Shelving / Checkpointing thoughts

First of all: thanks for working on shelving and checkpointing,
Julian. These could become very important and big features. Daunting
to take them on, but it's good to see someone having a go at it.

I've tried to read through all the docs and recent mail threads. Below
are a couple of thoughts.

TL;DR: I'd suggest first going for a very good Shelving
implementation, and not rushing for Checkpointing. Shelving can
already solve some checkpointing use-cases (see below), and
Checkpointing as a full-blown feature needs very careful thought (even
if limited, it's risky to paint ourselves into a corner), and will
quickly raise expectations of "local commits" or "local branching"
(which still seem far away).

== Shelving ==

Looks great so far. Of course a lot of challenges remain for all the
cases which are not yet (correctly) covered by 'svn diff' and 'svn
patch' (property changes, tree operations, binary files, unresolved
conflicts, etc.). Attaching a log message to a shelf is key, and the
association with changelists looks like a good approach.

- Shelves should (eventually) support directories as "versioned
items". Changelists currently don't support directories.

- Suggestion: 'svn shelve --keep', to create a shelf (patch) in the
"shelf storage" but not revert it. That would enable some crude way of
checkpointing your work, through simple patches (which can be applied
later by fuzzy patching, or ...):

    work on feature A
    svn shelve --keep --name "feature A"
    continue work on feature A
    turns out badly, lets go back
    svn revert; svn unshelve "feature A"

== Checkpointing ==

I think only "option 3" looks viable / interesting in the long run
(option 1, storing patches, looks a lot like simple shelving, so not
much more value imho). Or even a completely different approach which
is implemented in working copy storage (option 4? I haven't thought
this through, but I'm afraid of the disk space requirements, and init
I/O cost, of option 3 for large multi-GB working copies).

It's very hard for me to not think of checkpoints as local branches of
some sort. And my users will immediately want to use them in that way.
In all honesty, I think we should aim for powerful local branches (and
think of an architecture / design / ui for that), and then think about
how we can perhaps start with something simpler and more limited as a
first step, but which goes in that direction. I.e. a more holistic
design around "local branches", "local commits", "checkpointing".
What's the big picture?

- After reading the "Terminology" section of
https://wiki.apache.org/subversion/SavePoints, I agree with that
document that "Savepoints" might be a better name. But don't want to
bikeshed over it ...

- In a prior mail-thread between you and Nathan Hartman the "rebasing"
problem was mentioned. In [1] you said:

> Performing an 'update' with a checkpoint series is a bigger ask than it
> might at first seem. In effect, it requires rebasing the series of
> checkpoints on the new base, which gets ugly because of the need to
> handle conflicts (which is ugly enough already in the existing
> single-depth WC).

However, that seems to only sane way to go for me. Rebasing the
checkpoints one by one, and resolving conflicts along the way. Don't
know how you'd do that though, if the checkpoints are revisions 1, 2,
3 in a local repository (with immutable history etc). This is really
something where the "local repository" technique breaks down IMHO. In
contract with DVCS's, in SVN history is immutable. But mutability is
quite important for local branches / commits.

In that sense, a series of patches is more flexible: you can still
apply them with fuzz even if applying them to a different BASE state,
and often that will "just work" (and conflicts "just" need to be
resolved).

In that light, Nathan's latest post in that thread ([2]) was also
interesting, where he suggested to store the BASE together with the
WORK for every checkpoint. I'm not sure if that's the way to go (maybe
you only need a "description of BASE", not the pristines etc), but it
made me realize: don't expect checkpoints to work for "undo 'svn
update'" if you can't restore the original BASE tree (down to the
exact mixed-revisionness).

[1] https://svn.haxx.se/dev/archive-2017-07/0302.shtml
[2] https://svn.haxx.se/dev/archive-2017-08/0064.shtml

-- 
Johan