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Scenario: tracing Git commands

Use Git's tracing to inspect the repository operations git-splice runs, including the commits a push sends. The design principle explains why using the user's installed Git matters.

setup.bash builds a splice at lib/ with one commit to push:

$ build_scenario scenario_git_trace

Set GIT_TRACE to an absolute path so Git records each command in that file:

$ GIT_TRACE="$PWD/../trace" git splice status
On branch main

  ahead 1  lib

The trace includes discovery and config queries as well as writes. Filter it to inspect the rebuild: status creates an upstream-shaped commit to compare, but publishes no ref.

$ sed -n 's/.*trace: built-in: //p' ../trace | grep '^git commit-tree '
git commit-tree --no-gpg-sign 5f5ab219661c5dba1f8904855aaa5867a97f2dc9 -p bde416459fbcc09c9b585f3b65a94cab3f68bfcd

Use a file, not GIT_TRACE=1​

GIT_TRACE=1 traces to stderr, and git-splice silences stderr where a failure is an expected answer, such as asking for a key that isn't set. Those commands are missing from what you see, such as this one, which every command runs:

$ GIT_TRACE=1 git splice status 2>&1 |
>     grep -c -- --show-toplevel
0
[1]

In the file, it's there:

$ grep -c -- --show-toplevel ../trace
1

What a push sends​

A file also keeps the commands apart from the output. To see what a push writes, start with an empty file and look for the commands that write. commit-tree makes the rebuilt commit, push sends it, and update-ref records that the upstream has it now:

$ : >../trace &&
>   GIT_TRACE="$PWD/../trace" git splice push lib
ok   lib: pushed 0fd98a6 to main
$ sed -n 's/.*trace: built-in: //p' ../trace |
>     grep -E '^git (commit-tree|push|update-ref) '
git commit-tree --no-gpg-sign 5f5ab219661c5dba1f8904855aaa5867a97f2dc9 -p bde416459fbcc09c9b585f3b65a94cab3f68bfcd
git push --quiet -- https://git.example.com/lib.git 0fd98a6c01e5ee8559e8c043c4593ed9d8dd6271:refs/heads/main
git update-ref --create-reflog refs/splices/lib/heads/main 0fd98a6c01e5ee8559e8c043c4593ed9d8dd6271

The list has more than git-splice's own commands: Git adds the ones it runs itself, here pack-objects for the push.

What the trace doesn't show​

Three things git-splice does without a git command:

  • init and clone --from-submodule write <path>/.splice to the folder, then git add and git commit it, as you would. clone --from-submodule also removes the submodule's .git file and writes the edited .gitmodules the same way.
  • While it records an upstream's key, a folder .git/splice-keys.lock keeps other git-splice commands from picking the same one.
  • It writes temporary files outside the repository, for git config --file and a temporary GIT_INDEX_FILE.