[PATCH v2 6/6] unpack-objects: unpack large object in stream

[Han Xin <chiyutianyi@gmail.com>]

From: Han Xin <hanxin.hx@alibaba-inc.com>

When calling "unpack_non_delta_entry()", will allocate full memory for
the whole size of the unpacked object and write the buffer to loose file
on disk. This may lead to OOM for the git-unpack-objects process when
unpacking a very large object.

In function "unpack_delta_entry()", will also allocate full memory to
buffer the whole delta, but since there will be no delta for an object
larger than "core.bigFileThreshold", this issue is moderate.

To resolve the OOM issue in "git-unpack-objects", we can unpack large
object to file in stream, and use "core.bigFileThreshold" to avoid OOM
limits when called "get_data()".

Signed-off-by: Han Xin <hanxin.hx@alibaba-inc.com>
---
 builtin/unpack-objects.c          | 76 ++++++++++++++++++++++++-
 t/t5590-receive-unpack-objects.sh | 92 +++++++++++++++++++++++++++++++
 2 files changed, 167 insertions(+), 1 deletion(-)
 create mode 100755 t/t5590-receive-unpack-objects.sh

diff --git a/builtin/unpack-objects.c b/builtin/unpack-objects.c
index 4a9466295b..6c757d823b 100644
--- a/builtin/unpack-objects.c
+++ b/builtin/unpack-objects.c
@@ -320,11 +320,85 @@ static void added_object(unsigned nr, enum object_type type,
 	}
 }
 
+struct input_data_from_zstream {
+	git_zstream *zstream;
+	unsigned char buf[4096];
+	int status;
+};
+
+static const char *read_inflate_in_stream(void *data, unsigned long *readlen)
+{
+	struct input_data_from_zstream *input = data;
+	git_zstream *zstream = input->zstream;
+	void *in = fill(1);
+
+	if (!len || input->status == Z_STREAM_END) {
+		*readlen = 0;
+		return NULL;
+	}
+
+	zstream->next_out = input->buf;
+	zstream->avail_out = sizeof(input->buf);
+	zstream->next_in = in;
+	zstream->avail_in = len;
+
+	input->status = git_inflate(zstream, 0);
+	use(len - zstream->avail_in);
+	*readlen = sizeof(input->buf) - zstream->avail_out;
+
+	return (const char *)input->buf;
+}
+
+static void write_stream_blob(unsigned nr, unsigned long size)
+{
+	char hdr[32];
+	int hdrlen;
+	git_zstream zstream;
+	struct input_data_from_zstream data;
+	struct input_stream in_stream = {
+		.read = read_inflate_in_stream,
+		.data = &data,
+	};
+	struct object_id *oid = &obj_list[nr].oid;
+	int ret;
+
+	memset(&zstream, 0, sizeof(zstream));
+	memset(&data, 0, sizeof(data));
+	data.zstream = &zstream;
+	git_inflate_init(&zstream);
+
+	/* Generate the header */
+	hdrlen = xsnprintf(hdr, sizeof(hdr), "%s %"PRIuMAX, type_name(OBJ_BLOB), (uintmax_t)size) + 1;
+
+	if ((ret = write_loose_object(oid, hdr, hdrlen, &in_stream, dry_run, 0, 0)))
+		die(_("failed to write object in stream %d"), ret);
+
+	if (zstream.total_out != size || data.status != Z_STREAM_END)
+		die(_("inflate returned %d"), data.status);
+	git_inflate_end(&zstream);
+
+	if (strict && !dry_run) {
+		struct blob *blob = lookup_blob(the_repository, oid);
+		if (blob)
+			blob->object.flags |= FLAG_WRITTEN;
+		else
+			die("invalid blob object from stream");
+	}
+	obj_list[nr].obj = NULL;
+}
+
 static void unpack_non_delta_entry(enum object_type type, unsigned long size,
 				   unsigned nr)
 {
-	void *buf = get_data(size);
+	void *buf;
+
+	/* Write large blob in stream without allocating full buffer. */
+	if (type == OBJ_BLOB && size > big_file_threshold) {
+		write_stream_blob(nr, size);
+		return;
+	}
 
+	buf = get_data(size);
 	if (!dry_run && buf)
 		write_object(nr, type, buf, size);
 	else
diff --git a/t/t5590-receive-unpack-objects.sh b/t/t5590-receive-unpack-objects.sh
new file mode 100755
index 0000000000..7e63dfc0db
--- /dev/null
+++ b/t/t5590-receive-unpack-objects.sh
@@ -0,0 +1,92 @@
+#!/bin/sh
+#
+# Copyright (c) 2021 Han Xin
+#
+
+test_description='Test unpack-objects when receive pack'
+
+GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
+export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
+
+. ./test-lib.sh
+
+test_expect_success "create commit with big blobs (1.5 MB)" '
+	test-tool genrandom foo 1500000 >big-blob &&
+	test_commit --append foo big-blob &&
+	test-tool genrandom bar 1500000 >big-blob &&
+	test_commit --append bar big-blob &&
+	(
+		cd .git &&
+		find objects/?? -type f | sort
+	) >expect &&
+	git repack -ad
+'
+
+test_expect_success 'setup GIT_ALLOC_LIMIT to 1MB' '
+	GIT_ALLOC_LIMIT=1m &&
+	export GIT_ALLOC_LIMIT
+'
+
+test_expect_success 'prepare dest repository' '
+	git init --bare dest.git &&
+	git -C dest.git config core.bigFileThreshold 2m &&
+	git -C dest.git config receive.unpacklimit 100
+'
+
+test_expect_success 'fail to push: cannot allocate' '
+	test_must_fail git push dest.git HEAD 2>err &&
+	test_i18ngrep "remote: fatal: attempting to allocate" err &&
+	(
+		cd dest.git &&
+		find objects/?? -type f | sort
+	) >actual &&
+	! test_cmp expect actual
+'
+
+test_expect_success 'set a lower bigfile threshold' '
+	git -C dest.git config core.bigFileThreshold 1m
+'
+
+test_expect_success 'unpack big object in stream' '
+	git push dest.git HEAD &&
+	git -C dest.git fsck &&
+	(
+		cd dest.git &&
+		find objects/?? -type f | sort
+	) >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'setup for unpack-objects dry-run test' '
+	PACK=$(echo main | git pack-objects --progress --revs test) &&
+	unset GIT_ALLOC_LIMIT &&
+	git init --bare unpack-test.git
+'
+
+test_expect_success 'unpack-objects dry-run with large threshold' '
+	(
+		cd unpack-test.git &&
+		git config core.bigFileThreshold 2m &&
+		git unpack-objects -n <../test-$PACK.pack
+	) &&
+	(
+		cd unpack-test.git &&
+		find objects/ -type f
+	) >actual &&
+	test_must_be_empty actual
+'
+
+test_expect_success 'unpack-objects dry-run with small threshold' '
+	(
+		cd unpack-test.git &&
+		git config core.bigFileThreshold 1m &&
+		git unpack-objects -n <../test-$PACK.pack
+	) &&
+	(
+		cd unpack-test.git &&
+		find objects/ -type f
+	) >actual &&
+	test_must_be_empty actual
+'
+
+test_done
-- 
2.33.1.44.g9344627884.agit.6.5.4