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From: Derrick Stolee <stolee@gmail.com>
To: "René Scharfe" <l.s.r@web.de>,
	"Abhishek Kumar via GitGitGadget" <gitgitgadget@gmail.com>,
	git@vger.kernel.org, "Derrick Stolee" <dstolee@microsoft.com>
Cc: "Jakub Narębski" <jnareb@gmail.com>,
	"Abhishek Kumar" <abhishekkumar8222@gmail.com>
Subject: Re: [PATCH 3/6] commit-graph: consolidate fill_commit_graph_info
Date: Tue, 28 Jul 2020 11:58:28 -0400
Message-ID: <542e98f1-f793-5290-02ae-3e4706765b80@gmail.com> (raw)
In-Reply-To: <d4a613c1-f3e8-3789-2548-8344c4b976e9@web.de>

On 7/28/2020 11:19 AM, René Scharfe wrote:
> [Had to remove stolee@gmail.com because with it my mail provider
>  rejected this email with the following error message:
> 
>    Requested action not taken: mailbox unavailable
>    invalid DNS MX or A/AAAA resource record.]
> 
> Am 28.07.20 um 15:14 schrieb Derrick Stolee:
>> On 7/28/2020 5:13 AM, Abhishek Kumar via GitGitGadget wrote:
>>> From: Abhishek Kumar <abhishekkumar8222@gmail.com>
>>>
>>> Both fill_commit_graph_info() and fill_commit_in_graph() parse
>>> information present in commit data chunk. Let's simplify the
>>> implementation by calling fill_commit_graph_info() within
>>> fill_commit_in_graph().
>>>
>>> The test 'generate tar with future mtime' creates a commit with commit
>>> time of (2 ^ 36 + 1) seconds since EPOCH. The commit time overflows into
>>> generation number and has undefined behavior. The test used to pass as
>>> fill_commit_in_graph() did not read commit time from commit graph,
>>> reading commit date from odb instead.
>>
>> I was first confused as to why fill_commit_graph_info() did not
>> load the timestamp, but the reason is that it is only used by
>> two methods:
>>
>> 1. fill_commit_in_graph(): this actually leaves the commit in a
>>    "parsed" state, so the date must be correct. Thus, it parses
>>    the date out of the commit-graph.
>>
>> 2. load_commit_graph_info(): this only helps to guarantee we
>>    know the graph_pos and generation number values.
>>
>> Perhaps add this extra context: you will _need_ the commit date
>> from the commit-graph in order to populate the generation number
>> v2 in fill_commit_graph_info().
>>
>>> Let's fix that by setting commit time of (2 ^ 34 - 1) seconds.
>>
>> The timestamp limit placed in the commit-graph is more restrictive
>> than 64-bit timestamps, but as your test points out, the maximum
>> timestamp allowed takes place in the year 2514. That is far enough
>> away for all real data.
> 
> We all may feel like the end of the world is imminent, but do we really
> need to set such an arbitrary limit?  OK, that limit was already set two
> years ago, and I'm really late.  But still: It's sad to see anything
> else than signed 64-bit timestamps to be used in fresh code (after Y2K).
> The extra four bytes would fatten up the structures less than the
> transition from SHA-1 to SHA-256 will, and no bit twiddling would be
> required.  *sigh*

One thing to consider after generation number v2 is out long enough
is if we could drop the topo-levels and write zeroes for the topo-
level portion. This was valid data in the first version of the
commit-graph, so it would still be valid. Then, we could allow
full 64-bit timestamps again.

This is something to think about again in a year, maybe.

