Fix REF_DELTA chain bug in 'git index-pack'

Makefile

@@ -821,6 +821,7 @@ TEST_BUILTINS_OBJS += test-mergesort.o
 TEST_BUILTINS_OBJS += test-mktemp.o
 TEST_BUILTINS_OBJS += test-name-hash.o
 TEST_BUILTINS_OBJS += test-online-cpus.o
+TEST_BUILTINS_OBJS += test-pack-deltas.o
 TEST_BUILTINS_OBJS += test-pack-mtimes.o
 TEST_BUILTINS_OBJS += test-parse-options.o
 TEST_BUILTINS_OBJS += test-parse-pathspec-file.o

builtin/index-pack.c

@@ -1109,8 +1109,8 @@ static void *threaded_second_pass(void *data)
 		set_thread_data(data);
 	for (;;) {
 		struct base_data *parent = NULL;
-		struct object_entry *child_obj;
-		struct base_data *child;
+		struct object_entry *child_obj = NULL;
+		struct base_data *child = NULL;
 
 		counter_lock();
 		display_progress(progress, nr_resolved_deltas);
@@ -1137,15 +1137,18 @@ static void *threaded_second_pass(void *data)
 			parent = list_first_entry(&work_head, struct base_data,
 						  list);
 
-			if (parent->ref_first <= parent->ref_last) {
+			while (parent->ref_first <= parent->ref_last) {
 				int offset = ref_deltas[parent->ref_first++].obj_no;
 				child_obj = objects + offset;
-				if (child_obj->real_type != OBJ_REF_DELTA)
-					die("REF_DELTA at offset %"PRIuMAX" already resolved (duplicate base %s?)",
-					    (uintmax_t) child_obj->idx.offset,
-					    oid_to_hex(&parent->obj->idx.oid));
+				if (child_obj->real_type != OBJ_REF_DELTA) {
+					child_obj = NULL;
+					continue;
+				}
 				child_obj->real_type = parent->obj->real_type;
-			} else {
+				break;
+			}
+
+			if (!child_obj && parent->ofs_first <= parent->ofs_last) {
 				child_obj = objects +
 					ofs_deltas[parent->ofs_first++].obj_no;
 				assert(child_obj->real_type == OBJ_OFS_DELTA);
@@ -1178,29 +1181,32 @@ static void *threaded_second_pass(void *data)
 		}
 		work_unlock();
 
-		if (parent) {
-			child = resolve_delta(child_obj, parent);
-			if (!child->children_remaining)
-				FREE_AND_NULL(child->data);
-		} else {
-			child = make_base(child_obj, NULL);
-			if (child->children_remaining) {
-				/*
-				 * Since this child has its own delta children,
-				 * we will need this data in the future.
-				 * Inflate now so that future iterations will
-				 * have access to this object's data while
-				 * outside the work mutex.
-				 */
-				child->data = get_data_from_pack(child_obj);
-				child->size = child_obj->size;
+		if (child_obj) {
+			if (parent) {
+				child = resolve_delta(child_obj, parent);
+				if (!child->children_remaining)
+					FREE_AND_NULL(child->data);
+			} else{
+				child = make_base(child_obj, NULL);
+				if (child->children_remaining) {
+					/*
+					 * Since this child has its own delta children,
+					 * we will need this data in the future.
+					 * Inflate now so that future iterations will
+					 * have access to this object's data while
+					 * outside the work mutex.
+					 */
+					child->data = get_data_from_pack(child_obj);
+					child->size = child_obj->size;
+				}
 			}
 		}
 
 		work_lock();
 		if (parent)
 			parent->retain_data--;
-		if (child->data) {
+
+		if (child && child->data) {
 			/*
 			 * This child has its own children, so add it to
 			 * work_head.
@@ -1209,7 +1215,7 @@ static void *threaded_second_pass(void *data)
 			base_cache_used += child->size;
 			prune_base_data(NULL);
 			free_base_data(child);
-		} else {
+		} else if (child) {
 			/*
 			 * This child does not have its own children. It may be
 			 * the last descendant of its ancestors; free those

