sync: Share final error handling logic between sync modes

Dedupe error reporting logic for phased and interleaved sync modes by
extracting it into _ReportErrors.

Error reporting will now distinguish between network and local failures
and lists the specific repos that failed in each phase.

Bug: 421935613
Change-Id: I4604a83943dbbd71d979158d7a1c4b8c243347d2
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/484541
Tested-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Scott Lee <ddoman@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

git_superproject.py

@@ -147,7 +147,8 @@ class Superproject:
                 "git rev-parse call failed, command: git {}, "
                 "return code: {}, stderr: {}",
                 cmd,
-                p.stdwerr,
+                retval,
+                p.stderr,
             )
             return None
         return p.stdout
@@ -173,9 +174,11 @@ class Superproject:
         Then the repo_id would be:
             android/platform/superproject
         """
-        if review_url := self.remote.review:
+        review_url = self.remote.review
+        if review_url:
             parsed_url = urllib.parse.urlparse(review_url)
-            if netloc := parsed_url.netloc:
+            netloc = parsed_url.netloc
+            if netloc:
                 parts = netloc.split("-review", 1)
                 host = parts[0]
                 rev = GitRefs(self._work_git).get("HEAD")

hooks.py

@@ -22,6 +22,12 @@ from error import HookError
 from git_refs import HEAD
 
 
+# The API we've documented to hook authors.  Keep in sync with repo-hooks.md.
+_API_ARGS = {
+    "pre-upload": {"project_list", "worktree_list"},
+}
+
+
 class RepoHook:
     """A RepoHook contains information about a script to run as a hook.
 
@@ -56,6 +62,7 @@ class RepoHook:
         hooks_project,
         repo_topdir,
         manifest_url,
+        bug_url=None,
         bypass_hooks=False,
         allow_all_hooks=False,
         ignore_hooks=False,
@@ -75,6 +82,7 @@ class RepoHook:
                 run with CWD as this directory.
                 If you have a manifest, this is manifest.topdir.
             manifest_url: The URL to the manifest git repo.
+            bug_url: The URL to report issues.
             bypass_hooks: If True, then 'Do not run the hook'.
             allow_all_hooks: If True, then 'Run the hook without prompting'.
             ignore_hooks: If True, then 'Do not abort action if hooks fail'.
@@ -85,6 +93,7 @@ class RepoHook:
         self._hooks_project = hooks_project
         self._repo_topdir = repo_topdir
         self._manifest_url = manifest_url
+        self._bug_url = bug_url
         self._bypass_hooks = bypass_hooks
         self._allow_all_hooks = allow_all_hooks
         self._ignore_hooks = ignore_hooks
@@ -414,6 +423,20 @@ class RepoHook:
                 ignore the result through the option combinations as listed in
                 AddHookOptionGroup().
         """
+        # Make sure our own callers use the documented API.
+        exp_kwargs = _API_ARGS.get(self._hook_type, set())
+        got_kwargs = set(kwargs.keys())
+        if exp_kwargs != got_kwargs:
+            print(
+                "repo internal error: "
+                f"hook '{self._hook_type}' called incorrectly\n"
+                f"  got:      {sorted(got_kwargs)}\n"
+                f"  expected: {sorted(exp_kwargs)}\n"
+                f"Please file a bug: {self._bug_url}",
+                file=sys.stderr,
+            )
+            return False
+
         # Do not do anything in case bypass_hooks is set, or
         # no-op if there is no hooks project or if hook is disabled.
         if (
@@ -472,6 +495,7 @@ class RepoHook:
                 "manifest_url": manifest.manifestProject.GetRemote(
                     "origin"
                 ).url,
+                "bug_url": manifest.contactinfo.bugurl,
             }
         )
         return cls(*args, **kwargs)

man/repo-smartsync.1

@@ -1,5 +1,5 @@
 .\" DO NOT MODIFY THIS FILE!  It was generated by help2man.
-.TH REPO "1" "September 2024" "repo smartsync" "Repo Manual"
+.TH REPO "1" "June 2025" "repo smartsync" "Repo Manual"
 .SH NAME
 repo \- repo smartsync - manual page for repo smartsync
 .SH SYNOPSIS
@@ -20,11 +20,12 @@ number of CPU cores)
 .TP
 \fB\-\-jobs\-network\fR=\fI\,JOBS\/\fR
 number of network jobs to run in parallel (defaults to
-\fB\-\-jobs\fR or 1)
+\fB\-\-jobs\fR or 1). Ignored when \fB\-\-interleaved\fR is set
 .TP
 \fB\-\-jobs\-checkout\fR=\fI\,JOBS\/\fR
 number of local checkout jobs to run in parallel
-(defaults to \fB\-\-jobs\fR or 8)
+(defaults to \fB\-\-jobs\fR or 8). Ignored when \fB\-\-interleaved\fR
+is set
 .TP
 \fB\-f\fR, \fB\-\-force\-broken\fR
 obsolete option (to be deleted in the future)
@@ -58,6 +59,9 @@ only update working tree, don't fetch
 use the existing manifest checkout as\-is. (do not
 update to the latest revision)
 .TP
+\fB\-\-interleaved\fR
+fetch and checkout projects in parallel (experimental)
+.TP
 \fB\-n\fR, \fB\-\-network\-only\fR
 fetch only, don't update working tree
 .TP

