Re: [PATCH v2 1/6] object-file: refactor write_loose_object() to support inputstream

From: Jiang Xin <worldhello.net@gmail.com>
To: Han Xin <chiyutianyi@gmail.com>
Cc: Junio C Hamano <gitster@pobox.com>,
	Git List <git@vger.kernel.org>, Jeff King <peff@peff.net>,
	Jiang Xin <zhiyou.jx@alibaba-inc.com>,
	Philip Oakley <philipoakley@iee.email>,
	Han Xin <hanxin.hx@alibaba-inc.com>
Subject: Re: [PATCH v2 1/6] object-file: refactor write_loose_object() to support inputstream
Date: Thu, 18 Nov 2021 12:59:04 +0800	[thread overview]
Message-ID: <CANYiYbHiTnhpzyWLakVZ6tfmG0pKO=qHdZsgaocX6eJ=PN_06g@mail.gmail.com> (raw)
In-Reply-To: <20211112094010.73468-1-chiyutianyi@gmail.com>

On Fri, Nov 12, 2021 at 5:43 PM Han Xin <chiyutianyi@gmail.com> wrote:
>
> From: Han Xin <hanxin.hx@alibaba-inc.com>

It would be better to provide a cover letter describing changes in v2, such as:

* Make "write_loose_object()" a public method, so we can
   reuse it in "unpack_non_delta_entry()".
   (But I doubt we can use "write_object_file_flags()" public
     function, without make this change.)

* Add an new interface "input_stream" as an argument for
   "write_loose_object()", so that we can feed data to
   "write_loose_object()" from buffer or from zlib stream.

> Refactor write_loose_object() to support inputstream, in the same way
> that zlib reading is chunked.

In the beginning of your commit log, you should describe the problem, such as:

We used to read the full content of a blob into buffer in
"unpack_non_delta_entry()" by calling:

    void *buf = get_data(size);

This will consume lots of memory for a very big blob object.

> Using "in_stream" instead of "void *buf", we needn't to allocate enough
> memory in advance, and only part of the contents will be read when
> called "in_stream.read()".
>
> Helped-by: Jiang Xin <zhiyou.jx@alibaba-inc.com>
> Signed-off-by: Han Xin <hanxin.hx@alibaba-inc.com>
> ---
>  object-file.c  | 50 ++++++++++++++++++++++++++++++++++++++++++++++----
>  object-store.h |  5 +++++
>  2 files changed, 51 insertions(+), 4 deletions(-)
>
> diff --git a/object-file.c b/object-file.c
> index 02b7970274..1ad2cb579c 100644
> --- a/object-file.c
> +++ b/object-file.c
> @@ -1860,8 +1860,26 @@ static int create_tmpfile(struct strbuf *tmp, const char *filename)
>         return fd;
>  }
>
> +struct input_data_from_buffer {
> +       const char *buf;
> +       unsigned long len;
> +};
> +
> +static const char *read_input_stream_from_buffer(void *data, unsigned long *len)

Use "const void *" for the type of return variable, just like input
argument for write_loose_object()?

> +{
> +       struct input_data_from_buffer *input = (struct input_data_from_buffer *)data;
> +
> +       if (input->len == 0) {
> +               *len = 0;
> +               return NULL;
> +       }
> +       *len = input->len;
> +       input->len = 0;
> +       return input->buf;
> +}
> +
>  static int write_loose_object(const struct object_id *oid, char *hdr,
> -                             int hdrlen, const void *buf, unsigned long len,
> +                             int hdrlen, struct input_stream *in_stream,
>                               time_t mtime, unsigned flags)
>  {
>         int fd, ret;
> @@ -1871,6 +1889,8 @@ static int write_loose_object(const struct object_id *oid, char *hdr,
>         struct object_id parano_oid;
>         static struct strbuf tmp_file = STRBUF_INIT;
>         static struct strbuf filename = STRBUF_INIT;
> +       const char *buf;

Can we use the same prototype as the original:  "const void *buf" ?

