Verifying the sync status of source-git and dist-git repos

A source-git repo is in sync with the corresponding dist-git repo, if an attempt to sync the former to the later (by running packit source-git update-dist-git) does not produce any change in the dist-git repo.

Being out-of-sync can mean:

• There are commits in source-git which were not yet transferred to dist-git.
• There are commits in dist-git, which were not created as an update from source-git, and need to be synced back to it.
• Both of the above (source-git and dist-git history has diverged).

From an operational point of view, dist-git changes will need to be synced back to source-git, before accepting content to be transformed from source-git to dist-git, in order to be able to preserve dist-git as the "blessed" place to store the sources. In other words: all new changes to be synced from source-git to dist-git should be based on a commit matching the current state in dist-git.

In order to help with this, both update commands (packit source-git update-dist-git and packit source-git update-source-git) should refuse to update the destination repository if the head commit in the destination repository doesn't have it's pair commit among the ancestors of the current head commit in the source repository. Nevertheless, it should be possible to override this with a --force flag, to allow developers to override tooling or human errors and bring the repositories back in sync.

CLI proposals

The naïve way to check if the two repositories are in sync is to run packit source-git update-dist-git. But this goes over the entire process to transform content from source-git to dist-git, which might not be practical when looking just for the information whether the repos are in sync, and if they are not, to tell the (range of) commits to be synced.

Note, that the list of commits (or rather the range of commits) to be synced is relevant only when syncing from dist-git to source-git. Transformation in the other direction always considers the commits since upstream_ref, so in this case the range of outstanding commits would only serve as an information for the user.

The CLI for the sync-check can be implemented as a new sub-command for packit source-git:

    $ packit source-git status src/acl rpms/acl

The command would have the following outputs:

• Print the range of commits (as a Git-revision, i.e: HEAD~<n>..) in source-git which need to be synced to dist-git, if any.
• Print the range of commits (as a Git-revision, i.e: HEAD~<n>..) in dist-git which need to be synced to source-git, if any.
• Print a message that the repositories are in sync if that's the case.

Marking the origin of synced content

In order to be able to tell whether a commit in the dist-git or source-git history has originated from the corresponding sibling repository the full hash of the source commit should be recorded. This can be done with some dedicated Git-trailers, appended at the end of the commit messages.

Commits created by packit source-git update-dist-git in dist-git repositories should have:

    From-source-git-commit: <head-commit-hash>

Commits created by packit source-git update-source-git in source-git repositories, and the commit at the tip of the branch, created by packit source-git init should have:

    From-dist-git-commit: <head-commit-hash>

Identifying commits to sync

Search the commits of the current branch in each repository and look for the last (latest) commit which has the Git-trailers above.

$ git log -1 --grep='^From-source-git-commit: .\+$'
$ git log -1 --grep='^From-dist-git-commit: .\+$'

Identify the pair-commits from the other repository.

Tell which pair is the latest. The commits to be synced are the ones newer than the latest pair.

Example

Given the following history:

Source-git                    Dist-git

   E *
     |
   D *                           * 4
     |                           |
   C * (From: 3)  <------------  * 3
     |                           |
   B * ------------>  (From: B)  * 2
     |                           |
   A * (From: 1)  <------------- * 1

Commit C in source-git and commit 2 in dist-git would be found as the ones having the Git-trailers.

The commit pairs would be: B-2 and C-3.

The newest pair is C-3, so HEAD~2.. needs to be synced from source-git to dist-git and HEAD~1.. needs to be synced from dist-git to source-git. (In other words: the history of source-git and dist-git has diverged.)

Proposed tasks

1. Update source-git commands to mark the commit origin with Git-trailers
2. Modify source-git update-* commands to check the sync-status
3. Implement checking the sync-status