git-repo/docs/fetch-cmd.md

Fetch Command Contract

The repo.fetchcmd configuration allows specifying a custom command to be executed during repo sync to fetch objects, instead of using standard git fetch. This is particularly useful in environments with virtualized filesystems or lazy checkouts where fetching metadata and downloading file contents should be decoupled.

The checkout half of a sync has a counterpart, repo.reprojectcmd; see docs/reproject-cmd.md.

Configuration

To use this feature, set the following in .repo/manifests.git/config:

[repo]
	fetchcmd = "your custom command here"
	uselocalgitdirs = true

Setting repo.fetchcmd requires repo.uselocalgitdirs to be set to true.

Environment Variables

The custom command is executed in a subshell populated with standard project-context environment variables. For details on standard variables (such as REPO_PROJECT, REPO_PATH, REPO_PROJECT_FETCH_URL, etc.), see the Environment section in repo help forall or subcmds/forall.py.

The following environment variable is specific to repo.fetchcmd:

• REPO_TREV: The target commit hash when the revision is already pinned to a commit (e.g. via a superproject or a commit SHA in the manifest), or empty when REPO_RREV is an unpinned branch or tag to be resolved by the fetch command.

Contract

Postconditions on exit 0

After the fetch command exits with status 0, repo expects the following postconditions to be met:

1. When the manifest revision names a branch or tag, the mapped local tracking ref (e.g. refs/remotes/REPO_REMOTE/<branch> for a branch, or refs/tags/<tag> for a tag) must resolve to a valid commit in the object store (git rev-parse --verify <ref>^{commit}), and must equal REPO_TREV if REPO_TREV was non-empty.
2. FETCH_HEAD must resolve to the same target commit in the object store (git rev-parse --verify FETCH_HEAD^{commit}).
3. The commit graph from the target commit must be reachable far enough to compute merge bases with local branches.

Invariants

• The command should be idempotent; repeating the same fetch should be a no-op.
• Only FETCH_HEAD, refs/remotes/*, and refs/tags/* should be modified to preserve repo sync --network-only semantics. HEAD and local branches must not be touched by the fetch command.
• Dirty worktree state must be preserved.
• The command is not executed for MetaProjects (i.e. the internal repo repository itself at .repo/repo and the manifests repository at .repo/manifests).

Failure

• A non-zero exit status aborts the project's sync, and the command's stderr is surfaced to the user.
• repo verifies the tracking ref and target reachability after exit 0. Any mismatch is treated as a failure.