docs/smart-sync.md

# repo Smart Syncing

Repo normally fetches & syncs manifests from the same URL specified during
`repo init`, and that often fetches the latest revisions of all projects in
the manifest.  This flow works well for tracking and developing with the
latest code, but often it's desirable to sync to other points.  For example,
to get a local build matching a specific release or build to reproduce bugs
reported by other people.

Repo's sync subcommand has support for fetching manifests from a server over
an XML-RPC connection.  The local configuration and network API are defined by
repo, but individual projects have to host their own server for the client to
communicate with.

This process is called "smart syncing" -- instead of blindly fetching the latest
revision of all projects and getting an unknown state to develop against, the
client passes a request to the server and is given a matching manifest that
typically specifies specific commits for every project to fetch a known source
state.

[TOC]

## Manifest Configuration

The manifest specifies the server to communicate with via the
the [`<manifest-server>` element](manifest-format.md#Element-manifest_server)
element.  This is how the client knows what service to talk to.

```xml
  <manifest-server url="https://example.com/your/manifest/server/url"
                   helper="repo-remote-helper-name" />
```

If the URL starts with `persistent-`, then the
[`git-remote-persistent-https` helper](https://github.com/git/git/blob/HEAD/contrib/persistent-https/README)
is used to communicate with the server.

### Pluggable Remote Helpers

For custom proxying or authentication, Repo supports pluggable remote helpers.
You can declare a helper binary via the optional `helper` attribute on the
`<manifest-server>` element.

If the `helper` attribute is present in `<manifest-server>`:
1.  Repo searches your system `PATH` for the specified helper binary (e.g.,
    `repo-remote-sso`). If the helper cannot be found in `PATH`, Repo will raise
    a `SmartSyncError` and abort the sync.
2.  The helper is executed with the manifest server URL as its first argument:
    ```bash
    <helper-binary-name> <url>
    ```
3.  The helper must output a single-line JSON object on stdout and exit:
    *   On success, return `status: "ok"` and a loopback proxy URL (including
        scheme, e.g., `http://127.0.0.1:999`):
        ```json
        {"status": "ok", "message": "http://127.0.0.1:999"}
        ```
    *   On failure, return `status: "error"` and a detailed error message:
        ```json
        {"status": "error", "message": "unauthorized"}
        ```
4.  Repo routes the XML-RPC request through the returned proxy address.

**Timeout Constraint**: The remote helper must complete and exit within 10
seconds. If it hangs or exceeds this limit, Repo will force-terminate (`kill`)
the process and abort the synchronization.

## Credentials

Credentials may be specified directly in typical `username:password`
[URI syntax](https://en.wikipedia.org/wiki/URI#Syntax) in the
`<manifest-server>` element directly in the manifest.

If they are not specified, `repo sync` has `--manifest-server-username=USERNAME`
and `--manifest-server-password=PASSWORD` options.

If those are not used, then repo will look up the host in your
[`~/.netrc`](https://docs.python.org/3/library/netrc.html) database.

When making the connection, cookies matching the host are automatically loaded
from the cookiejar specified in
[Git's `http.cookiefile` setting](https://git-scm.com/docs/git-config#Documentation/git-config.txt-httpcookieFile).

## Manifest Server

Unfortunately, there are no public reference implementations.  Google has an
internal one for Android, but it is written using Google's internal systems,
so wouldn't be that helpful as a reference.

That said, the XML-RPC API is pretty simple, so any standard XML-RPC server
example would do.  Google's internal server uses Python's
[xmlrpc.server.SimpleXMLRPCDispatcher](https://docs.python.org/3/library/xmlrpc.server.html).

## Network API

The manifest server should implement the following RPC methods.

### GetApprovedManifestV2

> `GetApprovedManifestV2(request: dict) -> dict`

Like [GetApprovedManifest](#GetApprovedManifest), but takes a single struct of
named options so new options can be added without changing the method signature.
Unset options are omitted from the struct. Servers should ignore options they do
not understand.

Request options:

*   `branch` (`str`, required): Same as in `GetApprovedManifest`.
*   `target` (`str`, optional): Same as in `GetApprovedManifest`.
*   `best_effort` (`bool`, optional, default `False`): The client can proceed
    without a manifest, so the server may decline the request (e.g. during a
    staged rollout or to shed load). Repo sets this for Smart Syncs enabled by
    the manifest's `sync-smartsync` default, and falls back to a regular ToT
    sync if the request is declined. Servers must not decline requests without
    this option for these reasons.

Response fields:

*   `success` (`bool`): Whether a manifest was returned.
*   `manifest_xml` (`str`): The manifest. Required if `success` is true.
*   `error_message` (`str`): Why the request failed or was declined. Repo shows
    it to the user like `GetApprovedManifest` errors.

