Re: Can someone explain this git-blame/git-rev behavior to me?

Johannes Sixt [off-list ref] writes:

David Kastrup wrote:

quoted

I am looking for a descendant of tags/v1.4.4-rc1~72 with regard to
contrib/emacs/vc-git.el by scanning the output of the first command
in the following pipeline:

git-rev-list --parents HEAD ^tags/v1.4.4-rc1~72 -- vc-git.el|git-name-rev --stdin|fgrep tags/v1.4.4-rc1

This gives:
4de0f9f9b6ed1731de00b67952504e5a783f54a4 (tags/v1.5.0-rc0~31) 1259404c7e5cd88b7f6692986469cd20cbfacdad (tags/v1.4.4-rc1~71)
1259404c7e5cd88b7f6692986469cd20cbfacdad (tags/v1.4.4-rc1~71) b4aee09e610567529dc619d7324dc2fe85a11db5 (tags/v1.4.4-rc1~72) 0abc0260fa3419de649fcc1444e3d256a17ca6c7 (tags/v1.4.3.2~6)

And this looks like it would tell us that tags/v1.4.4-rc1~72 has with
regard to vc-git.el a child tags/v1.4.4-rc1~71 and a grandchild of
tags/v1.5.0-rc0~31.

Now if I instead do
git-rev-list --parents HEAD -- vc-git.el|git-name-rev --stdin|fgrep tags/v1.4.4-rc1

4de0f9f9b6ed1731de00b67952504e5a783f54a4 (tags/v1.5.0-rc0~31) b4aee09e610567529dc619d7324dc2fe85a11db5 (tags/v1.4.4-rc1~72)
b4aee09e610567529dc619d7324dc2fe85a11db5 (tags/v1.4.4-rc1~72) 474a90fef9ebcdedee041b2def4b9a98b94cd146 (tags/v1.4.3.2~8)

I get to see that the child of tags/v1.4.4-rc1~72 is actually
tags/v1.5.0-rc0~31, skipping tags/v1.4.4-rc1~71 which presumably did
not change vc-git.el.

So the ^tags/v1.4.4-rc1~72 specification caused an additional mention
of tags/v1.4.4-rc1~71 even though, with regard to vc-git.el, it would
be equivalent to tags/v1.4.4-rc1~72.

So is there a way I can get the information of the second call without
the performance loss of going through the entire history?

It seems that there is an unwanted interaction between --parents and
history simplification at the edge commits. I don't think that there
will be an easy solution.

After some testing, it would appear that indeed the --parents option
causes the problematic behavior.  I find this surprising since
--parents should not have an effect on which lines are getting
displayed, should it?  It should just make those lines that _are_
getting displayed longer.

-- 
David Kastrup, Kriemhildstr. 15, 44793 Bochum