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From: Jakub Narebski <jnareb@gmail.com>
To: Derrick Stolee <dstolee@microsoft.com>
Cc: git@vger.kernel.org, Jeff King <peff@peff.net>,
	Stefan Beller <sbeller@google.com>
Subject: Re: [RFC PATCH 12/12] commit-graph: update design document
Date: Fri, 20 Apr 2018 21:10:18 +0200	[thread overview]
Message-ID: <86r2n9d78l.fsf@gmail.com> (raw)
In-Reply-To: <20180417181028.198397-13-dstolee@microsoft.com> (Derrick Stolee's message of "Tue, 17 Apr 2018 18:10:45 +0000")

Derrick Stolee <dstolee@microsoft.com> writes:

> The commit-graph feature is now integrated with 'fsck' and 'gc',
> so remove those items from the "Future Work" section of the
> commit-graph design document.

See comments below, but this looks good to me.

What is missing from the "Future Work" section (and which was somewhat
implied by now removed "When this feature stabilizes enough to recommend
to most users") is safety against history [view] changing features:
git-replace, shallow clone and grafts file.  I wrote about this in
"Re: [PATCH v8 04/14] graph: add commit graph design document"
https://public-inbox.org/git/86vacsjdcg.fsf@gmail.com/

JN> The problem in my opinion lies in different direction, namely that
JN> commit grafts can change, changing the view of the history.  If we want
JN> commit-graph file to follow user-visible view of the history of the
JN> project, it needs to respect current version of commit grafts - but what
JN> if commit grafts changed since commit-graph file was generated?
JN> 
JN> Actually, there are currently three ways to affect the view of the
JN> history:
JN> 
JN> a. legacy commit grafts mechanism; it was first, but it is not safe,
JN>    cannot be transferred on fetch / push, and is now deprecated.
JN> 
JN> b. shallow clones, which are kind of specialized and limited grafts;
JN>    they used to limit available functionality, but restrictions are
JN>    being lifted (or perhaps even got lifted)
JN> 
JN> c. git-replace mechanism, where we can create an "overlay" of any
JN>    object, and is intended to be among others replacement for commit
JN>    grafts; safe, transferable, can be turned off with "git
JN>    --no-replace-objects <command>"
JN> 
JN> All those can change; some more likely than others.  The problem is if
JN> they change between writing commit-graph file (respecting old view of
JN> the history) and reading it (where we expect to see the new view).
JN> 
JN> a. grafts file can change: lines can be added, removed or changed
JN> 
JN> b. shallow clones can be deepened or shortened, or even make
JN>    not shallow
JN> 
JN> c. new replacements can be added, old removed, and existing edited
JN> 
JN> 
JN> There are, as far as I can see, two ways of handling the issue of Git
JN> features that can change the view of the project's history, namely:
JN> 
JN>  * Disable commit-graph reading when any of this features are used, and
JN>    always write full graph info.
JN> 
JN>    This may not matter much for shallow clones, where commit count
JN>    should be small anyway, but when using git-replace to stitch together
JN>    current repository with historical one, commit-graph would be
JN>    certainly useful.  Also, git-replace does not need to change history.
JN> 
JN>    On the other hand I think it is the easier solution.
JN> 
JN> Or
JN> 
JN>  * Detect somehow that the view of the history changed, and invalidate
JN>    commit-graph (perhaps even automatically regenerate it).
JN> 
JN>    For shallow clone changes I think one can still use the old
JN>    commit-graph file to generate the new one.  For other cases, the
JN>    metadata is simple to modify, but indices such as generation number
JN>    would need to be at least partially calculated anew.

Note that in all cases one can simply discard generation number
information (treating it as if it was ZERO), and use commit-graph as
values before applying history-changing feature: replacements, grafts or
shallow.

Well, at least for shallow you can do that for generation numbers: using
grafts (deprecated) or replacements to join repository with historical
one would mean that we are no longer have commit-graph transitively
closed under reachability.

>
> Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
> ---
>  Documentation/technical/commit-graph.txt | 9 ---------
>  1 file changed, 9 deletions(-)
>
> diff --git a/Documentation/technical/commit-graph.txt b/Documentation/technical/commit-graph.txt
> index d9f2713efa..d04657b781 100644
> --- a/Documentation/technical/commit-graph.txt
> +++ b/Documentation/technical/commit-graph.txt
> @@ -118,9 +118,6 @@ Future Work
>  - The commit graph feature currently does not honor commit grafts. This can
>    be remedied by duplicating or refactoring the current graft logic.
>  
> -- The 'commit-graph' subcommand does not have a "verify" mode that is
> -  necessary for integration with fsck.
> -

All right (though "verify" mode is actually done via "check" command).

