Re: git-note -C changes commit type?

On Wed, Feb 12, 2014 at 9:53 AM, Joachim Breitner
[off-list ref] wrote:

Am Mittwoch, den 12.02.2014, 00:52 +0100 schrieb Johan Herland:

quoted

You would have a notes ref "refs/notes/history" whose tree would
contain an entry named e1bfac434ebd3135a3784f6fc802f235098eebd0
pointing to a _commit_ (3d7de37...). Obviously, it would not make
sense to use refs/notes/history while displaying the commit log ("git
log --notes=history"), as the raw commit object would be shown in the
log. However, more fundamentally: a tree referring to a _commit_ is
usually how Git stores _submodule_ links (i.e. which revision of the
named submodule is to be used by this super-repo tree), and I'm (off
the top of my head) not at all sure that such a submodule link in a
notes tree is handled "sanely" by Git - or even that it makes sense at
all. For one, I'm not sure that Git requires (or even expects) the
commit object referenced by a tree to be present in the same object
DB. So if you share your notes, I don't know whether or not the
fetch/push machinery will include the commit object in the shared
notes... These are questions that should be answered before we decide
whether using commits directly as notes makes sense.

If that is the case, then my approach is indeed flawed. The main point
of the exercise is to have a tree that follows another commit (or, as a
next-best approximation, a note attached to that commit) around.

quoted

In that case, you might be better off using an explicit
ref to keep that history alive; e.g. you could create
refs/history/e1bfac4 to point to 3d7de37 ("git update-ref
refs/history/e1bfac4 3d7de37"), and keep everything
alive/reachable/shareable that way...

That’s an interesting idea; instead of relying on the notes feature
putting the hash in the ref name. But I wonder how that scales – imagine
every second feature merged into Linux¹ also having such a history ref?

Ah, that will probably not scale very well.

I guess having a way for a tree to reference commits in a way that is
followed by git gc, i.e. separate from submodules, would allow a less
noisy implementation, and possibly create the opportunity for many other
strange uses of git :-)

Here's another way to solve your problem, which should be fairly
transparent and performant:

Whenever you want to reference "history" of a commit (I'm using quotes
here, because we're not talking about the "regular" git sense of
history, i.e. the commit graph), you perform the following two steps:

1. Append the "historical" commit SHA-1 (3d7de37 in your example) to a
note on the "current" commit (e1bfac4). E.g.:

    git notes --ref history append -m 3d7de37... e1bfac4...

2. Perform some automated merge into a "history"-tracking ref (e.g.
refs/history), to keep the "historical" commits reachable.

(You can easily wrap both steps into a script to automate things.)

Step #1 encodes the "history" of a commit in a note, but does not keep
the "history" reachable.

Step #2 keeps all "historical" commits reachable by making them part
of the history (in the git sense - without quotes) of a proper ref
(refs/history). The actual result/outcome of the merge is not
interesting. It only exists to insert the "historical" commit
(3d7de37) into the ancestry for refs/history. Since the actual merge
itself is uninteresting, you should probably use a merge strategy that
never yields conflicts, e.g. "-s ours"

You can now share the "history" by pushing/fetching the two refs
refs/notes/history and refs/history.

(In theory, you might even be able to combine the two refs, by
performing the merge directly into refs/notes/history, always taking
care to retain the notes tree contents as the result of the merge. In
other words, after you do step #1 (append the note), you manually
rewrite the just-created tip of refs/notes/history to include 3d7de37
as a second parent. This keeps 3d7de37 reachable (and it will be
shared when you share refs/notes/history), and it should not interfere
with the notes infrastructure, as they only look at the current state
of the notes tree.)


Hope this helps,

...Johan

-- 
Johan Herland, [off-list ref]
www.herland.net