man/repo-sync.1

@@ -1,5 +1,5 @@
 .\" DO NOT MODIFY THIS FILE!  It was generated by help2man.
-.TH REPO "1" "September 2024" "repo sync" "Repo Manual"
+.TH REPO "1" "June 2025" "repo sync" "Repo Manual"
 .SH NAME
 repo \- repo sync - manual page for repo sync
 .SH SYNOPSIS
@@ -20,11 +20,12 @@ number of CPU cores)
 .TP
 \fB\-\-jobs\-network\fR=\fI\,JOBS\/\fR
 number of network jobs to run in parallel (defaults to
-\fB\-\-jobs\fR or 1)
+\fB\-\-jobs\fR or 1). Ignored when \fB\-\-interleaved\fR is set
 .TP
 \fB\-\-jobs\-checkout\fR=\fI\,JOBS\/\fR
 number of local checkout jobs to run in parallel
-(defaults to \fB\-\-jobs\fR or 8)
+(defaults to \fB\-\-jobs\fR or 8). Ignored when \fB\-\-interleaved\fR
+is set
 .TP
 \fB\-f\fR, \fB\-\-force\-broken\fR
 obsolete option (to be deleted in the future)
@@ -58,6 +59,9 @@ only update working tree, don't fetch
 use the existing manifest checkout as\-is. (do not
 update to the latest revision)
 .TP
+\fB\-\-interleaved\fR
+fetch and checkout projects in parallel (experimental)
+.TP
 \fB\-n\fR, \fB\-\-network\-only\fR
 fetch only, don't update working tree
 .TP

progress.py

@@ -195,6 +195,21 @@ class Progress:
                 )
             )
 
+    def display_message(self, msg):
+        """Clears the current progress line and prints a message above it.
+
+        The progress bar is then redrawn on the next line.
+        """
+        if not _TTY or IsTraceToStderr() or self._quiet:
+            return
+
+        # Erase the current line, print the message with a newline,
+        # and then immediately redraw the progress bar on the new line.
+        sys.stderr.write("\r" + CSI_ERASE_LINE)
+        sys.stderr.write(msg + "\n")
+        sys.stderr.flush()
+        self.update(inc=0)
+
     def end(self):
         self._update_event.set()
         if not _TTY or IsTraceToStderr() or self._quiet:

subcmds/sync.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import collections
+import contextlib
 import functools
 import http.cookiejar as cookielib
 import io
@@ -25,7 +26,7 @@ from pathlib import Path
 import sys
 import tempfile
 import time
-from typing import List, NamedTuple, Set, Union
+from typing import List, NamedTuple, Optional, Set, Tuple, Union
 import urllib.error
 import urllib.parse
 import urllib.request
@@ -194,6 +195,57 @@ class _CheckoutOneResult(NamedTuple):
     finish: float
 
 
+class _SyncResult(NamedTuple):
+    """Individual project sync result for interleaved mode.
+
+    Attributes:
+      project_index (int): The index of the project in the shared list.
+      relpath (str): The project's relative path from the repo client top.
+      remote_fetched (bool): True if the remote was actually queried.
+      fetch_success (bool): True if the fetch operation was successful.
+      fetch_error (Optional[Exception]): The Exception from a failed fetch,
+          or None.
+      fetch_start (Optional[float]): The time.time() when fetch started.
+      fetch_finish (Optional[float]): The time.time() when fetch finished.
+      checkout_success (bool): True if the checkout operation was
+          successful.
+      checkout_error (Optional[Exception]): The Exception from a failed
+          checkout, or None.
+      checkout_start (Optional[float]): The time.time() when checkout
+          started.
+      checkout_finish (Optional[float]): The time.time() when checkout
+          finished.
+      stderr_text (str): The combined output from both fetch and checkout.
+    """
+
+    project_index: int
+    relpath: str
+
+    remote_fetched: bool
+    fetch_success: bool
+    fetch_error: Optional[Exception]
+    fetch_start: Optional[float]
+    fetch_finish: Optional[float]
+
+    checkout_success: bool
+    checkout_error: Optional[Exception]
+    checkout_start: Optional[float]
+    checkout_finish: Optional[float]
+
+    stderr_text: str
+
+
+class _InterleavedSyncResult(NamedTuple):
+    """Result of an interleaved sync.
+
+    Attributes:
+      results (List[_SyncResult]): A list of results, one for each project
+          processed. Empty if the worker failed before creating results.
+    """
+
+    results: List[_SyncResult]
+
+
 class SuperprojectError(SyncError):
     """Superproject sync repo."""
 
