Re: [RFC/PATCH] Supporting non-blob notes
From: Johan Herland <hidden>
Date: 2016-06-15 22:59:57
On Mon, Feb 17, 2014 at 11:48 AM, [off-list ref] wrote:
The recent "git-note -C changes commit type?" thread (http://thread.gmane.org/gmane.comp.version-control.git/241950) looks like a good occasion to discuss possible uses of non-blob notes. The use-case we're thinking about is the storage of testrun logs as notes (think: being able to justify that a given set of tests were successfully run on a given revision).
I think this is a good use of notes, and organizing the testrun logs into a tree of files seems like a natural way to proceed.
Here is a proof-of-concept patch (that applies to 1.8.4.2) I've been playing with. Because of the -C behaviour described in this other thread, I opted for a new -o flag that would not mess with the object argument. This patch is very minimalist, and just allows storing a tree note (currently any type of object, but that's easy to restrict if we want to), and retrieving it.
I think we must think _very_ carefully about which object types we allow to be stored in notes trees. As far as I can see, you use case (storing testrun logs) is covered nicely by allowing tree objects as notes, and I think that's where we should start. The note tree is itself a tree object, and storing sub-trees of that is not new or unusual to Git at all. Reachability is nicely covered by how Git already handles sub-trees. Obviously we must flesh out how the notes-related parts of the code deal with trees (see below), but that does not really affect the rest of Git, and should therefore be relatively uncontroversial. If we go on to _commit_ objects, they are currently only referenced from tree objects as "gitlink"s (with a special "160000" mode). If you were to put one of these in a notes tree, you would get the same semantics as a "gitlink", i.e. git handles that part of the tree as a submodule where a different submodule repo is (to be) checked out. The commit is NOT considered/required to be reachable, and would therefore not be automatically communicated by a fetch or push. So if you want commits in a notes tree to be handled differently from commits-as-gitlinks, you would have to tweak all the code in Git that deal with gitlinks. You would have to introduce a differentiation between your "commits-as-gitlinks" and "commits-as-notes", either by reserving another special mode number, or by otherwise making the rest of Git notes-aware. All of this comes in addition to teaching the notes-related code how to deal with commits (i.e. how to display them, etc.). In other words, before you embark on this, you need a convincing argument for why allowing commits-as-notes is really necessary and worth it in the end. Please also consider that you _can_ support commits-as-notes by the mechanism I suggested in the previous thread: Store the commit SHA1 in a note-as-blob, and then amend the notes commit to include the commit SHA1 as an additional parent. It's not very elegant, but it solves the reachability problem. If we go even further and want to allow ANY git object as a note, then we must also consider tag objects, which AFAIK has never before been stored inside a tree. Here we are really entering uncharted territory... So for now (and in lieu of a convincing use case for notes-as-commits), I suggest you only look at notes-as-trees. The first consequence of this is probably that your added -o/--object option should be renamed. -t/--tree is not taken, AFAICS...
Johan Herland wrote:quoted
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.Currently, a non-blob commit is just not displayed at all. And rather than displaying the raw object, we have a number of options available, starting with object's sha1, to more elaborate presentations depending on the type of object (commit info, tree hierarchy, etc, as "git notes show" already does). This PoC shows that it can be dealt with later.
I'm only considering the notes-as-tree case here... I assume that if you organize your notes in tree objects, then you probably have more information in there than is useful to display in the textual output from "git log". Also, you probably have special-purpose scripts for initially generating those trees, and later digging into the information stored therein. Hence we should concentrate on getting the basics covered, to allow those scripts to do their thing, and adding bells and whistles to "git log" for displaying notes-as-trees is much less important. For now, "git log" should probably show a short summary when encountering a notes-as-tree. Whether that summary consists of merely the tree SHA1, or in providing a (relatively short) tree listing, I leave up to you. I also agree that this can be dealt with later (as long as the default behaviour is not actively harmful/confusing).
What I envision, would be viewers like gitk simply show the hyperlinked sha1, and (in the case of a tree note) open a tree view that allows to browse the contents of the note.
Agreed, although not important in v1.
My (necessarily incomplete) list for what's missing: * more audit of impact (eg. seen "git fetch" complain about a non-blob note on HEAD, have to check why; "git fetch" refuses to fetch a non-blob note, pack-objects considering it is a corruption) * support in "notes append/merge", safeguards for commands that won't know of to deal with non-blobs, tests
For trees, notes append/edit/merge sort of makes sense, in that is should be possible to edit a notes-as-tree object, and merge it with a different notes-as-tree object. Editing a notes-as-tree probably involved checking out that tree into a temporary directory, and then recording the new tree after the user have mucked about in there. This is somewhat similar to how "git notes merge" works today, and these operations probably will need to expand/reuse that code.
* let --show-note display the sha1 (make that optional ?)
Displaying the SHA1 by default is good enough for v1. More options can be added later.
* extend gitk (and gitweb ?) as described above
Not important for v1. I guess most users of notes-as-trees will have some domain-specific structure/convention imposed on their notes-as-trees, which probably makes general notes-as-trees browsing less useful, and domain-specific tools for processing the information more or less required.
* provide a <rev>^{note} / <rev>^{note:<noteref>} syntax for easy
access to notes contentsYes. This is good, although somewhat orthogonal to the rest of this topic... Hope this helps, ...Johan