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From: Derrick Stolee <stolee@gmail.com>
To: Ævar Arnfjörð Bjarmason <avarab@gmail.com>,
	Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>
Cc: git@vger.kernel.org, sandals@crustytoothpaste.net,
	Junio C Hamano <gitster@pobox.com>
Subject: Re: [PATCH v3 0/6] Create commit-graph file format v2
Date: Thu, 2 May 2019 09:26:04 -0400
Message-ID: <bb0c22f9-9d0b-0fa6-e826-8e2ac146c6f9@gmail.com> (raw)
In-Reply-To: <87lfzprkfc.fsf@evledraar.gmail.com>

On 5/1/2019 4:25 PM, Ævar Arnfjörð Bjarmason wrote:
> I won't repeat my outstanding v2 feedback about v1 & v2
> incompatibilities, except to say that I'd in principle be fine with
> having a v2 format the way this series is adding it. I.e. saying "here
> it is, it's never written by default, we'll figure out these compat
> issues later".
> 
> My only objection/nit on that point would be that the current
> docs/commit messages should make some mention of the really bad
> interactions between v1 and v2 on different git versions.

Good idea to add some warnings in the docs to say something like
"version 2 is not supported by Git 2.2x and earlier".

> However, having written this again I really don't understand why we need
> a v2 of this format at all.

[snip]

> How about we instead just don't change the header? I.e.:
> 
>  * Let's just live with "1" as the marker for SHA-1.
> 
>    Yeah it would be cute to use 0x73686131 instead like "struct
>    git_hash_algo", but we can live with a 1=0x73686131 ("sha1"),
>    2=0x73323536 ("s256") mapping somewhere. It's not like we're going to
>    be running into the 255 limit of hash algorithms Git will support any
>    time soon.

This was the intended direction of using the 1-byte value before, but
we have a preferred plan to use the 4-byte value in all future file formats.

>  * Don't add the reachability index version *to the header* or change
>    the reserved byte to be an error (see [1] again).

Since we can make the "corrected commit date" offset for a commit be
strictly larger than the offset of a parent, we can make it so an old client
will not give incorrect values when we use the new values. The only downside
would be that we would fail on 'git commit-graph verify' since the offsets
are not actually generation numbers in all cases.

> Instead we just add these things to new "chunks" as appropriate. As this
> patch of mine shows we can easily do that, and it doesn't error out on
> any existing version of git:
> https://github.com/avar/git/commit/3fca63e12a9d38867d4bc0a8a25d419c00a09d95

I like the idea of a "metadata" chunk. This can be useful for a lot of things.
If we start the chunk with a "number of items" and only append items to the
list, we can dynamically grow the chunk as we add values.

> I now can't imagine a situation where we'd ever need to change the
> format. We have 32 bits of chunk ids to play with, and can have 255 of
> these chunks at a time, and unknown chunks are ignored by existing
> versions and future version.

The solutions you have discussed work for 2 of the 3 problems at hand.
The incremental file format is a high-value feature, but _would_ break
existing clients if they don't understand the extra data. Unless I am
missing something for how to succeed here.

> 1. See feedback on the v2 patch in
>    https://public-inbox.org/git/87muk6q98k.fsf@evledraar.gmail.com/

My response [2] to that message includes the discussion of the
incremental file format.

