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sync: Implement fetchcmd for standard Git layouts

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Introduce support for `repo.fetchcmd` configuration, allowing users to
specify a custom command to fetch project data instead of
`_RemoteFetch`.

When `repo.fetchcmd` is specified, `repo` will execute it instead of
`_RemoteFetch` during the network half of sync. Note that
`repo.fetchcmd` requires `repo.uselocalgitdirs` to be enabled.

The command is executed in a subshell with project-context environment
variables, including the new `REPO_TREV` (target revision resolved to a
commit hash) and `REPO_PROJECT_FETCH_URL`.

After execution, `repo` verifies that the target commit is available and
that the tracking ref and FETCH_HEAD are correctly updated.

Tested with:
```
> ~/git-repo/repo init -u https://android-review.googlesource.com/platform/manifest \
    --repo-url file:///usr/local/google/home/gavinmak/git-repo \
    --groups developers \
    --no-repo-verify \
    --use-local-gitdirs
...

> git config --file .repo/manifests.git/config repo.fetchcmd 'mkdir -p $REPO_PATH && cd $REPO_PATH && if [ ! -d .git ]; then git init && git remote add aosp $REPO_PROJECT_FETCH_URL; fi && git fetch aosp $REPO_TREV && git reset --hard $REPO_TREV && mkdir -p .git/refs/remotes/aosp && echo $REPO_TREV > .git/refs/remotes/aosp/main && echo $REPO_TREV > .git/FETCH_HEAD'

> ~/git-repo/repo sync -j32
warning: repo is not tracking a remote branch, so it will not receive updates; run `repo init --repo-rev=stable` to fix.
You are currently enrolled in Git submodules experiment (go/android-submodules-quickstart).  Use --no-use-superproject to override.

Syncing: 100% (4/4), done in 1m15.686s
Finalizing sync state...
repo sync has finished successfully.
```

Bug: 513329573
Change-Id: I754d3f3c78e86fdeee1a72115297a75b571bc497
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/583883
Reviewed-by: Becky Siegel <beckysiegel@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>
This commit is contained in:
Gavin Mak
2026-06-15 23:23:04 +00:00
committed by gerrit-scoped@luci-project-accounts.iam.gserviceaccount.com
parent e7cac4bca6
commit ead4b2d7aa
3 changed files with 311 additions and 31 deletions
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# 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.
## 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 revision resolved to a full commit hash.
## Contract
### Postconditions on exit 0
After the fetch command exits with status 0, `repo` expects the following
postconditions to be met:
1. `git cat-file -e REPO_TREV` succeeds (the commit must exist in the object
store).
2. The mapped local tracking ref (e.g. `refs/remotes/REPO_REMOTE/<branch>`
for a branch revision, or the tag ref itself for a tag) must point to
`REPO_TREV`.
3. `FETCH_HEAD` must point to `REPO_TREV`.
4. The commit graph from `REPO_TREV` must be reachable far enough to compute
merge bases with local branches.
### Invariants
* The command should be idempotent; fetching the same `REPO_TREV` twice should
be a no-op.
* Only `FETCH_HEAD` and `refs/remotes/*` 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.