@@ -359,7 +411,7 @@ later is required to fix a server side protocol bug.
             type=int,
             metavar="JOBS",
             help="number of network jobs to run in parallel (defaults to "
-            "--jobs or 1)",
+            "--jobs or 1). Ignored when --interleaved is set",
         )
         p.add_option(
             "--jobs-checkout",
@@ -367,7 +419,8 @@ later is required to fix a server side protocol bug.
             type=int,
             metavar="JOBS",
             help="number of local checkout jobs to run in parallel (defaults "
-            f"to --jobs or {DEFAULT_LOCAL_JOBS})",
+            f"to --jobs or {DEFAULT_LOCAL_JOBS}). Ignored when --interleaved "
+            "is set",
         )
 
         p.add_option(
@@ -423,6 +476,11 @@ later is required to fix a server side protocol bug.
             help="use the existing manifest checkout as-is. "
             "(do not update to the latest revision)",
         )
+        p.add_option(
+            "--interleaved",
+            action="store_true",
+            help="fetch and checkout projects in parallel (experimental)",
+        )
         p.add_option(
             "-n",
             "--network-only",
@@ -832,15 +890,7 @@ later is required to fix a server side protocol bug.
         )
 
         sync_event = _threading.Event()
-
-        def _MonitorSyncLoop():
-            while True:
-                pm.update(inc=0, msg=self._GetSyncProgressMessage())
-                if sync_event.wait(timeout=1):
-                    return
-
-        sync_progress_thread = _threading.Thread(target=_MonitorSyncLoop)
-        sync_progress_thread.daemon = True
+        sync_progress_thread = self._CreateSyncProgressThread(pm, sync_event)
 
         def _ProcessResults(pool, pm, results_sets):
             ret = True
@@ -939,25 +989,16 @@ later is required to fix a server side protocol bug.
         Returns:
             List of all projects that should be checked out.
         """
-        rp = manifest.repoProject
-
         to_fetch = []
-        now = time.time()
-        if _ONE_DAY_S <= (now - rp.LastFetch):
-            to_fetch.append(rp)
         to_fetch.extend(all_projects)
         to_fetch.sort(key=self._fetch_times.Get, reverse=True)
 
         result = self._Fetch(to_fetch, opt, err_event, ssh_proxy, errors)
         success = result.success
         fetched = result.projects
-
         if not success:
             err_event.set()
 
-        # Call self update, unless requested not to
-        if os.environ.get("REPO_SKIP_SELF_UPDATE", "0") == "0":
-            _PostRepoFetch(rp, opt.repo_verify)
         if opt.network_only:
             # Bail out now; the rest touches the working tree.
             if err_event.is_set():
@@ -1136,6 +1177,16 @@ later is required to fix a server side protocol bug.
         self._local_sync_state.Save()
         return proc_res and not err_results
 
+    def _PrintManifestNotices(self, opt):
+        """Print all manifest notices, but only once."""
+        printed_notices = set()
+        # Print all manifest notices, but only once.
+        # Sort by path_prefix to ensure consistent ordering.
+        for m in sorted(self.ManifestList(opt), key=lambda x: x.path_prefix):
+            if m.notice and m.notice not in printed_notices:
+                print(m.notice)
+                printed_notices.add(m.notice)
+
     @staticmethod
     def _GetPreciousObjectsState(project: Project, opt):
         """Get the preciousObjects state for the project.
@@ -1308,6 +1359,61 @@ later is required to fix a server side protocol bug.
             t.join()
         pm.end()
 