[2] https://public-inbox.org/git/87muk6q98k.fsf@evledraar.gmail.com/



  reply index

Thread overview: 89+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-23 21:59 [PATCH " Derrick Stolee via GitGitGadget
2019-01-23 21:59 ` [PATCH 1/6] commit-graph: return with errors during write Derrick Stolee via GitGitGadget
2019-01-23 21:59 ` [PATCH 2/6] commit-graph: collapse parameters into flags Derrick Stolee via GitGitGadget
2019-01-23 21:59 ` [PATCH 3/6] commit-graph: create new version flags Derrick Stolee via GitGitGadget
2019-01-23 21:59 ` [PATCH 4/6] commit-graph: add --version=<n> option Derrick Stolee via GitGitGadget
2019-01-24  9:31   ` Ævar Arnfjörð Bjarmason
2019-01-23 21:59 ` [PATCH 5/6] commit-graph: implement file format version 2 Derrick Stolee via GitGitGadget
2019-01-23 23:56   ` Jonathan Tan
2019-01-24  9:40   ` Ævar Arnfjörð Bjarmason
2019-01-24 14:34     ` Derrick Stolee
2019-03-21  9:21   ` Ævar Arnfjörð Bjarmason
2019-01-23 21:59 ` [PATCH 6/6] commit-graph: test verifying a corrupt v2 header Derrick Stolee via GitGitGadget
2019-01-23 23:59   ` Jonathan Tan
2019-01-24 23:05 ` [PATCH 0/6] Create commit-graph file format v2 Junio C Hamano
2019-01-24 23:39 ` Junio C Hamano
2019-01-25 13:54   ` Derrick Stolee
2019-04-24 19:58 ` [PATCH v2 0/5] " Derrick Stolee via GitGitGadget
2019-04-24 19:58   ` [PATCH v2 1/5] commit-graph: return with errors during write Derrick Stolee via GitGitGadget
2019-04-24 19:58   ` [PATCH v2 2/5] commit-graph: collapse parameters into flags Derrick Stolee via GitGitGadget
2019-04-25  5:21     ` Junio C Hamano
2019-04-24 19:58   ` [PATCH v2 3/5] commit-graph: create new version flags Derrick Stolee via GitGitGadget
2019-04-25  5:29     ` Junio C Hamano
2019-04-25 11:09       ` Derrick Stolee
2019-04-25 21:31     ` Ævar Arnfjörð Bjarmason
2019-04-26  2:20       ` Junio C Hamano
2019-04-24 19:58   ` [PATCH v2 4/5] commit-graph: add --version=<n> option Derrick Stolee via GitGitGadget
2019-04-24 19:58   ` [PATCH v2 5/5] commit-graph: implement file format version 2 Derrick Stolee via GitGitGadget
2019-04-25 22:09   ` [PATCH v2 0/5] Create commit-graph file format v2 Ævar Arnfjörð Bjarmason
2019-04-26  2:28     ` Junio C Hamano
2019-04-26  8:33       ` Ævar Arnfjörð Bjarmason
2019-04-26 12:06         ` Derrick Stolee
2019-04-26 13:55           ` Ævar Arnfjörð Bjarmason
2019-04-27 12:57     ` Ævar Arnfjörð Bjarmason
2019-05-01 13:11   ` [PATCH v3 0/6] " Derrick Stolee via GitGitGadget
2019-05-01 13:11     ` [PATCH v3 1/6] commit-graph: return with errors during write Derrick Stolee via GitGitGadget
2019-05-01 14:46       ` Ævar Arnfjörð Bjarmason
2019-05-01 13:11     ` [PATCH v3 2/6] commit-graph: collapse parameters into flags Derrick Stolee via GitGitGadget
2019-05-01 13:11     ` [PATCH v3 3/6] commit-graph: create new version parameter Derrick Stolee via GitGitGadget
2019-05-01 13:11     ` [PATCH v3 4/6] commit-graph: add --version=<n> option Derrick Stolee via GitGitGadget
2019-05-01 13:11     ` [PATCH v3 5/6] commit-graph: implement file format version 2 Derrick Stolee via GitGitGadget
2019-05-01 19:12       ` Ævar Arnfjörð Bjarmason
2019-05-01 19:56         ` Derrick Stolee
2019-05-01 13:11     ` [PATCH v3 6/6] commit-graph: remove Future Work section Derrick Stolee via GitGitGadget
2019-05-01 14:58       ` Ævar Arnfjörð Bjarmason
2019-05-01 19:59         ` Derrick Stolee
2019-05-01 20:25     ` [PATCH v3 0/6] Create commit-graph file format v2 Ævar Arnfjörð Bjarmason
2019-05-02 13:26       ` Derrick Stolee [this message]
2019-05-02 18:02         ` Ævar Arnfjörð Bjarmason
2019-05-03 12:47           ` Derrick Stolee
2019-05-03 13:41             ` Ævar Arnfjörð Bjarmason
