# 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`:
```ini
[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 `MetaProject`s (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.