[PATCH 1 of 4 V2] obsolete: track node versions

[Jun Wu]

Per discussion on IRC. I'll drop this series from patchwork and send a new
version with better documentation and some planned fixes.

Excerpts from Jun Wu's message of 2017-03-27 01:49:03 -0700:
> Thanks for the detailed reply. I have replies on multiple subtopics. But I
> deleted topics that I think are less important, to make the most important
> topic clear, and make the discussion more efficient. If you think I need to
> respond to more topics, feel free to point me at them.
> 
> Excerpts from Pierre-Yves David's message of 2017-03-27 09:14:53 +0200:
> 
> [snip]
> 
> > Simple practical example with date:
> > -----------------------------------
> > 
> > [time-1] repo-B: changeset C-A is pulled from repo-A
> > [time-2] repo-A: changeset C-A is rewritten as C-B (time-2)
> > [time-3] repo-B: changeset C-C is marked as superceeded by C-A (time-3)
> > [time-4] repo-B: Changeset C-B (with marker) is pulled from repo-A
> > 
> > The markers pointing from C-B to C-A have a version "time-2" but "C-A" 
> > now have a version of "time-3". "C-A" become wrongly un-obsoleted (and 
> 
> I think we basically disagree here. You said that "C-A" being visible does
> not match "the meaning of the user action", could you define "the meaning of
> the user action" formally ?
> 
> In the "node version" approach, the "meaning of a user action", where action
> is "creating a marker", is defined as below:
> 
>   When the user replace changeset X with Y, the marker X -> Y gets created,
>   they want Y to be visible, regardless of what previous (local or remote)
>   attempts hiding it.
> 
> So the un-obsolete behavior is expected.
> 
> > C-B is in strange state). The fact C-A is un-obsoleted here is a bug and 
> 
> Strictly speaking, C-B still has an obsoleted precursor of C-A. If we draw a
> 2-D graph, where Y-axis is revision, X-axis is time, and markers are edges,
> the above case could be represented like:
> 
>    ^ rev
>    |      C-C
>    |        \
>    |  C-A   C-A
>    |     \
>    |     C-B
>    |
>    +---------------> time
> 
> Note that there are 2 "C-A"s, the right one is visible, the left one is not.
> We show users the right one. Internally, the code could distinguish the
> different of 2 "C-A"s, if it wants.
> 
> > do not match the meaning of the user action. This is a regression from 
> > what evolution is currently able to achieve.
> 
> [snip]