>  - After computing and storing generation numbers, we must make graph
>    walks aware of generation numbers to gain the performance benefits they
>    enable. This will mostly be accomplished by swapping a commit-date-ordered
> @@ -142,12 +139,6 @@ Future Work
>    such as "ensure_tree_loaded(commit)" that fully loads a tree before
>    using commit->tree.
>  
> -- The current design uses the 'commit-graph' subcommand to generate the graph.
> -  When this feature stabilizes enough to recommend to most users, we should
> -  add automatic graph writes to common operations that create many commits.
> -  For example, one could compute a graph on 'clone', 'fetch', or 'repack'
> -  commands.
> -

I'm not sure if this paragraph should be deleted as a whole; it depends
on whether we decide that using git-gc to do automatic graph writes is
enough (git-gc should be ran automatically by git if we get many new
objects anyway, so this gives us almost "compute a graph on 'clone',
'fetch', or 'repack'").

>  - A server could provide a commit graph file as part of the network protocol
>    to avoid extra calculations by clients. This feature is only of benefit if
>    the user is willing to trust the file, because verifying the file is correct

  reply	other threads:[~2018-04-20 19:10 UTC|newest]

Thread overview: 149+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-17 18:10 [RFC PATCH 00/12] Integrate commit-graph into 'fsck' and 'gc' Derrick Stolee
2018-04-17 18:10 ` [RFC PATCH 01/12] fixup! commit-graph: always load commit-graph information Derrick Stolee
2018-04-17 18:10 ` [RFC PATCH 03/12] commit-graph: check file header information Derrick Stolee
2018-04-19 15:58   ` Jakub Narebski
2018-04-17 18:10 ` [RFC PATCH 02/12] commit-graph: add 'check' subcommand Derrick Stolee
2018-04-19 13:24   ` Jakub Narebski
2018-04-17 18:10 ` [RFC PATCH 04/12] commit-graph: parse commit from chosen graph Derrick Stolee
2018-04-19 17:21   ` Jakub Narebski
2018-04-17 18:10 ` [RFC PATCH 06/12] commit: force commit to parse from object database Derrick Stolee
2018-04-20 12:13   ` Jakub Narebski
2018-04-17 18:10 ` [RFC PATCH 05/12] commit-graph: check fanout and lookup table Derrick Stolee
2018-04-20  7:27   ` Jakub Narebski
2018-04-17 18:10 ` [RFC PATCH 07/12] commit-graph: load a root tree from specific graph Derrick Stolee
2018-04-20 12:18   ` Jakub Narebski
2018-04-17 18:10 ` [RFC PATCH 08/12] commit-graph: verify commit contents against odb Derrick Stolee
2018-04-20 16:47   ` Jakub Narebski
2018-04-17 18:10 ` [RFC PATCH 09/12] fsck: check commit-graph Derrick Stolee
2018-04-20 16:59   ` Jakub Narebski
2018-04-17 18:10 ` [RFC PATCH 10/12] commit-graph: add '--reachable' option Derrick Stolee
2018-04-20 17:17   ` Jakub Narebski
2018-04-17 18:10 ` [RFC PATCH 12/12] commit-graph: update design document Derrick Stolee
2018-04-20 19:10   ` Jakub Narebski [this message]
2018-04-17 18:10 ` [RFC PATCH 11/12] gc: automatically write commit-graph files Derrick Stolee
2018-04-20 17:34   ` Jakub Narebski
2018-04-20 18:33     ` Ævar Arnfjörð Bjarmason
2018-04-17 18:50 ` [RFC PATCH 00/12] Integrate commit-graph into 'fsck' and 'gc' Derrick Stolee
2018-05-10 17:34 ` [PATCH 00/12] Integrate commit-graph into fsck, gc, and fetch Derrick Stolee
2018-05-10 17:34   ` [PATCH 01/12] commit-graph: add 'verify' subcommand Derrick Stolee
2018-05-10 18:15     ` Martin Ågren
2018-05-10 17:34   ` [PATCH 02/12] commit-graph: verify file header information Derrick Stolee
2018-05-10 18:21     ` Martin Ågren
2018-05-10 17:34   ` [PATCH 03/12] commit-graph: parse commit from chosen graph Derrick Stolee
2018-05-10 17:34   ` [PATCH 04/12] commit-graph: verify fanout and lookup table Derrick Stolee