If the server does not implement this method, repo calls `GetApprovedManifest`
instead and drops the options it cannot express.

### GetApprovedManifest

> `GetApprovedManifest(branch: str, target: Optional[str]) -> str`

The meaning of `branch` and `target` is not strictly defined.  The server may
interpret them however it wants.  The recommended interpretation is that the
`branch` matches the manifest branch, and `target` is an identifier for your
project that matches something users would build.

See the client section below for how repo typically generates these values.

The server will return a manifest or an error.  If it's an error, repo will
show the output directly to the user to provide a limited feedback channel.

If the user's request is ambiguous and could match multiple manifests, the
server has to decide whether to pick one automatically (and silently such that
the user won't know there were multiple matches), or return an error and force
the user to be more specific.

### GetManifest

> `GetManifest(tag: str) -> str`

The meaning of `tag` is not strictly defined.  Projects are encouraged to use
a system where the tag matches a unique source state.

See the client section below for how repo typically generates these values.

The server will return a manifest or an error.  If it's an error, repo will
show the output directly to the user to provide a limited feedback channel.

If the user's request is ambiguous and could match multiple manifests, the
server has to decide whether to pick one automatically (and silently such that
the user won't know there were multiple matches), or return an error and force
the user to be more specific.

## Client Options

Once repo has successfully downloaded the manifest from the server, it saves a
copy into `.repo/manifests/smart_sync_override.xml` so users can examine it.
The next time `repo sync` is run, this file is automatically replaced or removed
based on the current set of options.

With `-l`/`--local-only` and no explicit `-s`/`--smart-sync` or
`-t`/`--smart-tag`, repo does not contact the manifest server. Instead, it keeps
using the existing `smart_sync_override.xml`, if there is one, whether it came
from an explicit `-s`/`--smart-sync` or from the manifest's `sync-smartsync`
default. This way, `repo sync -n` followed by `repo sync -l` ends up on the same
revisions as the equivalent single `repo sync` (e.g. `repo sync -s -n` +
`repo sync -l` matches `repo sync -s`). Pass `--no-smart-sync` to discard it and
use the default manifest instead.

`--no-manifest-update` only skips updating the manifest checkout. It does not
affect Smart Sync: the default Smart Sync from `sync-smartsync` (see below)
still runs, and an existing `smart_sync_override.xml` is replaced or removed as
in a regular sync.

### sync-smartsync (Manifest Attribute)

The manifest can enable Smart Sync by default for all `repo sync` invocations
by setting `sync-smartsync="true"` on the `<default>` element:

```xml
  <default sync-smartsync="true" ... />
```

When enabled in the manifest, `repo sync` will automatically perform a Smart
Sync unless explicitly overridden on the command line. Since the user did not
ask for a Smart Sync, any failure to obtain a manifest from the manifest server
(e.g. the server is unreachable, or it returns a manifest that cannot be parsed)
is silently ignored and `repo sync` falls back to a regular ToT sync; pass
`--verbose` to see the reason. Repo marks these requests as `best_effort` (see
[GetApprovedManifestV2](#GetApprovedManifestV2)), so the server may also decline
them, e.g. during a staged rollout. This is only a fallback for the
manifest-driven default: an explicit `-s`/`--smart-sync` or `-t`/`--smart-tag`
still fails the sync, as before. Options that specify an explicit target or
manifest source (such as `-t`/`--smart-tag`, `-m`/`--manifest-name` or
`--superproject-revision`) will also disable the default Smart Sync behavior.

### --smart-sync / -s

Explicitly enables Smart Sync. Repo will call
`GetApprovedManifestV2({branch[, target]})`, or
`GetApprovedManifest(branch[, target])` if the server does not implement it.

The `branch` is determined by the current manifest branch as specified by
`--manifest-branch=BRANCH` when running `repo init`.  If `${SYNC_BRANCH}` is
defined and non-empty, its value is sent instead.  This allows selecting a
branch other than the one the checkout tracks, e.g. a server-side (CI) branch
name that builds from the same manifest branch.

The `target` is defined by environment variables in the order below.  If none
of them match, then `target` is omitted.  These variables were decided as they
match the settings Android build environments automatically setup.

1.  `${SYNC_TARGET}`: If defined, the value is used directly.
2.  `${TARGET_PRODUCT}-${TARGET_RELEASE}-${TARGET_BUILD_VARIANT}`: If these
    variables are all defined, then they are merged with `-` and used.
3.  `${TARGET_PRODUCT}-${TARGET_BUILD_VARIANT}`: If these variables are all
    defined, then they are merged with `-` and used.

### --no-smart-sync

Explicitly disables Smart Sync, overriding any `sync-smartsync="true"` setting
declared in the manifest `<default>` element.

### --smart-tag=TAG / -t

Repo will call `GetManifest(TAG)`.