sync: Support manifest-driven default smart sync

Add support for enabling smart sync by default via the
sync-smartsync="true" attribute on the manifest <default> element.

When sync-smartsync is set to true in the manifest defaults:
- Plain `repo sync` triggers Smart Sync automatically.
- If that implicit Smart Sync fails (manifest server unreachable, or
  the returned manifest cannot be parsed), repo silently falls back to
  a regular ToT sync. The reason is logged with --verbose.
- `--no-smart-sync` CLI flag overrides the manifest default and
  syncs to ToT.
- Passing explicit `-s` / `--smart-sync` forces Smart Sync as before,
  and its failures are still fatal.
- `-t`, `--smart-tag`, `-m`, `--manifest-name`, and
  `--superproject-revision` disable the default smart sync.
- `-l` / `--local-only` and `--no-manifest-update` skip the manifest
  server and reuse the existing smart_sync_override.xml, if any.

Update docs, completion, regenerate man pages, and add tests for XML
parsing, serialization, CLI option resolution, and the fallback and
reuse paths.

Bug: 545137090
Change-Id: If75569c58dd525d170e9801e70ccbf02206a52f2
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/617641
Tested-by: Rahul Yadav <yadavrah@google.com>
Commit-Queue: Rahul Yadav <yadavrah@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

subcmds/sync.py

@@ -58,6 +58,7 @@ from command import DEFAULT_LOCAL_JOBS
 from command import MirrorSafeCommand
 from command import WORKER_BATCH_SIZE
 from error import GitError
+from error import ManifestParseError
 from error import RepoChangedException
 from error import RepoError
 from error import RepoExitError
@@ -764,9 +765,16 @@ later is required to fix a server side protocol bug.
                 "-s",
                 "--smart-sync",
                 action="store_true",
+                default=None,
                 help="smart sync using manifest from the latest known good "
                 "build",
             )
+            p.add_option(
+                "--no-smart-sync",
+                dest="smart_sync",
+                action="store_false",
+                help="disable smart sync and sync to ToT instead",
+            )
             p.add_option(
                 "-t",
                 "--smart-tag",
@@ -2249,10 +2257,6 @@ later is required to fix a server side protocol bug.
         if opt.manifest_name and opt.smart_tag:
             self.OptionParser.error("cannot combine -m and -t")
         if opt.manifest_server_username or opt.manifest_server_password:
-            if not (opt.smart_sync or opt.smart_tag):
-                self.OptionParser.error(
-                    "-u and -p may only be combined with -s or -t"
-                )
             if None in [
                 opt.manifest_server_username,
                 opt.manifest_server_password,
@@ -2413,6 +2417,34 @@ later is required to fix a server side protocol bug.
                 "failed to sync manifest project", aggregate_errors=[e]
             )
 
+    def _ResolveSmartSyncOption(
+        self, opt: optparse.Values, manifest: XmlManifest
+    ) -> bool:
+        """Resolve opt.smart_sync from the CLI flags and manifest default.
+
+        Returns:
+            True if smart_sync was enabled implicitly by the manifest default,
+            False otherwise.
+        """
+        implicit = (
+            opt.smart_sync is None
+            and not opt.smart_tag
+            and not opt.manifest_name
+            and not opt.superproject_revision
+            and not opt.local_only
+            and opt.mp_update
+            and getattr(manifest.default, "sync_smartsync", False) is True
+        )
+        if opt.smart_sync is None:
+            opt.smart_sync = implicit
+
+        if (
+            opt.manifest_server_username or opt.manifest_server_password
+        ) and not (opt.smart_sync or opt.smart_tag):
+            raise SmartSyncError("-u and -p may only be combined with -s or -t")
+
+        return implicit
+
     def _ExecuteHelper(self, opt, args, errors):
         manifest = self.outer_manifest
         if not opt.outer_manifest:
@@ -2427,11 +2459,58 @@ later is required to fix a server side protocol bug.
         if opt.clone_bundle is None:
             opt.clone_bundle = manifest.CloneBundle
 
+        # An onboarded client run with -l or --no-manifest-update keeps the
+        # smart sync manifest it last synced to instead of contacting the
+        # manifest server. Computed before _ResolveSmartSyncOption, which
+        # makes an unset opt.smart_sync indistinguishable from --no-smart-sync.
+        reuse_smart_sync_override = (
+            (opt.local_only or not opt.mp_update)
+            and opt.smart_sync is None
+            and not opt.smart_tag
+            and not opt.manifest_name
+            and not opt.superproject_revision
+            and getattr(manifest.default, "sync_smartsync", False) is True
+            and os.path.isfile(smart_sync_manifest_path)
+        )
+
+        implicit_smart_sync = self._ResolveSmartSyncOption(opt, manifest)
+
         if opt.smart_sync or opt.smart_tag:
-            manifest_name = self._SmartSyncSetup(
-                opt, smart_sync_manifest_path, manifest
-            )
-        else:
+            try:
+                manifest_name = self._SmartSyncSetup(
+                    opt, smart_sync_manifest_path, manifest
+                )
+            except (SmartSyncError, ManifestParseError) as e:
+                if not implicit_smart_sync:
+                    raise
+                # The user did not ask for a smart sync, so failing to get a
+                # usable manifest from the server should not fail the sync.
+                # Fall back to a regular ToT sync instead.
+                opt.smart_sync = False
+                # Override() records the override before parsing it, so an
+                # unparsable manifest would otherwise leave the client pinned
+                # to a file it cannot load.
+                manifest.ClearOverride()
+                if opt.verbose:
+                    logger.warning(
+                        "warning: smart sync failed; falling back to ToT: %s", e
+                    )
+        elif reuse_smart_sync_override:
+            manifest_name = os.path.basename(smart_sync_manifest_path)
+            try:
+                self._ReloadManifest(manifest_name, manifest)
+            except ManifestParseError as e:
+                reuse_smart_sync_override = False
+                manifest_name = None
+                manifest.ClearOverride()
+                if opt.verbose:
+                    logger.warning(
+                        "warning: failed to load existing smart sync manifest; "
+                        "falling back to ToT: %s",
+                        e,
+                    )
+
+        if not (opt.smart_sync or opt.smart_tag or reuse_smart_sync_override):
             if os.path.isfile(smart_sync_manifest_path):
                 try:
                     platform_utils.remove(smart_sync_manifest_path)