2018-05-10 18:29     ` Martin Ågren
2018-05-11 15:17       ` Derrick Stolee
2018-05-10 17:34   ` [PATCH 05/12] commit: force commit to parse from object database Derrick Stolee
2018-05-10 17:34   ` [PATCH 06/12] commit-graph: load a root tree from specific graph Derrick Stolee
2018-05-10 17:34   ` [PATCH 07/12] commit-graph: verify commit contents against odb Derrick Stolee
2018-05-10 17:34   ` [PATCH 08/12] fsck: verify commit-graph Derrick Stolee
2018-05-10 17:34   ` [PATCH 09/12] commit-graph: add '--reachable' option Derrick Stolee
2018-05-10 17:34   ` [PATCH 10/12] gc: automatically write commit-graph files Derrick Stolee
2018-05-10 17:34   ` [PATCH 11/12] fetch: compute commit-graph by default Derrick Stolee
2018-05-10 17:34   ` [PATCH 12/12] commit-graph: update design document Derrick Stolee
2018-05-10 19:05   ` [PATCH 00/12] Integrate commit-graph into fsck, gc, and fetch Martin Ågren
2018-05-10 19:22     ` Stefan Beller
2018-05-11 17:23       ` Derrick Stolee
2018-05-11 17:30         ` Martin Ågren
2018-05-10 19:17   ` Ævar Arnfjörð Bjarmason
2018-05-11 17:23     ` Derrick Stolee
2018-05-11 21:15   ` [PATCH v2 00/12] Integrate commit-graph into fsck and gc Derrick Stolee
2018-05-11 21:15     ` [PATCH v2 01/12] commit-graph: add 'verify' subcommand Derrick Stolee
2018-05-12 13:31       ` Martin Ågren
2018-05-14 13:27         ` Derrick Stolee
2018-05-20 12:10       ` Jakub Narebski
2018-05-11 21:15     ` [PATCH v2 02/12] commit-graph: verify file header information Derrick Stolee
2018-05-12 13:35       ` Martin Ågren
2018-05-14 13:31         ` Derrick Stolee
2018-05-20 20:00       ` Jakub Narebski
2018-05-11 21:15     ` [PATCH v2 03/12] commit-graph: test that 'verify' finds corruption Derrick Stolee
2018-05-12 13:43       ` Martin Ågren
2018-05-21 18:53       ` Jakub Narebski
2018-05-24 16:28         ` Derrick Stolee
2018-05-11 21:15     ` [PATCH v2 04/12] commit-graph: parse commit from chosen graph Derrick Stolee
2018-05-12 20:50       ` Martin Ågren
2018-05-11 21:15     ` [PATCH v2 05/12] commit-graph: verify fanout and lookup table Derrick Stolee
2018-05-11 21:15     ` [PATCH v2 06/12] commit: force commit to parse from object database Derrick Stolee
2018-05-12 20:54       ` Martin Ågren
2018-05-11 21:15     ` [PATCH v2 07/12] commit-graph: load a root tree from specific graph Derrick Stolee
2018-05-12 20:55       ` Martin Ågren
2018-05-11 21:15     ` [PATCH v2 08/12] commit-graph: verify commit contents against odb Derrick Stolee
2018-05-12 21:17       ` Martin Ågren
2018-05-14 13:44         ` Derrick Stolee
2018-05-15 21:12       ` Martin Ågren
2018-05-11 21:15     ` [PATCH v2 09/12] fsck: verify commit-graph Derrick Stolee
2018-05-17 18:13       ` Martin Ågren
2018-05-11 21:15     ` [PATCH v2 11/12] gc: automatically write commit-graph files Derrick Stolee
2018-05-17 18:20       ` Martin Ågren
2018-05-11 21:15     ` [PATCH v2 10/12] commit-graph: add '--reachable' option Derrick Stolee
2018-05-17 18:16       ` Martin Ågren
2018-05-11 21:15     ` [PATCH v2 12/12] commit-graph: update design document Derrick Stolee
2018-05-24 16:25     ` [PATCH v3 00/20] Integrate commit-graph into 'fsck' and 'gc' Derrick Stolee
2018-05-24 16:25       ` [PATCH v3 01/20] commit-graph: UNLEAK before die() Derrick Stolee
2018-05-24 22:47         ` Stefan Beller
2018-05-25  0:08           ` Derrick Stolee
2018-05-24 16:25       ` [PATCH v3 02/20] commit-graph: fix GRAPH_MIN_SIZE Derrick Stolee
2018-05-26 18:46         ` Jakub Narebski
2018-05-26 20:30           ` brian m. carlson
2018-06-02 19:43             ` Jakub Narebski
2018-05-24 16:25       ` [PATCH v3 03/20] commit-graph: parse commit from chosen graph Derrick Stolee
