Re: gsoc - Better git log --follow support

[Jeff King <peff@peff.net>]

On Sat, Mar 19, 2011 at 08:24:20PM +0100, Michał Łowicki wrote:

> I'm looking at idea about better git log --follow support from
> https://git.wiki.kernel.org/index.php/SoC2011Ideas .There is something
> like this - "[.. ] it does not interact well with git's usual history
> simplification [...]". Can someone elaborate this? I've found History
> Simplification in git rev-list man but don't know yet about issues
> with --follow.

In short, history simplification is a way of looking at a subset of the
commit history graph, but in a way that makes it look like a complete
graph. Imagine I have a linear history like this:

  A--B--C

where "A" modifies "file1", "B" modifies "file2", and "C" modifies
"file1" again. If I ask for the history of "file1" with "git log file1",
then git will pretend as if the graph looks like:

  A--C

including rewriting the parent of "C" to point to "A" (because the
parent pointer is basically an edge in the graph).

If you are just doing a straight "git log", the actual parentage is not
that interesting. We either show commits or we don't, and we don't show
links between them. But try "git log --graph" or "gitk", which do care
about the edges. They want to show you a whole connected graph.

Now consider --follow. It doesn't happen during the commit limiting
phase, but instead it happens while we're showing commits. And if it
decides a commit isn't interesting, we don't show it. That works OK for
"git log", but it makes the graph for other things disjointed.

You can see it in this example:

  # make the A-B-C repo we mentioned above
  git init repo && cd repo
  echo content >file1 && git add file1 && git commit -m one
  echo content >file2 && git add file2 && git commit -m two
  echo content >>file1 && git add file1 && git commit -m three

  # Now look at it in gitk; we see a nice linear graph.
  gitk

  # Now let's try it with path limiting. We see a nice subgraph that
  # pretends to be linear, because we "squished" out the uninteresting
  # nodes.
  gitk file1

  # Now let's make some more commits with a rename.
  echo content >>file2 && git commit -a -m four
  git mv file1 newfile && git commit -m five
  echo content >>newfile && git commit -a -m six

  # If we use path limiting, we'll only see the two most recent commits.
  # We get stopped at the rename because path limiting is just about the
  # pathname.
  gitk newfile

  # So we can use --follow to follow the rename. First let's try simple
  # output. You should see commits 1, 3, 5, and 6, which touched either
  # newfile or its rename source, file1. 
  git log --oneline --follow newfile

  # But now look at it in gitk. Commit 4 is included as a boundary
  # commit, but we fail to notice that it connects to three. And we
  # don't see commit 3 connecting to anything, and commit 1 is missing
  # entirely.
  gitk --follow newfile

Obviously this a pretty simplistic example. But you can imagine in a
history with a lot of branching how useful this simplification is to
understanding what happened to a subset of the tree.

Jakub mentioned another example with gitweb's subtree merge not being
found by --follow. I haven't looked into that case, but it may be
related (or it may simply be a defect in follow finding the right
source).

-Peff