+    def _UpdateRepoProject(self, opt, manifest, errors):
+        """Fetch the repo project and check for updates."""
+        if opt.local_only:
+            return
+
+        rp = manifest.repoProject
+        now = time.time()
+        # If we've fetched in the last day, don't bother fetching again.
+        if (now - rp.LastFetch) < _ONE_DAY_S:
+            return
+
+        with multiprocessing.Manager() as manager:
+            with ssh.ProxyManager(manager) as ssh_proxy:
+                ssh_proxy.sock()
+                start = time.time()
+                buf = TeeStringIO(sys.stdout if opt.verbose else None)
+                sync_result = rp.Sync_NetworkHalf(
+                    quiet=opt.quiet,
+                    verbose=opt.verbose,
+                    output_redir=buf,
+                    current_branch_only=self._GetCurrentBranchOnly(
+                        opt, manifest
+                    ),
+                    force_sync=opt.force_sync,
+                    clone_bundle=opt.clone_bundle,
+                    tags=opt.tags,
+                    archive=manifest.IsArchive,
+                    optimized_fetch=opt.optimized_fetch,
+                    retry_fetches=opt.retry_fetches,
+                    prune=opt.prune,
+                    ssh_proxy=ssh_proxy,
+                    clone_filter=manifest.CloneFilter,
+                    partial_clone_exclude=manifest.PartialCloneExclude,
+                    clone_filter_for_depth=manifest.CloneFilterForDepth,
+                )
+                if sync_result.error:
+                    errors.append(sync_result.error)
+
+                finish = time.time()
+                self.event_log.AddSync(
+                    rp,
+                    event_log.TASK_SYNC_NETWORK,
+                    start,
+                    finish,
+                    sync_result.success,
+                )
+                if not sync_result.success:
+                    logger.error("error: Cannot fetch repo tool %s", rp.name)
+                    return
+
+        # After fetching, check if a new version of repo is available and
+        # restart. This is only done if the user hasn't explicitly disabled it.
+        if os.environ.get("REPO_SKIP_SELF_UPDATE", "0") == "0":
+            _PostRepoFetch(rp, opt.repo_verify)
+
     def _ReloadManifest(self, manifest_name, manifest):
         """Reload the manfiest from the file specified by the |manifest_name|.
 
@@ -1772,8 +1878,6 @@ later is required to fix a server side protocol bug.
                         e,
                     )
 
-        err_event = multiprocessing.Event()
-
         rp = manifest.repoProject
         rp.PreSync()
         cb = rp.CurrentBranch
@@ -1812,6 +1916,9 @@ later is required to fix a server side protocol bug.
         # might be in the manifest.
         self._ValidateOptionsWithManifest(opt, mp)
 
+        # Update the repo project and check for new versions of repo.
+        self._UpdateRepoProject(opt, manifest, errors)
+
         superproject_logging_data = {}
         self._UpdateProjectsRevisionId(
             opt, args, superproject_logging_data, manifest
@@ -1825,10 +1932,6 @@ later is required to fix a server side protocol bug.
             all_manifests=not opt.this_manifest_only,
         )
 
-        err_network_sync = False
-        err_update_projects = False
-        err_update_linkfiles = False
-
         # Log the repo projects by existing and new.
         existing = [x for x in all_projects if x.Exists]
         mp.config.SetString("repo.existingprojectcount", str(len(existing)))
@@ -1838,6 +1941,185 @@ later is required to fix a server side protocol bug.
 
