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

[Jun Wu]

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]