t/helper/meson.build

@@ -36,6 +36,7 @@ test_tool_sources = [
   'test-mktemp.c',
   'test-name-hash.c',
   'test-online-cpus.c',
+  'test-pack-deltas.c',
   'test-pack-mtimes.c',
   'test-parse-options.c',
   'test-parse-pathspec-file.c',

t/helper/test-pack-deltas.c

@@ -0,0 +1,140 @@
+#define USE_THE_REPOSITORY_VARIABLE
+
+#include "test-tool.h"
+#include "git-compat-util.h"
+#include "delta.h"
+#include "git-zlib.h"
+#include "hash.h"
+#include "hex.h"
+#include "pack.h"
+#include "pack-objects.h"
+#include "setup.h"
+#include "strbuf.h"
+#include "string-list.h"
+
+static const char usage_str[] = "test-tool pack-deltas <n>";
+
+static unsigned long do_compress(void **pptr, unsigned long size)
+{
+	git_zstream stream;
+	void *in, *out;
+	unsigned long maxsize;
+
+	git_deflate_init(&stream, 1);
+	maxsize = git_deflate_bound(&stream, size);
+
+	in = *pptr;
+	out = xmalloc(maxsize);
+	*pptr = out;
+
+	stream.next_in = in;
+	stream.avail_in = size;
+	stream.next_out = out;
+	stream.avail_out = maxsize;
+	while (git_deflate(&stream, Z_FINISH) == Z_OK)
+		; /* nothing */
+	git_deflate_end(&stream);
+
+	free(in);
+	return stream.total_out;
+}
+
+static void write_ref_delta(struct hashfile *f,
+			    struct object_id *oid,
+			    struct object_id *base)
+{
+	unsigned char header[MAX_PACK_OBJECT_HEADER];
+	unsigned long size, base_size, delta_size, compressed_size, hdrlen;
+	enum object_type type;
+	void *base_buf, *delta_buf;
+	void *buf = repo_read_object_file(the_repository,
+					  oid, &type,
+					  &size);
+
+	if (!buf)
+		die("unable to read %s", oid_to_hex(oid));
+
+	base_buf = repo_read_object_file(the_repository,
+					 base, &type,
+					 &base_size);
+
+	if (!base_buf)
+		die("unable to read %s", oid_to_hex(base));
+
+	delta_buf = diff_delta(base_buf, base_size,
+			       buf, size, &delta_size, 0);
+
+	compressed_size = do_compress(&delta_buf, delta_size);
+
+	hdrlen = encode_in_pack_object_header(header, sizeof(header),
+					      OBJ_REF_DELTA, delta_size);
+	hashwrite(f, header, hdrlen);
+	hashwrite(f, base->hash, the_repository->hash_algo->rawsz);
+	hashwrite(f, delta_buf, compressed_size);
+
+	free(buf);
+	free(base_buf);
+	free(delta_buf);
+}
+
+int cmd__pack_deltas(int argc, const char **argv)
+{
+	int N;
+	struct hashfile *f;
+	struct strbuf line = STRBUF_INIT;
+
+	if (argc != 2) {
+		usage(usage_str);
+		return -1;
+	}
+
+	N = atoi(argv[1]);
+
+	setup_git_directory();
+
+	f = hashfd(the_repository->hash_algo, 1, "<stdout>");
+	write_pack_header(f, N);
+
+	/* Read each line from stdin into 'line' */
+	while (strbuf_getline_lf(&line, stdin) != EOF) {
+		const char *type_str, *content_oid_str, *base_oid_str = NULL;
+		struct object_id content_oid, base_oid;
+		struct string_list items = STRING_LIST_INIT_NODUP;
+		/*
+		 * Tokenize into two or three parts:
+		 * 1. REF_DELTA, OFS_DELTA, or FULL.
+		 * 2. The object ID for the content object.
+		 * 3. The object ID for the base object (optional).
+		 */
+		if (string_list_split_in_place(&items, line.buf, " ", 3) < 0)
+			die("invalid input format: %s", line.buf);
+
+		if (items.nr < 2)
+			die("invalid input format: %s", line.buf);
+
+		type_str = items.items[0].string;
+		content_oid_str = items.items[1].string;
+
+		if (get_oid_hex(content_oid_str, &content_oid))
+			die("invalid object: %s", content_oid_str);
+		if (items.nr >= 3) {
+			base_oid_str = items.items[2].string;
+			if (get_oid_hex(base_oid_str, &base_oid))
+				die("invalid object: %s", base_oid_str);
+		}
+
+		if (!strcmp(type_str, "REF_DELTA"))
+			write_ref_delta(f, &content_oid, &base_oid);
+		else if (!strcmp(type_str, "OFS_DELTA"))
+			die("OFS_DELTA not implemented");
+		else if (!strcmp(type_str, "FULL"))
+			die("FULL not implemented");
+		else
+			die("unknown pack type: %s", type_str);
+	}
+
+	finalize_hashfile(f, NULL, FSYNC_COMPONENT_PACK,
+			  CSUM_HASH_IN_STREAM | CSUM_FSYNC | CSUM_CLOSE);
+	strbuf_release(&line);
+	return 0;
+}

