# Re: \[PATCH v7 1/5\] unpack-objects.c: add dry\_run mode for get\_data()

- From: René Scharfe \<hidden\>
- Date: 2021-12-21 14:43:47 
- List: git
- Page: https://ratatoskr.run/git/2021/12/7916467
- Thread: https://ratatoskr.run/git/2021/10/7904958/t/2.md

```text
Am 21.12.21 um 15:09 schrieb Ævar Arnfjörð Bjarmason:
>
> On Tue, Dec 21 2021, Han Xin wrote:
>
>> From: Han Xin <redacted>
>>
>> In dry_run mode, "get_data()" is used to verify the inflation of data,
>> and the returned buffer will not be used at all and will be freed
>> immediately. Even in dry_run mode, it is dangerous to allocate a
>> full-size buffer for a large blob object. Therefore, only allocate a
>> low memory footprint when calling "get_data()" in dry_run mode.
>>
>> Suggested-by: Jiang Xin <redacted>
>> Signed-off-by: Han Xin <redacted>
>> ---
>>  builtin/unpack-objects.c            | 23 +++++++++---
>>  t/t5590-unpack-non-delta-objects.sh | 57 +++++++++++++++++++++++++++++
>>  2 files changed, 74 insertions(+), 6 deletions(-)
>>  create mode 100755 t/t5590-unpack-non-delta-objects.sh
>>
>> diff --git a/builtin/unpack-objects.c b/builtin/unpack-objects.c
>> index 4a9466295b..9104eb48da 100644
>> --- a/builtin/unpack-objects.c
>> +++ b/builtin/unpack-objects.c
>> @@ -96,15 +96,21 @@ static void use(int bytes)
>>  	display_throughput(progress, consumed_bytes);
>>  }
>>
>> -static void *get_data(unsigned long size)
>> +static void *get_data(size_t size, int dry_run)
>>  {
>>  	git_zstream stream;
>> -	void *buf = xmallocz(size);
>> +	size_t bufsize;
>> +	void *buf;
>>
>>  	memset(&stream, 0, sizeof(stream));
>> +	if (dry_run && size > 8192)
>> +		bufsize = 8192;
>> +	else
>> +		bufsize = size;
>> +	buf = xmallocz(bufsize);
>
> Maybe I'm misunderstanding this, but the commit message says it would be
> dangerous to allocate a very larger buffer, but here we only limit the
> size under "dry_run".

This patch reduces the memory usage of dry runs, as its commit message
says.  The memory usage of one type of actual (non-dry) unpack is reduced
by patch 5.

> Removing that "&& size > 8192" makes all the tests pass still, so there
> seems to be some missing coverage here in any case.

How would you test that an 8KB buffer is allocated even though a smaller
one would suffice?  And why?  Wasting a few KB shouldn't be noticeable.

René
```