Re: Re: [PATCH] git tag --contains : avoid stack overflow

Jeff King [off-list ref] writes:

Yeah. We tolerate a certain amount of skew (24 hours for --name-rev, and
5 broken commits in a row for --since). But the big ones are usually
software bugs (the big kernel ones were from broken "guilt", I think) or
broken imports (when I published a bunch of skew statistics last year,
the interesting ones were all imports; I don't know if they were
software bugs, or just garbage in, garbage out).

I was hoping that 2e6bdd3 (test-generation: compute generation
numbers and clock skews, 2012-09-04) may be a good first step to
come up with a practical and cheap solution on top of it.

The traversal can be fooled by clock skews when it sees a commit
that has a timestamp that is older than it should, causing it to
give up, incorrectly thinking that there won't be newer commits that
it is interested in behind the problematic commit.

The logic implemented by the change is to identify these problematic
commits, and we could record these commits with the value of the
timestamps they should have had (e.g. the timestamp of the newest
ancestor for each of these commits) in a notes tree.  Then the
traversal logic (commit-list-insert-by-date) could be updated use
that "corrected" timestamp instead not to be fooled by the clock
skew.

Such a notes tree can be built once and updated by only "appending",
as a commit will never acquire more ancestors in its parents chain
once it is made.

Is it too simplistic, or too costly?  In git.git we have three such
commits whose timestamp need to be corrected, while in the Linux
kernel there were 2.2k skewed commits when I counted them a few
months ago.

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