Re: [idea] File history tracking hints

On Mon, Sep 11, 2017 at 11:11 AM, Stefan Beller [off-list ref] wrote:

On Mon, Sep 11, 2017 at 12:11 AM, Pavel Kretov [off-list ref] wrote:

quoted

Hi all,

Excuse me if the topic I'm going to raise here has been already discussed
on the mailing list, forums, or IRC, but I couldn't find anything related.


The problem:

Git, being "a stupid content tracker", doesn't try to keep an eye on
operations which happens to individual files; things like file renames
aren't recorded during commit, but heuristically detected later.

Unfortunately, the heuristic can only deal with simple file renames with
no substantial content changes; it's helpless when you:

 - rename file and change it's content significantly;
 - split single file into several files;
 - merge several files into another;
 - copy entire file from another commit, and do other things like these.

However, if we're able to preserve this information, it's possible
not only to do more accurate 'git blame', but also merge revisions with
fewer conflicts.


The proposal:

The idea is to let user give hints about what was changed during
the commit. For example, if user did a rename which wasn't automatically
detected, he would append something like the following to his commit
message:

    Tracking-hints: rename dev-vcs/git/git-1.0.ebuild ->
dev-vcs/git/git-2.0.ebuild

or (if full paths of affected files can be unambiguously omitted):

    Tracking-hints: rename git-1.0.ebuild -> git-2.0.ebuild

There may be other hint types:

    Tracking-hint: recreate LICENSE.txt
    Tracking-hint: split main.c -> main.c cmdline.c
    Tracking-hint: merge linalg.py <- vector.py matrix.py

or even something like this:

    Tracking-hint: copy json.py <-
libs/json.py@4db88291251151d8c5c8e4f20430fa4def2cb2ed

If file transformation cannot be described by a single tracking hint, it shall
be possible to specify a sequence of hints at once:

    Tracking-hint:
        split Utils.java -> AppHelpers.java StringHelpers.java
        recreate Utils.java

Note that in the above example the order of operations really matters, so
both lines have to reside in one 'Tracking-hint' block.

* * *

How do you think, is this idea worth implementing?
Any other thoughts on this?

-- Pavel Kretov.

This was discussed a couple of times on the mailing list
(though not recently).

I searched for "rename tracking files site:public-inbox.org/git"
and came up with
https://public-inbox.org/git/Pine.LNX.4.58.0504141102430.7211@ppc970.osdl.org/
(the nearby emails seem to also be relevant to this discussion)

tl:dr: When encoding these hints, you do it at commit time,
but the heuristic can be improved upon later.
So you can assume the heuristic is better for the
common case, as someone will fix the heuristic for the
common case. Also Gits model is to track objects.

Linus has a pretty long post about this, it's somewhere in that
discussion. Essentially, if you bake in rename detection (or other
hints) at commit time, then you're stuck with it forever.

Additionally, there are similar but not *quite* the same operations
which you probably wouldn't bake into at the start, and the types of
questions a user wants to ask isn't known at commit time, but rather
known at *debug* time in the future when you're digging up history. In
this time frame, the user does know what to care about and what kind
of questions to ask, so it's already natural to ask these questions at
that time.

Additionally, if you have to generate the heuristic every commit,
you're increasing time "wasted" every commit, where as doing the
lookup later when a user starts asking questions like during blame or
diff would only add time during an operation the user already expects
to take some time.

Thanks,
Jake