Thanks,
-Stolee


  reply index

Thread overview: 107+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-28  9:13 [PATCH 0/6] [GSoC] Implement Corrected Commit Date Abhishek Kumar via GitGitGadget
2020-07-28  9:13 ` [PATCH 1/6] commit-graph: fix regression when computing bloom filter Abhishek Kumar via GitGitGadget
2020-07-28 15:28   ` Taylor Blau
2020-07-30  5:24     ` Abhishek Kumar
2020-08-04  0:46   ` Jakub Narębski
2020-08-04  0:56     ` Taylor Blau
2020-08-04 10:10       ` Jakub Narębski
2020-08-04  7:55     ` Jakub Narębski
2020-07-28  9:13 ` [PATCH 2/6] revision: parse parent in indegree_walk_step() Abhishek Kumar via GitGitGadget
2020-07-28 13:00   ` Derrick Stolee
2020-07-28 15:30     ` Taylor Blau
2020-08-05 23:16   ` Jakub Narębski
2020-07-28  9:13 ` [PATCH 3/6] commit-graph: consolidate fill_commit_graph_info Abhishek Kumar via GitGitGadget
2020-07-28 13:14   ` Derrick Stolee
2020-07-28 15:19     ` René Scharfe
2020-07-28 15:58       ` Derrick Stolee [this message]
2020-07-28 16:01     ` Taylor Blau
2020-07-30  6:07     ` Abhishek Kumar
2020-07-28  9:13 ` [PATCH 4/6] commit-graph: consolidate compare_commits_by_gen Abhishek Kumar via GitGitGadget
2020-07-28 16:03   ` Taylor Blau
2020-07-28  9:13 ` [PATCH 5/6] commit-graph: implement generation data chunk Abhishek Kumar via GitGitGadget
2020-07-28 16:12   ` Taylor Blau
2020-07-30  6:52     ` Abhishek Kumar
2020-07-28  9:13 ` [PATCH 6/6] commit-graph: implement corrected commit date offset Abhishek Kumar via GitGitGadget
2020-07-28 15:55   ` Derrick Stolee
2020-07-28 16:23     ` Taylor Blau
2020-07-30  7:27     ` Abhishek Kumar
2020-07-28 14:54 ` [PATCH 0/6] [GSoC] Implement Corrected Commit Date Taylor Blau
2020-07-30  7:47   ` Abhishek Kumar
2020-07-28 16:35 ` Derrick Stolee
2020-08-09  2:53 ` [PATCH v2 00/10] " Abhishek Kumar via GitGitGadget
2020-08-09  2:53   ` [PATCH v2 01/10] commit-graph: fix regression when computing bloom filter Abhishek Kumar via GitGitGadget
2020-08-09  2:53   ` [PATCH v2 02/10] revision: parse parent in indegree_walk_step() Abhishek Kumar via GitGitGadget
2020-08-09  2:53   ` [PATCH v2 03/10] commit-graph: consolidate fill_commit_graph_info Abhishek Kumar via GitGitGadget
2020-08-09  2:53   ` [PATCH v2 04/10] commit-graph: consolidate compare_commits_by_gen Abhishek Kumar via GitGitGadget
2020-08-09  2:53   ` [PATCH v2 05/10] commit-graph: implement generation data chunk Abhishek Kumar via GitGitGadget
2020-08-10 16:28     ` Derrick Stolee
2020-08-11 11:03       ` Abhishek Kumar
2020-08-11 12:27         ` Derrick Stolee
2020-08-11 18:58           ` Taylor Blau
2020-08-09  2:53   ` [PATCH v2 06/10] commit-graph: return 64-bit generation number Abhishek Kumar via GitGitGadget
2020-08-09  2:53   ` [PATCH v2 07/10] commit-graph: implement corrected commit date Abhishek Kumar via GitGitGadget
2020-08-10 14:23     ` Derrick Stolee
2020-08-14  4:59       ` Abhishek Kumar
2020-08-14 12:24         ` Derrick Stolee
2020-08-09  2:53   ` [PATCH v2 08/10] commit-graph: handle mixed generation commit chains Abhishek Kumar via GitGitGadget
2020-08-10 16:42     ` Derrick Stolee
2020-08-11 11:36       ` Abhishek Kumar
2020-08-11 12:43         ` Derrick Stolee
2020-08-09  2:53   ` [PATCH v2 09/10] commit-reach: use corrected commit dates in paint_down_to_common() Abhishek Kumar via GitGitGadget
2020-08-09  2:53   ` [PATCH v2 10/10] doc: add corrected commit date info Abhishek Kumar via GitGitGadget
2020-08-10 16:47   ` [PATCH v2 00/10] [GSoC] Implement Corrected Commit Date Derrick Stolee
