Re: [PATCH v2] pack-format.txt: more details on pack file format

From: Stefan Beller <sbeller@google.com>
Date: May 10, 2018, 17:06 UTC
Message-ID: <CAGZ79kYYFBE+39aeHX22w3ARmujFcgE8YDUQpp6Hps264r0wfQ@mail.gmail.com>
In-Reply-To: <20180510150939.25399-1-pclouds@gmail.com>

On Thu, May 10, 2018 at 8:09 AM, Nguyễn Thái Ngọc Duy <pclouds@gmail.com> wrote:

Show 44 quoted lines

> The current document mentions OBJ_* constants without their actual
> values. A git developer would know these are from cache.h but that's
> not very friendly to a person who wants to read this file to implement
> a pack file parser.
>
> Similarly, the deltified representation is not documented at all (the
> "document" is basically patch-delta.c). Translate that C code to
> English with a bit more about what ofs-delta and ref-delta mean.
>
> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
> ---
>  This is a much better description than v1. I hope.
>
>  Documentation/technical/pack-format.txt | 78 +++++++++++++++++++++++++
>  cache.h                                 |  5 ++
>  2 files changed, 83 insertions(+)
>
> diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt
> index 8e5bf60be3..d20bf592aa 100644
> --- a/Documentation/technical/pack-format.txt
> +++ b/Documentation/technical/pack-format.txt
> @@ -36,6 +36,84 @@ Git pack format
>
>    - The trailer records 20-byte SHA-1 checksum of all of the above.
>
> +=== Object types
> +
> +Valid object types are:
> +
> +- OBJ_COMMIT (1)
> +- OBJ_TREE (2)
> +- OBJ_BLOB (3)
> +- OBJ_TAG (4)
> +- OBJ_OFS_DELTA (6)
> +- OBJ_REF_DELTA (7)
> +
> +Type 5 is reserved for future expansion. Type 0 is invalid.
> +
> +=== Deltified representation
> +
> +Conceptually there are only four object types: commit, tree, tag and
> +blob. However to save space, an object could be stored as a "delta" of
> +another "base" object. These representations are assigned new types
> +ofs-delta and ref-delta, which is only valid in a pack file.

...only valid...

as opposed to loose objects or as opposed to referencing cross-packs? I would think the former, not the latter.

> +Both ofs-delta and ref-delta store the "delta" against another
> +object. The difference between them is, ref-delta directly encodes
> +20-byte base object name. If the base object is in the same pack,
> +ofs-delta encodes the offset of the base object in the pack instead.

Reading this paragraph clears up the question from before. The ref delta is a delta to another "reference by hash id (sha1)". What abbreviation is OFS? OFfSet ?

> +The delta data is a sequence of instructions to reconstruct an object
> +from the base object.

As said before the base object is of type 1..4, we do not "delta-on-delta" yet, but to construct the object we have to create the base object first, which itself can be represented as a deltified object leading to a delta chain.

>     Each instruction appends more and more data to
> +the target object until it's complete. There are two supported
> +instructions so far: one for copy a byte range from the source object
> +and one for inserting new data embedded in the instruction itself.

ok. So there are 2 types of instructions, "copy from (offset, size)" and "new data follows".

The next paragraphs seem to describe the copy instruction, maybe add a sub-headline here?

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> +Each instruction has variable length. Instruction type is determined
> +by the seventh bit of the first octet. The following diagrams follow
> +the convention in RFC 1951 (Deflate compressed data format).
> +
> +  +----------+---------+---------+---------+---------+-------+-------+-------+
> +  | 1xxxxxxx | offset1 | offset2 | offset3 | offset4 | size1 | size2 | size3 |
> +  +----------+---------+---------+---------+---------+-------+-------+-------+
> +
> +This is the instruction format to copy a byte range from the source
> +object. It encodes the offset to copy from any the number of bytes to
> +copy. Offset and size are in little-endian order.
> +
> +All offset and size bytes are optional. This is to reduce the
> +instruction size when encoding small offsets or sizes. The first seven
> +bits in the first octet determines which of the next seven octets is
> +present. If bit zero is set, offset1 is present. If bit one is set
> +offset2 is present and so on.
> +
> +Note that a more compact instruction does not change offset and size
> +encoding. For example, if only offset2 is omitted like below, offset3
> +still contains bits 16-23. It does not become offset2 and contains
> +bits 8-15 even if it's right next to offset1.
> +
> +  +----------+---------+---------+
> +  | 10000101 | offset1 | offset3 |
> +  +----------+---------+---------+

It reads very fluently to here.

> +In its most compact form, this instruction only takes up one byte
> +(0x80) with both offset and size omitted, which will have default
> +values zero. There is another exception: size zero is automatically
> +converted to 0x10000.

This "another exception" sounds a bit tacked on, but is still understandable. I would imagine that the size of 0 is used frequently to copy large blocks and coincidentally it is represented using the lowest number of bytes for size. Cute!

Before the next diagram we could have a sub-headline, indicating that the other instruction "new data follows" will now be described.

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> +  +----------+============+
> +  | 0xxxxxxx |    data    |
> +  +----------+============+
> +
> +This is the instruction to construct target object without the base
> +object. The following data is appended to the target object. The first
> +seven bits of the first octet determines the size of data in
> +bytes. The size must be non-zero.

This command sounds very easy. However we can have at most 127 bytes of new data, so if someone adds a larger part of new code, we'd have many "insert new data" instructions, all at the max of 127, such that the overhead for instruction bytes is 1/127 = 0.7 %. Sounds efficient.

Show 5 quoted lines

> +  +----------+============
> +  | 00000000 |
> +  +----------+============
> +
> +This is the instruction reserved for future expansion.

Thanks for pointing this out.

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>
> +/*
> + * Values in this enum (except those outside the 3 bit range) are part
> + * of pack file format. See Documentation/technical/pack-format.txt
> + * for more information.
> + */

Makes sense.

I really like this patch very much. Thanks for writing it.
My annotations are just to add the cherry onto the cake,
the current form is
Reviewed-by: Stefan Beller <sbeller@google.com>

Thanks!