From: Maciej Ciemborowicz <maciej.ciemborowicz@gmail.com>
Date: Sat, 19 Sep 2026 13:34:15 GMT
Subject: [BUG] reference-transaction reports zero OIDs for branch and tag deletion
Message-ID: <CACQ=SRGTTdQ+dHXhN6F52dBv5KxZBRfk_Em2fvmEmGJDoB6oTg@mail.gmail.com>

Since Git 2.31, the reference-transaction hook receives all-zero old and
new object IDs when a branch or tag is deleted with a high-level command.

For example, `git branch -d topic` produces:

    0000000000000000000000000000000000000000 \
    0000000000000000000000000000000000000000 \
    refs/heads/topic

The same happens with `git branch -D`, `git tag -d`, and deletion of a
remote-tracking ref by `git remote prune`.

Git 2.28 through 2.30 reported the previous object ID followed by the
all-zero object ID. Starting with Git 2.31, that information is lost.
The behavior is still present in Git 2.55 with both the files and
reftable backends.

A direct deletion with:

    git update-ref -d refs/heads/topic "$old_oid"

continues to report the useful payload:

    <old-oid> 0000000000000000000000000000000000000000 refs/heads/topic

Minimal reproducer:

    #!/bin/sh
    set -eu

    root=$(mktemp -d)
    trap 'rm -rf "$root"' EXIT

    repo=$root/repo
    hooks=$root/hooks
    log=$root/transactions

    git init -q "$repo"
    git -C "$repo" config user.name Reproducer
    git -C "$repo" config user.email repro@example.com
    git -C "$repo" commit --allow-empty -qm initial

    git -C "$repo" branch topic
    git -C "$repo" tag v1

    mkdir "$hooks"
    cat >"$hooks/reference-transaction" <<'HOOK'
    #!/bin/sh
    printf '%s\n' "--- $1" >>"$HOOK_LOG"
    cat >>"$HOOK_LOG"
    HOOK
    chmod +x "$hooks/reference-transaction"

    git -C "$repo" config core.hooksPath "$hooks"
    export HOOK_LOG=$log
    : >"$log"

    git -C "$repo" branch -d topic
    git -C "$repo" tag -d v1

    cat "$log"

Actual output for the committed transactions is equivalent to:

    0000000000000000000000000000000000000000 \
    0000000000000000000000000000000000000000 \
    refs/heads/topic
    0000000000000000000000000000000000000000 \
    0000000000000000000000000000000000000000 \
    refs/tags/v1

I expected:

    <old-oid> 0000000000000000000000000000000000000000 refs/heads/topic
    <old-oid> 0000000000000000000000000000000000000000 refs/tags/v1

I understand that the reference-transaction documentation permits an
all-zero old value when a ref is force-updated without checking its
current value. However, `git branch -d` is a safety-checked deletion,
Git has already resolved the branch being deleted, and Git 2.28–2.30
provided its old object ID.

Was this loss of information intentional? If not, could the previous
object ID be restored for these deletion paths? If it is intentional,
the documentation may need to clarify that high-level deletion commands
can provide a zero-to-zero record.

The behavior was tested across Git 2.28–2.55. The compatibility results
and test implementation are available here:

https://github.com/ciembor/git-hooks-ext/blob/v0.2.0/tests/compat/README.md
https://github.com/ciembor/git-hooks-ext/blob/v0.2.0/tests/compat/e2e.sh

Thanks,
Maciej Ciemborowicz