2020-08-15 16:39   ` [PATCH v3 00/11] " Abhishek Kumar via GitGitGadget
2020-08-15 16:39     ` [PATCH v3 01/11] commit-graph: fix regression when computing bloom filter Abhishek Kumar via GitGitGadget
2020-08-17 22:30       ` Jakub Narębski
2020-08-15 16:39     ` [PATCH v3 02/11] revision: parse parent in indegree_walk_step() Abhishek Kumar via GitGitGadget
2020-08-18 14:18       ` Jakub Narębski
2020-08-15 16:39     ` [PATCH v3 03/11] commit-graph: consolidate fill_commit_graph_info Abhishek Kumar via GitGitGadget
2020-08-19 17:54       ` Jakub Narębski
2020-08-21  4:11         ` Abhishek Kumar
2020-08-25 11:11           ` Jakub Narębski
2020-09-01 11:35             ` Abhishek Kumar
2020-08-15 16:39     ` [PATCH v3 04/11] commit-graph: consolidate compare_commits_by_gen Abhishek Kumar via GitGitGadget
2020-08-17 13:22       ` Derrick Stolee
2020-08-21 11:05       ` Jakub Narębski
2020-08-15 16:39     ` [PATCH v3 05/11] commit-graph: return 64-bit generation number Abhishek Kumar via GitGitGadget
2020-08-21 13:14       ` Jakub Narębski
2020-08-25  5:04         ` Abhishek Kumar
2020-08-25 12:18           ` Jakub Narębski
2020-09-01 12:06             ` Abhishek Kumar
2020-09-03 13:42               ` Jakub Narębski
2020-09-05 17:21                 ` Abhishek Kumar
2020-09-13 15:39                   ` Jakub Narębski
2020-08-15 16:39     ` [PATCH v3 06/11] commit-graph: add a slab to store topological levels Abhishek Kumar via GitGitGadget
2020-08-21 18:43       ` Jakub Narębski
2020-08-25  6:14         ` Abhishek Kumar
2020-08-25  7:33           ` Jakub Narębski
2020-08-25  7:56             ` Jakub Narębski
2020-09-01 10:26               ` Abhishek Kumar
2020-09-03  9:25                 ` Jakub Narębski
2020-08-15 16:39     ` [PATCH v3 07/11] commit-graph: implement corrected commit date Abhishek Kumar via GitGitGadget
2020-08-22  0:05       ` Jakub Narębski
2020-08-25  6:49         ` Abhishek Kumar
2020-08-25 10:07           ` Jakub Narębski
2020-09-01 11:01             ` Abhishek Kumar
2020-08-15 16:39     ` [PATCH v3 08/11] commit-graph: implement generation data chunk Abhishek Kumar via GitGitGadget
2020-08-22 13:09       ` Jakub Narębski
2020-08-15 16:39     ` [PATCH v3 09/11] commit-graph: use generation v2 only if entire chain does Abhishek Kumar via GitGitGadget
2020-08-22 17:14       ` Jakub Narębski
2020-08-26  7:15         ` Abhishek Kumar
2020-08-26 10:38           ` Jakub Narębski
2020-08-15 16:39     ` [PATCH v3 10/11] commit-reach: use corrected commit dates in paint_down_to_common() Abhishek Kumar via GitGitGadget
2020-08-22 19:09       ` Jakub Narębski
2020-09-01 10:08         ` Abhishek Kumar
2020-09-03 19:11           ` Jakub Narębski
2020-08-15 16:39     ` [PATCH v3 11/11] doc: add corrected commit date info Abhishek Kumar via GitGitGadget
2020-08-22 22:20       ` Jakub Narębski
2020-08-27  6:39         ` Abhishek Kumar
2020-08-27 12:43           ` Jakub Narębski
2020-08-27 13:15           ` Derrick Stolee
2020-09-01 13:01             ` Abhishek Kumar
2020-08-17  0:13     ` [PATCH v3 00/11] [GSoC] Implement Corrected Commit Date Jakub Narębski
     [not found]       ` <CANQwDwdKp7oKy9BeKdvKhwPUiq0R5MS8TCw-eWGCYCoMGv=G-g@mail.gmail.com>
2020-08-17  1:32         ` Fwd: " Taylor Blau
2020-08-17  7:56           ` Jakub Narębski
2020-08-18  6:12       ` Abhishek Kumar
2020-08-23 15:27       ` Jakub Narębski
2020-08-24  2:49         ` Abhishek Kumar

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