Re: [PATCH v7 5/5] unpack-objects: unpack_non_delta_entry() read data in a stream

From: Han Xin <redacted>

We used to call "get_data()" in "unpack_non_delta_entry()" to read the
entire contents of a blob object, no matter how big it is. This
implementation may consume all the memory and cause OOM.

By implementing a zstream version of input_stream interface, we can use
a small fixed buffer for "unpack_non_delta_entry()".

However, unpack non-delta objects from a stream instead of from an
entrie buffer will have 10% performance penalty. Therefore, only unpack
object larger than the "core.BigFileStreamingThreshold" in zstream. See
the following benchmarks:

    hyperfine \
      --setup \
      'if ! test -d scalar.git; then git clone --bare https://github.com/microsoft/scalar.git; cp scalar.git/objects/pack/*.pack small.pack; fi' \
      --prepare 'rm -rf dest.git && git init --bare dest.git'

    Summary
      './git -C dest.git -c core.bigfilethreshold=512m unpack-objects <small.pack' in 'origin/master'
        1.01 ± 0.04 times faster than './git -C dest.git -c core.bigfilethreshold=512m unpack-objects <small.pack' in 'HEAD~1'
        1.01 ± 0.04 times faster than './git -C dest.git -c core.bigfilethreshold=512m unpack-objects <small.pack' in 'HEAD~0'
        1.03 ± 0.10 times faster than './git -C dest.git -c core.bigfilethreshold=16k unpack-objects <small.pack' in 'origin/master'
        1.02 ± 0.07 times faster than './git -C dest.git -c core.bigfilethreshold=16k unpack-objects <small.pack' in 'HEAD~0'
        1.10 ± 0.04 times faster than './git -C dest.git -c core.bigfilethreshold=16k unpack-objects <small.pack' in 'HEAD~1'

Helped-by: Ævar Arnfjörð Bjarmason [off-list ref]
Helped-by: Derrick Stolee [off-list ref]
Helped-by: Jiang Xin [off-list ref]
Signed-off-by: Han Xin <redacted>
---
 Documentation/config/core.txt       | 11 +++++
 builtin/unpack-objects.c            | 73 ++++++++++++++++++++++++++++-
 cache.h                             |  1 +
 config.c                            |  5 ++
 environment.c                       |  1 +
 t/t5590-unpack-non-delta-objects.sh | 36 +++++++++++++-
 6 files changed, 125 insertions(+), 2 deletions(-)

diff --git a/Documentation/config/core.txt b/Documentation/config/core.txt
index c04f62a54a..601b7a2418 100644
--- a/Documentation/config/core.txt
+++ b/Documentation/config/core.txt

@@ -424,6 +424,17 @@ be delta compressed, but larger binary media files won't be.
 +
 Common unit suffixes of 'k', 'm', or 'g' are supported.
 
+core.bigFileStreamingThreshold::
+	Files larger than this will be streamed out to a temporary
+	object file while being hashed, which will when be renamed
+	in-place to a loose object, particularly if the
+	`core.bigFileThreshold' setting dictates that they're always
+	written out as loose objects.
++
+Default is 128 MiB on all platforms.
++
+Common unit suffixes of 'k', 'm', or 'g' are supported.
+
 core.excludesFile::
 	Specifies the pathname to the file that contains patterns to
 	describe paths that are not meant to be tracked, in addition

diff --git a/builtin/unpack-objects.c b/builtin/unpack-objects.c
index 9104eb48da..72d8616e00 100644
--- a/builtin/unpack-objects.c
+++ b/builtin/unpack-objects.c

@@ -331,11 +331,82 @@ static void added_object(unsigned nr, enum object_type type,
 	}
 }
 
+struct input_zstream_data {
+	git_zstream *zstream;
+	unsigned char buf[8192];
+	int status;
+};
+
+static const void *feed_input_zstream(struct input_stream *in_stream,
+				      unsigned long *readlen)
+{
+	struct input_zstream_data *data = in_stream->data;
+	git_zstream *zstream = data->zstream;
+	void *in = fill(1);
+
+	if (!len || data->status == Z_STREAM_END) {
+		*readlen = 0;
+		return NULL;
+	}
+
+	zstream->next_out = data->buf;
+	zstream->avail_out = sizeof(data->buf);
+	zstream->next_in = in;
+	zstream->avail_in = len;
+
+	data->status = git_inflate(zstream, 0);
+	use(len - zstream->avail_in);
+	*readlen = sizeof(data->buf) - zstream->avail_out;
+
+	return data->buf;
+}
+
+static void write_stream_blob(unsigned nr, size_t size)
+{
+	git_zstream zstream;
+	struct input_zstream_data data;
+	struct input_stream in_stream = {
+		.read = feed_input_zstream,
+		.data = &data,
+	};
+
+	memset(&zstream, 0, sizeof(zstream));
+	memset(&data, 0, sizeof(data));

+	data.zstream = &zstream;
+	git_inflate_init(&zstream);
+
+	if (write_stream_object_file(&in_stream, size, OBJ_BLOB, 0, 0,
+				     &obj_list[nr].oid))

+		die(_("failed to write object in stream"));

diff --git a/environment.c b/environment.c
index 0d06a31024..04bba593de 100644
--- a/environment.c
+++ b/environment.c

@@ -47,6 +47,7 @@ size_t packed_git_window_size = DEFAULT_PACKED_GIT_WINDOW_SIZE;
 size_t packed_git_limit = DEFAULT_PACKED_GIT_LIMIT;
 size_t delta_base_cache_limit = 96 * 1024 * 1024;
 unsigned long big_file_threshold = 512 * 1024 * 1024;
+unsigned long big_file_streaming_threshold = 128 * 1024 * 1024;
 int pager_use_color = 1;
 const char *editor_program;
 const char *askpass_program;

diff --git a/t/t5590-unpack-non-delta-objects.sh b/t/t5590-unpack-non-delta-objects.sh
index 48c4fb1ba3..8436cbf8db 100755
--- a/t/t5590-unpack-non-delta-objects.sh
+++ b/t/t5590-unpack-non-delta-objects.sh

@@ -13,6 +13,11 @@ export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 prepare_dest () {
 	test_when_finished "rm -rf dest.git" &&
 	git init --bare dest.git
+	if test -n "$1"
+	then
+		git -C dest.git config core.bigFileStreamingThreshold $1
+		git -C dest.git config core.bigFileThreshold $1
+	fi