get commit ID from a tree object ID

Hello,

let's say I have made an annotated tag on a certain treeish:

$ git tag -a -m msg tagname HEAD:

Now, I can try to see the content of the tag:

$ git tag -v tagname object 42a1c36553a50ceae2f75ffc4b1446c6c393eae7 type tree tag tagname tagger clime <clime@redhat.com> 1521288727 +0100

msg error: no signature found

Can I use that object ID 42a1c36553a50ceae2f75ffc4b1446c6c393eae7 to get back to a particular commit from which the tag was created? The reason is that I would eventually like to checkout that commit.

Thank you clime

On Sat, Mar 17, 2018 at 01:17:12PM +0100, Michal Novotny wrote:

In general, you can't, and that's because there can be any number of commits referencing that tree. A typical case is a tree object representing a subdirectory of your project which changes rarily, if ever - in this case, each commit which includes the same state of this subdirectory will refer the same tree object.

Another point to consider is that the commit itself only refers to a single tree object -- that one which records the state of the top-level project directory, and it usually refers to other three objects which may, in turn, refer to others and so on - all the way down.

So actually a generic approach to what you need is a full scan of all the commits in the repository with recursive traversing of the hierarchy of trees of each of them (via `git ls-tree`) and looking for the SHA-1 name of the reference tree object. As you can see, this is not going to be fast on repos of realistic size.

On Sat, Mar 17, 2018 at 04:01:28PM +0300, Konstantin Khomoutov wrote:

If you assume that the tree is a root tree (which is by no means certain, but a good guess), it's not _too_ bad to do:

  git rev-list --all --format='%T %H' | grep ^$desired_tree

That's linear in the number of commits, but still takes only about 7 seconds on linux.git.

If you want to dig further, you can use the diff machinery to show which commit introduced a particular tree, like:

  git rev-list --all |
  git diff-tree --stdin --pretty=raw --raw -t -r |
  less +/$desired_tree

That "less" will find the mentioned tree, and then you'll have to manually read the commit. It would be possible to do it mechanically with a short perl script, but I'll leave that as an exercise for the reader.

-Peff

Jeff King <peff@peff.net> writes:

Before Stefan jumps in ;-) I wonder if a recently materialized "find-object" option to the diff family can be used here as a sugar-coated way.

On Sat, Mar 17, 2018 at 10:57 AM Junio C Hamano <gitster@pobox.com> wrote:

I am late to jump in, but testing the 'git log --find-object' seems to have issues with trees named by sha1 here, but the named tree via <commit>:<path> still seems to work.

It seems reasonable to be able to find trees using the find-object flag, though.

Stefan

Hi,

Stefan Beller wrote:

Experimenting a little more, I wondered if "-t" wasn't being passed by default:

  $ git --oneline log -t --find-object=$(git rev-parse HEAD~30:Documentation/technical)
  $

No, that's not it.  Could it have to do with merges?

  $ git log --oneline -m --first-parent --find-object=$(git rev-parse HEAD~30:Documentation/technical)
  df6cfe8c30 Merge branch 'debian-experimental' into google3
  f86d5fd2e4 Merge branch 'debian-experimental' into google3

Yes.

That doesn't explain why <commit>:<path> worked for you, though. :)

Thanks, Jonathan