         self._fetch_times = _FetchTimes(manifest)
         self._local_sync_state = LocalSyncState(manifest)
+
+        if opt.interleaved:
+            sync_method = self._SyncInterleaved
+        else:
+            sync_method = self._SyncPhased
+
+        sync_method(
+            opt,
+            args,
+            errors,
+            manifest,
+            mp,
+            all_projects,
+            superproject_logging_data,
+        )
+
+        # Log the previous sync analysis state from the config.
+        self.git_event_log.LogDataConfigEvents(
+            mp.config.GetSyncAnalysisStateData(), "previous_sync_state"
+        )
+
+        # Update and log with the new sync analysis state.
+        mp.config.UpdateSyncAnalysisState(opt, superproject_logging_data)
+        self.git_event_log.LogDataConfigEvents(
+            mp.config.GetSyncAnalysisStateData(), "current_sync_state"
+        )
+
+        self._local_sync_state.PruneRemovedProjects()
+        if self._local_sync_state.IsPartiallySynced():
+            logger.warning(
+                "warning: Partial syncs are not supported. For the best "
+                "experience, sync the entire tree."
+            )
+
+        if not opt.quiet:
+            print("repo sync has finished successfully.")
+
+    def _CreateSyncProgressThread(
+        self, pm: Progress, stop_event: _threading.Event
+    ) -> _threading.Thread:
+        """Creates and returns a daemon thread to update a Progress object.
+
+        The returned thread is not yet started. The thread will periodically
+        update the progress bar with information from _GetSyncProgressMessage
+        until the stop_event is set.
+
+        Args:
+            pm: The Progress object to update.
+            stop_event: The threading.Event to signal the monitor to stop.
+
+        Returns:
+            The configured _threading.Thread object.
+        """
+
+        def _monitor_loop():
+            """The target function for the monitor thread."""
+            while True:
+                # Update the progress bar with the current status message.
+                pm.update(inc=0, msg=self._GetSyncProgressMessage())
+                # Wait for 1 second or until the stop_event is set.
+                if stop_event.wait(timeout=1):
+                    return
+
+        return _threading.Thread(target=_monitor_loop, daemon=True)
+
+    def _UpdateManifestLists(
+        self,
+        opt: optparse.Values,
+        err_event: multiprocessing.Event,
+        errors: List[Exception],
+    ) -> Tuple[bool, bool]:
+        """Updates project lists and copy/link files for all manifests.
+
+        Args:
+            opt: Program options from optparse.
+            err_event: An event to set if any error occurs.
+            errors: A list to append any encountered exceptions to.
+
+        Returns:
+            A tuple (err_update_projects, err_update_linkfiles) indicating
+                an error for each task.
+        """
+        err_update_projects = False
+        err_update_linkfiles = False
+        for m in self.ManifestList(opt):
+            if m.IsMirror or m.IsArchive:
+                continue
+
+            try:
+                self.UpdateProjectList(opt, m)
+            except Exception as e:
+                err_event.set()
+                err_update_projects = True
+                errors.append(e)
+                if isinstance(e, DeleteWorktreeError):
+                    errors.extend(e.aggregate_errors)
+                if opt.fail_fast:
+                    logger.error("error: Local checkouts *not* updated.")
+                    raise SyncFailFastError(aggregate_errors=errors)
+
+            try:
+                self.UpdateCopyLinkfileList(m)
+            except Exception as e:
+                err_event.set()
+                err_update_linkfiles = True
+                errors.append(e)
+                if opt.fail_fast:
+                    logger.error(
+                        "error: Local update copyfile or linkfile failed."
+                    )
+                    raise SyncFailFastError(aggregate_errors=errors)
+        return err_update_projects, err_update_linkfiles
+
+    def _ReportErrors(
+        self,
+        errors,
+        err_network_sync=False,
+        failing_network_repos=None,
+        err_checkout=False,
+        failing_checkout_repos=None,
+        err_update_projects=False,
+        err_update_linkfiles=False,
+    ):
+        """Logs detailed error messages and raises a SyncError."""
+
+        def print_and_log(err_msg):
+            self.git_event_log.ErrorEvent(err_msg)
+            logger.error("%s", err_msg)
+
+        print_and_log("error: Unable to fully sync the tree")
+        if err_network_sync:
+            print_and_log("error: Downloading network changes failed.")
+            if failing_network_repos:
+                logger.error(
+                    "Failing repos (network):\n%s",
+                    "\n".join(sorted(failing_network_repos)),
+                )
+        if err_update_projects:
+            print_and_log("error: Updating local project lists failed.")
+        if err_update_linkfiles:
+            print_and_log("error: Updating copyfiles or linkfiles failed.")
+        if err_checkout:
+            print_and_log("error: Checking out local projects failed.")
+            if failing_checkout_repos:
+                logger.error(
+                    "Failing repos (checkout):\n%s",
+                    "\n".join(sorted(failing_checkout_repos)),
+                )
+        logger.error(
+            'Try re-running with "-j1 --fail-fast" to exit at the first error.'
+        )
+        raise SyncError(aggregate_errors=errors)
+
+    def _SyncPhased(
+        self,
+        opt,
+        args,
+        errors,
+        manifest,
+        mp,
+        all_projects,
+        superproject_logging_data,
+    ):
+        """Sync projects by separating network and local operations.
+
+        This method performs sync in two distinct, sequential phases:
+        1.  Network Phase: Fetches updates for all projects from their remotes.
+        2.  Local Phase: Checks out the updated revisions into the local
+            worktrees for all projects.
+
+        This approach ensures that the local work-tree is not modified until
+        all network operations are complete, providing a transactional-like
+        safety net for the checkout state.
+        """
+        err_event = multiprocessing.Event()
+        err_network_sync = False
+        err_update_projects = False
+        err_update_linkfiles = False
+
         if not opt.local_only:
             with multiprocessing.Manager() as manager:
                 with ssh.ProxyManager(manager) as ssh_proxy:
@@ -1870,34 +2152,11 @@ later is required to fix a server side protocol bug.
                     )
                     raise SyncFailFastError(aggregate_errors=errors)
 
-        for m in self.ManifestList(opt):
-            if m.IsMirror or m.IsArchive:
-                # Bail out now, we have no working tree.
-                continue
-
-            try:
-                self.UpdateProjectList(opt, m)
-            except Exception as e:
-                err_event.set()
-                err_update_projects = True
-                errors.append(e)
-                if isinstance(e, DeleteWorktreeError):
-                    errors.extend(e.aggregate_errors)
-                if opt.fail_fast:
-                    logger.error("error: Local checkouts *not* updated.")
-                    raise SyncFailFastError(aggregate_errors=errors)
-
-            try:
-                self.UpdateCopyLinkfileList(m)
-            except Exception as e:
-                err_update_linkfiles = True
-                errors.append(e)
-                err_event.set()
-                if opt.fail_fast:
-                    logger.error(
-                        "error: Local update copyfile or linkfile failed."
-                    )
-                    raise SyncFailFastError(aggregate_errors=errors)
+        err_update_projects, err_update_linkfiles = self._UpdateManifestLists(
+            opt,
+            err_event,
+            errors,
+        )
 
         err_results = []
         # NB: We don't exit here because this is the last step.
@@ -1907,61 +2166,416 @@ later is required to fix a server side protocol bug.
         if err_checkout:
             err_event.set()
 
