Subject: [PATCH v9 1/5] unpack-objects: low memory footprint for get_data() in dry_run mode

From: Han Xin <[email-masked]>
Date: Thu, 20 Jan 2022 19:21:10 +0800

From: Han Xin <[email-masked]>

As the name implies, "get_data(size)" will allocate and return a given
size of memory. Allocating memory for a large blob object may cause the
system to run out of memory. Before preparing to replace calling of
"get_data()" to unpack large blob objects in latter commits, refactor
"get_data()" to reduce memory footprint for dry_run mode.

Because in dry_run mode, "get_data()" is only used to check the
integrity of data, and the returned buffer is not used at all, we can
allocate a smaller buffer and reuse it as zstream output. Therefore,
in dry_run mode, "get_data()" will release the allocated buffer and
return NULL instead of returning garbage data.

Suggested-by: Jiang Xin <[email-masked]>
Signed-off-by: Han Xin <[email-masked]>
---
 builtin/unpack-objects.c        | 39 +++++++++++++++++++--------
 t/t5328-unpack-large-objects.sh | 48 +++++++++++++++++++++++++++++++++
 2 files changed, 76 insertions(+), 11 deletions(-)
 create mode 100755 t/t5328-unpack-large-objects.sh

diff --git a/builtin/unpack-objects.c b/builtin/unpack-objects.c
index 4a9466295b..c6d6c17072 100644
--- a/builtin/unpack-objects.c
+++ b/builtin/unpack-objects.c
@@ -96,15 +96,31 @@ static void use(int bytes)
 	display_throughput(progress, consumed_bytes);
 }
 
+/*
+ * Decompress zstream from stdin and return specific size of data.
+ * The caller is responsible to free the returned buffer.
+ *
+ * But for dry_run mode, "get_data()" is only used to check the
+ * integrity of data, and the returned buffer is not used at all.
+ * Therefore, in dry_run mode, "get_data()" will release the small
+ * allocated buffer which is reused to hold temporary zstream output
+ * and return NULL instead of returning garbage data.
+ */
 static void *get_data(unsigned long size)
 {
 	git_zstream stream;
-	void *buf = xmallocz(size);
+	unsigned long bufsize;
+	void *buf;
 
 	memset(&stream, 0, sizeof(stream));
+	if (dry_run && size > 8192)
+		bufsize = 8192;
+	else
+		bufsize = size;
+	buf = xmallocz(bufsize);
 
 	stream.next_out = buf;
-	stream.avail_out = size;
+	stream.avail_out = bufsize;
 	stream.next_in = fill(1);
 	stream.avail_in = len;
 	git_inflate_init(&stream);
@@ -124,8 +140,15 @@ static void *get_data(unsigned long size)
 		}
 		stream.next_in = fill(1);
 		stream.avail_in = len;
+		if (dry_run) {
+			/* reuse the buffer in dry_run mode */
+			stream.next_out = buf;
+			stream.avail_out = bufsize;
+		}
 	}
 	git_inflate_end(&stream);
+	if (dry_run)
+		FREE_AND_NULL(buf);
 	return buf;
 }
 
@@ -325,10 +348,8 @@ static void unpack_non_delta_entry(enum object_type type, unsigned long size,
 {
 	void *buf = get_data(size);
 
-	if (!dry_run && buf)
+	if (buf)
 		write_object(nr, type, buf, size);
-	else
-		free(buf);
 }
 
 static int resolve_against_held(unsigned nr, const struct object_id *base,
@@ -358,10 +379,8 @@ static void unpack_delta_entry(enum object_type type, unsigned long delta_size,
 		oidread(&base_oid, fill(the_hash_algo->rawsz));
 		use(the_hash_algo->rawsz);
 		delta_data = get_data(delta_size);
-		if (dry_run || !delta_data) {
-			free(delta_data);
+		if (!delta_data)
 			return;
-		}
 		if (has_object_file(&base_oid))
 			; /* Ok we have this one */
 		else if (resolve_against_held(nr, &base_oid,
@@ -397,10 +416,8 @@ static void unpack_delta_entry(enum object_type type, unsigned long delta_size,
 			die("offset value out of bound for delta base object");
 
 		delta_data = get_data(delta_size);
-		if (dry_run || !delta_data) {
-			free(delta_data);
+		if (!delta_data)
 			return;
-		}
 		lo = 0;
 		hi = nr;
 		while (lo < hi) {
diff --git a/t/t5328-unpack-large-objects.sh b/t/t5328-unpack-large-objects.sh
new file mode 100755
index 0000000000..45a3316e06
--- /dev/null
+++ b/t/t5328-unpack-large-objects.sh
@@ -0,0 +1,48 @@
+#!/bin/sh
+#
+# Copyright (c) 2022 Han Xin
+#
+
+test_description='git unpack-objects with large objects'
+
+. ./test-lib.sh
+
+prepare_dest () {
+	test_when_finished "rm -rf dest.git" &&
+	git init --bare dest.git
+}
+
+test_no_loose () {
+	glob=dest.git/objects/?? &&
+	echo "$glob" >expect &&
+	eval "echo $glob" >actual &&
+	test_cmp expect actual
+}
+
+test_expect_success "create large objects (1.5 MB) and PACK" '
+	test-tool genrandom foo 1500000 >big-blob &&
+	test_commit --append foo big-blob &&
+	test-tool genrandom bar 1500000 >big-blob &&
+	test_commit --append bar big-blob &&
+	PACK=$(echo HEAD | git pack-objects --revs test)
+'
+
+test_expect_success 'set memory limitation to 1MB' '
+	GIT_ALLOC_LIMIT=1m &&
+	export GIT_ALLOC_LIMIT
+'
+
+test_expect_success 'unpack-objects failed under memory limitation' '
+	prepare_dest &&
+	test_must_fail git -C dest.git unpack-objects <test-$PACK.pack 2>err &&
+	grep "fatal: attempting to allocate" err
+'
+
+test_expect_success 'unpack-objects works with memory limitation in dry-run mode' '
+	prepare_dest &&
+	git -C dest.git unpack-objects -n <test-$PACK.pack &&
+	test_no_loose &&
+	test_dir_is_empty dest.git/objects/pack
+'
+
+test_done
-- 
2.34.1.52.gc288e771b4.agit.6.5.6