2019-05-06  8:27               ` Christian Couder
2019-05-06 13:47                 ` Derrick Stolee
2019-05-03 14:16             ` SZEDER Gábor
2019-05-03 15:11               ` Derrick Stolee
2019-05-09 14:22     ` [PATCH v4 00/11] Commit-graph write refactor (was: Create commit-graph file format v2) Derrick Stolee via GitGitGadget
2019-05-09 14:22       ` [PATCH v4 01/11] commit-graph: fix the_repository reference Derrick Stolee via GitGitGadget
2019-05-13  2:56         ` Junio C Hamano
2019-05-09 14:22       ` [PATCH v4 02/11] commit-graph: return with errors during write Derrick Stolee via GitGitGadget
2019-05-13  3:13         ` Junio C Hamano
2019-05-13 11:04           ` Derrick Stolee
2019-05-13 11:22             ` Derrick Stolee
2019-05-09 14:22       ` [PATCH v4 03/11] commit-graph: collapse parameters into flags Derrick Stolee via GitGitGadget
2019-05-13  3:44         ` Junio C Hamano
2019-05-13 11:07           ` Derrick Stolee
2019-05-09 14:22       ` [PATCH v4 04/11] commit-graph: remove Future Work section Derrick Stolee via GitGitGadget
2019-05-09 14:22       ` [PATCH v4 05/11] commit-graph: create write_commit_graph_context Derrick Stolee via GitGitGadget
2019-05-09 14:22       ` [PATCH v4 06/11] commit-graph: extract fill_oids_from_packs() Derrick Stolee via GitGitGadget
2019-05-13  5:05         ` Junio C Hamano
2019-05-09 14:22       ` [PATCH v4 07/11] commit-graph: extract fill_oids_from_commit_hex() Derrick Stolee via GitGitGadget
2019-05-09 14:22       ` [PATCH v4 08/11] commit-graph: extract fill_oids_from_all_packs() Derrick Stolee via GitGitGadget
2019-05-09 14:22       ` [PATCH v4 09/11] commit-graph: extract count_distinct_commits() Derrick Stolee via GitGitGadget
2019-05-09 14:22       ` [PATCH v4 10/11] commit-graph: extract copy_oids_to_commits() Derrick Stolee via GitGitGadget
2019-05-09 14:22       ` [PATCH v4 11/11] commit-graph: extract write_commit_graph_file() Derrick Stolee via GitGitGadget
2019-05-13  5:09         ` Junio C Hamano
2019-05-09 17:58       ` [PATCH v4 00/11] Commit-graph write refactor (was: Create commit-graph file format v2) Josh Steadmon
2019-06-12 13:29       ` [PATCH v5 " Derrick Stolee via GitGitGadget
2019-06-12 13:29         ` [PATCH v5 01/11] commit-graph: fix the_repository reference Derrick Stolee via GitGitGadget
2019-06-12 13:29         ` [PATCH v5 02/11] commit-graph: return with errors during write Derrick Stolee via GitGitGadget
2019-06-29 17:23           ` SZEDER Gábor
2019-07-01 12:19             ` Derrick Stolee
2019-06-12 13:29         ` [PATCH v5 03/11] commit-graph: collapse parameters into flags Derrick Stolee via GitGitGadget
2019-06-12 13:29         ` [PATCH v5 04/11] commit-graph: remove Future Work section Derrick Stolee via GitGitGadget
2019-06-12 13:29         ` [PATCH v5 05/11] commit-graph: create write_commit_graph_context Derrick Stolee via GitGitGadget
2019-06-12 13:29         ` [PATCH v5 06/11] commit-graph: extract fill_oids_from_packs() Derrick Stolee via GitGitGadget
2019-06-12 13:29         ` [PATCH v5 08/11] commit-graph: extract fill_oids_from_all_packs() Derrick Stolee via GitGitGadget
2019-06-12 13:29         ` [PATCH v5 07/11] commit-graph: extract fill_oids_from_commit_hex() Derrick Stolee via GitGitGadget
2019-06-12 13:29         ` [PATCH v5 09/11] commit-graph: extract count_distinct_commits() Derrick Stolee via GitGitGadget
2019-06-12 13:29         ` [PATCH v5 10/11] commit-graph: extract copy_oids_to_commits() Derrick Stolee via GitGitGadget
2019-06-12 13:29         ` [PATCH v5 11/11] commit-graph: extract write_commit_graph_file() Derrick Stolee via GitGitGadget

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