-        printed_notices = set()
-        # If there's a notice that's supposed to print at the end of the sync,
-        # print it now...  But avoid printing duplicate messages, and preserve
-        # order.
-        for m in sorted(self.ManifestList(opt), key=lambda x: x.path_prefix):
-            if m.notice and m.notice not in printed_notices:
-                print(m.notice)
-                printed_notices.add(m.notice)
+        self._PrintManifestNotices(opt)
 
         # If we saw an error, exit with code 1 so that other scripts can check.
         if err_event.is_set():
-
-            def print_and_log(err_msg):
-                self.git_event_log.ErrorEvent(err_msg)
-                logger.error("%s", err_msg)
-
-            print_and_log("error: Unable to fully sync the tree")
-            if err_network_sync:
-                print_and_log("error: Downloading network changes failed.")
-            if err_update_projects:
-                print_and_log("error: Updating local project lists failed.")
-            if err_update_linkfiles:
-                print_and_log("error: Updating copyfiles or linkfiles failed.")
-            if err_checkout:
-                print_and_log("error: Checking out local projects failed.")
-                if err_results:
-                    # Don't log repositories, as it may contain sensitive info.
-                    logger.error("Failing repos:\n%s", "\n".join(err_results))
-            # Not useful to log.
-            logger.error(
-                'Try re-running with "-j1 --fail-fast" to exit at the first '
-                "error."
-            )
-            raise SyncError(aggregate_errors=errors)
-
-        # Log the previous sync analysis state from the config.
-        self.git_event_log.LogDataConfigEvents(
-            mp.config.GetSyncAnalysisStateData(), "previous_sync_state"
-        )
-
-        # Update and log with the new sync analysis state.
-        mp.config.UpdateSyncAnalysisState(opt, superproject_logging_data)
-        self.git_event_log.LogDataConfigEvents(
-            mp.config.GetSyncAnalysisStateData(), "current_sync_state"
-        )
-
-        self._local_sync_state.PruneRemovedProjects()
-        if self._local_sync_state.IsPartiallySynced():
-            logger.warning(
-                "warning: Partial syncs are not supported. For the best "
-                "experience, sync the entire tree."
+            self._ReportErrors(
+                errors,
+                err_network_sync=err_network_sync,
+                err_checkout=err_checkout,
+                failing_checkout_repos=err_results,
+                err_update_projects=err_update_projects,
+                err_update_linkfiles=err_update_linkfiles,
             )
 