t/helper/test-tool.c

@@ -46,6 +46,7 @@ static struct test_cmd cmds[] = {
 	{ "mktemp", cmd__mktemp },
 	{ "name-hash", cmd__name_hash },
 	{ "online-cpus", cmd__online_cpus },
+	{ "pack-deltas", cmd__pack_deltas },
 	{ "pack-mtimes", cmd__pack_mtimes },
 	{ "parse-options", cmd__parse_options },
 	{ "parse-options-flags", cmd__parse_options_flags },

t/helper/test-tool.h

@@ -39,6 +39,7 @@ int cmd__mergesort(int argc, const char **argv);
 int cmd__mktemp(int argc, const char **argv);
 int cmd__name_hash(int argc, const char **argv);
 int cmd__online_cpus(int argc, const char **argv);
+int cmd__pack_deltas(int argc, const char **argv);
 int cmd__pack_mtimes(int argc, const char **argv);
 int cmd__parse_options(int argc, const char **argv);
 int cmd__parse_options_flags(int argc, const char **argv);

t/t5309-pack-delta-cycles.sh

@@ -60,7 +60,10 @@ test_expect_success 'index-pack detects REF_DELTA cycles' '
 test_expect_success 'failover to an object in another pack' '
 	clear_packs &&
 	git index-pack --stdin <ab.pack &&
-	test_must_fail git index-pack --stdin --fix-thin <cycle.pack
+
+	# This cycle does not fail since the existence of A & B in
+	# the repo allows us to resolve the cycle.
+	git index-pack --stdin --fix-thin <cycle.pack
 '
 
 test_expect_success 'failover to a duplicate object in the same pack' '
@@ -72,7 +75,36 @@ test_expect_success 'failover to a duplicate object in the same pack' '
 		pack_obj $A
 	} >recoverable.pack &&
 	pack_trailer recoverable.pack &&
-	test_must_fail git index-pack --fix-thin --stdin <recoverable.pack
+
+	# This cycle does not fail since the existence of a full copy
+	# of A in the pack allows us to resolve the cycle.
+	git index-pack --fix-thin --stdin <recoverable.pack
+'
+
+test_expect_success 'index-pack works with thin pack A->B->C with B on disk' '
+	git init server &&
+	(
+		cd server &&
+		test_commit_bulk 4
+	) &&
+
+	A=$(git -C server rev-parse HEAD^{tree}) &&
+	B=$(git -C server rev-parse HEAD~1^{tree}) &&
+	C=$(git -C server rev-parse HEAD~2^{tree}) &&
+	git -C server reset --hard HEAD~1 &&
+
+	cat >in <<-EOF &&
+	REF_DELTA $A $B
+	REF_DELTA $B $C
+	EOF
+
+	test-tool -C server pack-deltas 2 <in >thin.pack &&
+
+	git clone "file://$(pwd)/server" client &&
+	(
+		cd client &&
+		git index-pack --fix-thin --stdin <../thin.pack
+	)
 '
 
 test_done