2018-05-27 10:23         ` Jakub Narebski
2018-05-29 12:31           ` Derrick Stolee
2018-05-24 16:25       ` [PATCH v3 04/20] commit: force commit to parse from object database Derrick Stolee
2018-05-27 18:04         ` Jakub Narebski
2018-05-24 16:25       ` [PATCH v3 05/20] commit-graph: load a root tree from specific graph Derrick Stolee
2018-05-27 19:12         ` Jakub Narebski
2018-05-24 16:25       ` [PATCH v3 06/20] commit-graph: add 'verify' subcommand Derrick Stolee
2018-05-27 22:55         ` Jakub Narebski
2018-05-30 16:07           ` Derrick Stolee
2018-06-02 21:19             ` Jakub Narebski
2018-06-04 11:30               ` Derrick Stolee
2018-05-24 16:25       ` [PATCH v3 07/20] commit-graph: verify catches corrupt signature Derrick Stolee
2018-05-28 14:05         ` Jakub Narebski
2018-05-29 12:43           ` Derrick Stolee
2018-06-02 22:30             ` Jakub Narebski
2018-05-24 16:25       ` [PATCH v3 08/20] commit-graph: verify required chunks are present Derrick Stolee
2018-05-28 17:11         ` Jakub Narebski
2018-05-24 16:25       ` [PATCH v3 09/20] commit-graph: verify corrupt OID fanout and lookup Derrick Stolee
2018-05-30 13:34         ` Jakub Narebski
2018-05-30 16:18           ` Derrick Stolee
2018-06-02  4:38         ` Duy Nguyen
2018-06-04 11:32           ` Derrick Stolee
2018-06-04 14:42             ` Duy Nguyen
2018-05-24 16:25       ` [PATCH v3 10/20] commit-graph: verify objects exist Derrick Stolee
2018-05-30 19:22         ` Jakub Narebski
2018-05-31 12:53           ` Derrick Stolee
2018-05-24 16:25       ` [PATCH v3 11/20] commit-graph: verify root tree OIDs Derrick Stolee
2018-05-30 22:24         ` Jakub Narebski
2018-05-31 13:16           ` Derrick Stolee
2018-06-02 22:50             ` Jakub Narebski
2018-05-24 16:25       ` [PATCH v3 12/20] commit-graph: verify parent list Derrick Stolee
2018-06-01 23:21         ` Jakub Narebski
2018-05-24 16:25       ` [PATCH v3 13/20] commit-graph: verify generation number Derrick Stolee
2018-06-02 12:23         ` Jakub Narebski
2018-06-04 11:47           ` Derrick Stolee
2018-05-24 16:25       ` [PATCH v3 14/20] commit-graph: verify commit date Derrick Stolee
2018-06-02 12:29         ` Jakub Narebski
2018-05-24 16:25       ` [PATCH v3 15/20] commit-graph: test for corrupted octopus edge Derrick Stolee
2018-06-02 12:39         ` Jakub Narebski
2018-06-04 13:08           ` Derrick Stolee
2018-05-24 16:26       ` [PATCH v3 16/20] commit-graph: verify contents match checksum Derrick Stolee
2018-05-30 12:35         ` SZEDER Gábor
2018-06-02 15:52         ` Jakub Narebski
2018-06-04 11:55           ` Derrick Stolee
2018-05-24 16:26       ` [PATCH v3 17/20] fsck: verify commit-graph Derrick Stolee
2018-06-02 16:17         ` Jakub Narebski
2018-06-04 11:59           ` Derrick Stolee
2018-05-24 16:26       ` [PATCH v3 18/20] commit-graph: add '--reachable' option Derrick Stolee
2018-06-02 17:34         ` Jakub Narebski
2018-06-04 12:44           ` Derrick Stolee
2018-05-24 16:26       ` [PATCH v3 19/20] gc: automatically write commit-graph files Derrick Stolee
2018-06-02 18:03         ` Jakub Narebski
2018-06-04 12:51           ` Derrick Stolee
2018-05-24 16:26       ` [PATCH v3 20/20] commit-graph: update design document Derrick Stolee
2018-06-02 18:27         ` Jakub Narebski
2018-05-24 21:15       ` [PATCH v3 00/20] Integrate commit-graph into 'fsck' and 'gc' Ævar Arnfjörð Bjarmason
2018-05-25  4:11       ` Junio C Hamano
2018-05-29  4:27       ` Junio C Hamano
2018-05-29 12:37         ` Derrick Stolee
2018-05-29 13:41           ` Junio C Hamano

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