> +       unsigned long len;
>
>         loose_object_path(the_repository, &filename, oid);
>
> @@ -1898,6 +1918,7 @@ static int write_loose_object(const struct object_id *oid, char *hdr,
>         the_hash_algo->update_fn(&c, hdr, hdrlen);
>
>         /* Then the data itself.. */
> +       buf = in_stream->read(in_stream->data, &len);
>         stream.next_in = (void *)buf;
>         stream.avail_in = len;
>         do {
> @@ -1960,6 +1981,13 @@ int write_object_file_flags(const void *buf, unsigned long len,
>  {
>         char hdr[MAX_HEADER_LEN];
>         int hdrlen = sizeof(hdr);
> +       struct input_stream in_stream = {
> +               .read = read_input_stream_from_buffer,
> +               .data = (void *)&(struct input_data_from_buffer) {
> +                       .buf = buf,
> +                       .len = len,
> +               },
> +       };
>
>         /* Normally if we have it in the pack then we do not bother writing
>          * it out into .git/objects/??/?{38} file.
> @@ -1968,7 +1996,7 @@ int write_object_file_flags(const void *buf, unsigned long len,
>                                   &hdrlen);
>         if (freshen_packed_object(oid) || freshen_loose_object(oid))
>                 return 0;
> -       return write_loose_object(oid, hdr, hdrlen, buf, len, 0, flags);
> +       return write_loose_object(oid, hdr, hdrlen, &in_stream, 0, flags);
>  }
>
>  int hash_object_file_literally(const void *buf, unsigned long len,
> @@ -1977,6 +2005,13 @@ int hash_object_file_literally(const void *buf, unsigned long len,
>  {
>         char *header;
>         int hdrlen, status = 0;
> +       struct input_stream in_stream = {
> +               .read = read_input_stream_from_buffer,
> +               .data = (void *)&(struct input_data_from_buffer) {
> +                       .buf = buf,
> +                       .len = len,
> +               },
> +       };
>
>         /* type string, SP, %lu of the length plus NUL must fit this */
>         hdrlen = strlen(type) + MAX_HEADER_LEN;
> @@ -1988,7 +2023,7 @@ int hash_object_file_literally(const void *buf, unsigned long len,
>                 goto cleanup;
>         if (freshen_packed_object(oid) || freshen_loose_object(oid))
>                 goto cleanup;
> -       status = write_loose_object(oid, header, hdrlen, buf, len, 0, 0);
> +       status = write_loose_object(oid, header, hdrlen, &in_stream, 0, 0);
>
>  cleanup:
>         free(header);
> @@ -2003,14 +2038,21 @@ int force_object_loose(const struct object_id *oid, time_t mtime)
>         char hdr[MAX_HEADER_LEN];
>         int hdrlen;
>         int ret;
> +       struct input_data_from_buffer data;
> +       struct input_stream in_stream = {
> +               .read = read_input_stream_from_buffer,
> +               .data = &data,
> +       };
>
>         if (has_loose_object(oid))
>                 return 0;
>         buf = read_object(the_repository, oid, &type, &len);
>         if (!buf)
>                 return error(_("cannot read object for %s"), oid_to_hex(oid));
> +       data.buf = buf;
> +       data.len = len;
>         hdrlen = xsnprintf(hdr, sizeof(hdr), "%s %"PRIuMAX , type_name(type), (uintmax_t)len) + 1;
> -       ret = write_loose_object(oid, hdr, hdrlen, buf, len, mtime, 0);
> +       ret = write_loose_object(oid, hdr, hdrlen, &in_stream, mtime, 0);
>         free(buf);
>
>         return ret;
> diff --git a/object-store.h b/object-store.h
> index 952efb6a4b..f1b67e9100 100644
> --- a/object-store.h
> +++ b/object-store.h
> @@ -34,6 +34,11 @@ struct object_directory {
>         char *path;
>  };
>
> +struct input_stream {
> +       const char *(*read)(void* data, unsigned long *len);
> +       void *data;
> +};
> +
>  KHASH_INIT(odb_path_map, const char * /* key: odb_path */,
>         struct object_directory *, 1, fspathhash, fspatheq)
>
> --
> 2.33.1.44.g9344627884.agit.6.5.4
>