+		if (!base)
+			continue;
+
+		delta_data = unpack_compressed_entry(p, &w_curs, curpos, delta_size);
+
+		if (!delta_data) {
+			error("failed to unpack compressed delta "
+			      "at offset %"PRIuMAX" from %s",
+			      (uintmax_t)curpos, p->pack_name);
+			free(base);
+			data = NULL;
+			continue;
+		}
+
+		data = patch_delta(base, base_size,
+				   delta_data, delta_size,
+				   &size);
+		if (!data)
+			die("failed to apply delta");
+
+		free (delta_data);
+	}
+
+	*final_type = type;
+	*final_size = size;
+
 	unuse_pack(&w_curs);
 	return data;
 }
-- 
1.8.2.266.g8176668