Re: [idea] File history tracking hints

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Sep 13, 2017, 11:38 UTC
Message-ID: <alpine.DEB.2.21.1.1709131322470.4132@virtualbox>
In-Reply-To: <E8C827ED458648F78F263F2F2712493B@PhilipOakley>

Hi Philip,

On Mon, 11 Sep 2017, Philip Oakley wrote:

Show 73 quoted lines

> From: "Pavel Kretov" <firegurafiku@gmail.com>
> > 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.
> 
> Maybe use the "interpret-trailers" methods for standardising your hints
> locally (in your team / workplace) to see how it goes and flesh out what works
> and what doesn't. Trying to decide, a-priori, what are the right hints is
> likely to be the hard part.

I think this adds a very valuable insight to this discussion: the current state of Git's rename handling is based on the idea that you either record the renames, or you detect them. Like, there is either "on" or "off". No middle ground.

However, if you understand that there is also the possibility of hints that can help any erroneous rename detection (and *everybody* who seriously worked on a massive code base has seen that rename detection fail in the most inopportune ways [*1*]), then you are on to something.

So I totally like the idea of introducing hints, possibly as trailers in the commit message (or as refs/notes/rename/* or whatever) that can be picked up by Git versions that know about them, and can be ignored by Git versions that insist on the rename detection du jour. With a config option to control the behavior, maybe, too.

Ciao, Dscho

Footnote *1*: Just to name a couple of examples from my personal experience, off the top of my head:

- license boiler plates often let Git detect renames/copies where there
  are none,

- even something as trivial as moving Java classes (and their dependent
  classes) between packages changes every line referring to said packages,
  causing Git's rename detection to go for a drink instead of doing its
  job,

- indentation changes overwhelm Git's rename detection,

- when rename detection would matter most, like, really a lot, to lift the
  burden of the human beings in front of the computer pouring over
  hundreds of thousands of files moved from one directory tree to another,
  that's exactly when Git's rename detection says that there are too many
  files, here are my union rights, I am going home, good luck to you.

In light of such experiences, I have to admit that the notion that the rename detection can always be improved in hindsight puts quite a bit of insult to injury for those developers who are bitten by it.