How to efficiently find the newest shared commit of a branch while running server pre-receive hook?

Question

While the server pre-receive hook script is executing, suppose it receive a new branch called baz. Branch foo, qqaz and bar are existing branch. How to efficiently find the newest shared commit of baz(It's commit 3 in the example below. not commit 2 and 1)?

---0---1                   foo
        \
         \   8---9---10    qqaz
          \ /
           2---3---4       bar
                \
                 5
                  \
                   6---7   baz

Answer 1

A bit complex, but should give the correct result:

git rev-list baz | grep -Ff <(git rev-list foo qqax bar) | head -1

If your shell does not understand the <(...) operator, you can use a temp file:

git rev-list foo qqax bar > /tmp/commit-list.txt
git rev-list baz | grep -Ff /tmp/commit-list.txt | head -1

---

Another way can be to successively take the merge-base of baz and the target branches, and keep the latest of those commits.

Answer 2

To do this correctly in all cases is anywhere from "very tricky" to "impossible", because git push can receive requests to change, add, and/or multiple branch names all in one go. That is, just because your receiving Git has branches named foo , qqaz , and bar identifying commits 1, 10, and 4 respectively now does not mean that this will be the case when the push finishes: it might contain a request to set foo to commit 0, to add a new commit 11 and set qqaz to point to that commit, to add several new commits past 4 and update bar , and so on.

If, however, you limit a git push to a single branch-name update—which you can do if you control the various hooks—the problem becomes considerably more tractable. The trick is to recognize that the pre-receive hook itself runs before any changes to any branch names are made . If baz is a new branch name , then there is no name yet, in all of the branch names in the repository, for commit 7. (There might already be a tag name for any of these commits, or a tag name creation or update in the list of references to be updated by this push: be sure to consider these when designing your algorithm.)

If we make use of that, then:

• the push request has a line of the form 000...000 <hash-of-7> refs/heads/baz in it, indicating that the branch name baz will be added, if this push is accepted;
• a git rev-list <hash-of-7> --not --branches --topo-order will list commits 7, 6, and 5, in that order;
• the parent of 5 is 3.

Should there be any merge commits in the new commit sequence, things get more complicated:

---0---1                   foo
        \
         \   8---9---10    qqaz
          \ /
           2---3---4       bar
            \   \
             ----5
                  \
                   6---7   baz

Here commit 5 is a merge, combining commits 2 and 3. I'm not sure which commit(s) you want to find in this case.

In general, when you wish to examine the commit graph, git rev-list is the Git tool for doing so. The --branches option tells it to use all branch names; --tags tells it to use all tag names; --all tells it to use all references. A --not in the options causes subsequent references to be "negative references", as if you had listed, say, ^refs/heads/dev to exclude branch dev . ¹ You can list particular patterns to include or exclude as well. See the git rev-list documentation for details.

---

¹ A later --not cancels an earlier one, so that:

git rev-list HEAD --not br1 br2 br3 --not br4

means the same thing as:

git rev-list HEAD ^br1 ^br2 ^br3 br4

and in general the order of positive and negative references does not matter.