-        if not opt.quiet:
-            print("repo sync has finished successfully.")
+    @classmethod
+    def _SyncOneProject(cls, opt, project_index, project) -> _SyncResult:
+        """Syncs a single project for interleaved sync."""
+        fetch_success = False
+        remote_fetched = False
+        fetch_error = None
+        fetch_start = None
+        fetch_finish = None
+        network_output = ""
+
+        if opt.local_only:
+            fetch_success = True
+        else:
+            fetch_start = time.time()
+            network_output_capture = io.StringIO()
+            try:
+                ssh_proxy = cls.get_parallel_context().get("ssh_proxy")
+                sync_result = project.Sync_NetworkHalf(
+                    quiet=opt.quiet,
+                    verbose=opt.verbose,
+                    output_redir=network_output_capture,
+                    current_branch_only=cls._GetCurrentBranchOnly(
+                        opt, project.manifest
+                    ),
+                    force_sync=opt.force_sync,
+                    clone_bundle=opt.clone_bundle,
+                    tags=opt.tags,
+                    archive=project.manifest.IsArchive,
+                    optimized_fetch=opt.optimized_fetch,
+                    retry_fetches=opt.retry_fetches,
+                    prune=opt.prune,
+                    ssh_proxy=ssh_proxy,
+                    clone_filter=project.manifest.CloneFilter,
+                    partial_clone_exclude=project.manifest.PartialCloneExclude,
+                    clone_filter_for_depth=project.manifest.CloneFilterForDepth,
+                )
+                fetch_success = sync_result.success
+                remote_fetched = sync_result.remote_fetched
+                fetch_error = sync_result.error
+            except KeyboardInterrupt:
+                logger.error(
+                    "Keyboard interrupt while processing %s", project.name
+                )
+            except GitError as e:
+                fetch_error = e
+                logger.error("error.GitError: Cannot fetch %s", e)
+            except Exception as e:
+                fetch_error = e
+                logger.error(
+                    "error: Cannot fetch %s (%s: %s)",
+                    project.name,
+                    type(e).__name__,
+                    e,
+                )
+            finally:
+                fetch_finish = time.time()
+                network_output = network_output_capture.getvalue()
+
+        checkout_success = False
+        checkout_error = None
+        checkout_start = None
+        checkout_finish = None
+        checkout_stderr = ""
+
+        if fetch_success and not opt.network_only:
+            checkout_start = time.time()
+            stderr_capture = io.StringIO()
+            try:
+                with contextlib.redirect_stderr(stderr_capture):
+                    syncbuf = SyncBuffer(
+                        project.manifest.manifestProject.config,
+                        detach_head=opt.detach_head,
+                    )
+                    local_half_errors = []
+                    project.Sync_LocalHalf(
+                        syncbuf,
+                        force_sync=opt.force_sync,
+                        force_checkout=opt.force_checkout,
+                        force_rebase=opt.rebase,
+                        errors=local_half_errors,
+                        verbose=opt.verbose,
+                    )
+                    checkout_success = syncbuf.Finish()
+                    if local_half_errors:
+                        checkout_error = SyncError(
+                            aggregate_errors=local_half_errors
+                        )
+            except KeyboardInterrupt:
+                logger.error(
+                    "Keyboard interrupt while processing %s", project.name
+                )
+            except GitError as e:
+                checkout_error = e
+                logger.error(
+                    "error.GitError: Cannot checkout %s: %s", project.name, e
+                )
+            except Exception as e:
+                checkout_error = e
+                logger.error(
+                    "error: Cannot checkout %s: %s: %s",
+                    project.name,
+                    type(e).__name__,
+                    e,
+                )
+            finally:
+                checkout_finish = time.time()
+                checkout_stderr = stderr_capture.getvalue()
+        elif fetch_success:
+            checkout_success = True
+
+        # Consolidate all captured output.
+        captured_parts = []
+        if network_output:
+            captured_parts.append(network_output)
+        if checkout_stderr:
+            captured_parts.append(checkout_stderr)
+        stderr_text = "\n".join(captured_parts)
+
+        return _SyncResult(
+            project_index=project_index,
+            relpath=project.relpath,
+            fetch_success=fetch_success,
+            remote_fetched=remote_fetched,
+            checkout_success=checkout_success,
+            fetch_error=fetch_error,
+            checkout_error=checkout_error,
+            stderr_text=stderr_text.strip(),
+            fetch_start=fetch_start,
+            fetch_finish=fetch_finish,
+            checkout_start=checkout_start,
+            checkout_finish=checkout_finish,
+        )
+
+    @classmethod
+    def _SyncProjectList(cls, opt, project_indices) -> _InterleavedSyncResult:
+        """Worker for interleaved sync.
+
+        This function is responsible for syncing a group of projects that share
+        a git object directory.
+
+        Args:
+            opt: Program options returned from optparse.  See _Options().
+            project_indices: A list of indices into the projects list stored in
+                the parallel context.
+
+        Returns:
+            An `_InterleavedSyncResult` containing the results for each project.
+        """
+        results = []
+        context = cls.get_parallel_context()
+        projects = context["projects"]
+        sync_dict = context["sync_dict"]
+
+        assert project_indices, "_SyncProjectList called with no indices."
+
+        # Use the first project as the representative for the progress bar.
+        first_project = projects[project_indices[0]]
+        key = f"{first_project.name} @ {first_project.relpath}"
+        sync_dict[key] = time.time()
+
+        try:
+            for idx in project_indices:
+                project = projects[idx]
+                results.append(cls._SyncOneProject(opt, idx, project))
+        finally:
+            del sync_dict[key]
+
+        return _InterleavedSyncResult(results=results)
+
+    def _ProcessSyncInterleavedResults(
+        self,
+        synced_relpaths: Set[str],
+        err_event: _threading.Event,
+        errors: List[Exception],
+        opt: optparse.Values,
+        pool: Optional[multiprocessing.Pool],
+        pm: Progress,
+        results_sets: List[_InterleavedSyncResult],
+    ):
+        """Callback to process results from interleaved sync workers."""
+        ret = True
+        projects = self.get_parallel_context()["projects"]
+        for result_group in results_sets:
+            for result in result_group.results:
+                pm.update()
+                project = projects[result.project_index]
+
+                if opt.verbose and result.stderr_text:
+                    pm.display_message(result.stderr_text)
+
+                if result.fetch_start:
+                    self._fetch_times.Set(
+                        project,
+                        result.fetch_finish - result.fetch_start,
+                    )
+                    self._local_sync_state.SetFetchTime(project)
+                    self.event_log.AddSync(
+                        project,
+                        event_log.TASK_SYNC_NETWORK,
+                        result.fetch_start,
+                        result.fetch_finish,
+                        result.fetch_success,
+                    )
+                if result.checkout_start:
+                    if result.checkout_success:
+                        self._local_sync_state.SetCheckoutTime(project)
+                    self.event_log.AddSync(
+                        project,
+                        event_log.TASK_SYNC_LOCAL,
+                        result.checkout_start,
+                        result.checkout_finish,
+                        result.checkout_success,
+                    )
+
+                if result.fetch_success and result.checkout_success:
+                    synced_relpaths.add(result.relpath)
+                else:
+                    ret = False
+                    err_event.set()
+                    if result.fetch_error:
+                        errors.append(result.fetch_error)
+                        self._interleaved_err_network = True
+                        self._interleaved_err_network_results.append(
+                            result.relpath
+                        )
+                    if result.checkout_error:
+                        errors.append(result.checkout_error)
+                        self._interleaved_err_checkout = True
+                        self._interleaved_err_checkout_results.append(
+                            result.relpath
+                        )
+
+            if not ret and opt.fail_fast:
+                if pool:
+                    pool.close()
+                break
+        return ret
+
+    def _SyncInterleaved(
+        self,
+        opt,
+        args,
+        errors,
+        manifest,
+        mp,
+        all_projects,
+        superproject_logging_data,
+    ):
+        """Sync projects by performing network and local operations in parallel.
+
+        This method processes each project (or groups of projects that share git
+        objects) independently. For each project, it performs the fetch and
+        checkout operations back-to-back. These independent tasks are run in
+        parallel.
+
+        It respects two constraints for correctness:
+        1.  Projects in nested directories (e.g. 'foo' and 'foo/bar') are
+            processed in hierarchical order.
+        2.  Projects that share git objects are processed serially to prevent
+            race conditions.
+        """
+        # Temporary state for tracking errors in interleaved mode.
+        self._interleaved_err_network = False
+        self._interleaved_err_network_results = []
+        self._interleaved_err_checkout = False
+        self._interleaved_err_checkout_results = []
+
+        err_event = multiprocessing.Event()
+        synced_relpaths = set()
+        project_list = list(all_projects)
+        pm = Progress(
+            "Syncing",
+            len(project_list),
+            delay=False,
+            quiet=opt.quiet,
+            show_elapsed=True,
+            elide=True,
+        )
+        previously_pending_relpaths = set()
+
+        sync_event = _threading.Event()
+        sync_progress_thread = self._CreateSyncProgressThread(pm, sync_event)
+
+        with multiprocessing.Manager() as manager, ssh.ProxyManager(
+            manager
+        ) as ssh_proxy:
+            ssh_proxy.sock()
+            with self.ParallelContext():
+                self.get_parallel_context()["ssh_proxy"] = ssh_proxy
+                # TODO(gavinmak): Use multprocessing.Queue instead of dict.
+                self.get_parallel_context()[
+                    "sync_dict"
+                ] = multiprocessing.Manager().dict()
+                sync_progress_thread.start()
+
+                try:
+                    # Outer loop for dynamic project discovery. This continues
+                    # until no unsynced projects remain.
+                    while True:
+                        projects_to_sync = [
+                            p
+                            for p in project_list
+                            if p.relpath not in synced_relpaths
+                        ]
+                        if not projects_to_sync:
+                            break
+
+                        pending_relpaths = {p.relpath for p in projects_to_sync}
+                        if previously_pending_relpaths == pending_relpaths:
+                            logger.error(
+                                "Stall detected in interleaved sync, not all "
+                                "projects could be synced."
+                            )
+                            err_event.set()
+                            break
+                        previously_pending_relpaths = pending_relpaths
+
+                        self.get_parallel_context()[
+                            "projects"
+                        ] = projects_to_sync
+                        project_index_map = {
+                            p: i for i, p in enumerate(projects_to_sync)
+                        }
+
+                        # Inner loop to process projects in a hierarchical
+                        # order. This iterates through levels of project
+                        # dependencies (e.g. 'foo' then 'foo/bar'). All projects
+                        # in one level can be processed in parallel, but we must
+                        # wait for a level to complete before starting the next.
+                        for level_projects in _SafeCheckoutOrder(
+                            projects_to_sync
+                        ):
+                            if not level_projects:
+                                continue
+
+                            objdir_project_map = collections.defaultdict(list)
+                            for p in level_projects:
+                                objdir_project_map[p.objdir].append(
+                                    project_index_map[p]
+                                )
+
+                            work_items = list(objdir_project_map.values())
+                            if not work_items:
+                                continue
+
+                            jobs = max(1, min(opt.jobs, len(work_items)))
+                            callback = functools.partial(
+                                self._ProcessSyncInterleavedResults,
+                                synced_relpaths,
+                                err_event,
+                                errors,
+                                opt,
+                            )
+                            if not self.ExecuteInParallel(
+                                jobs,
+                                functools.partial(self._SyncProjectList, opt),
+                                work_items,
+                                callback=callback,
+                                output=pm,
+                                chunksize=1,
+                            ):
+                                err_event.set()
+
+                            if err_event.is_set() and opt.fail_fast:
+                                raise SyncFailFastError(aggregate_errors=errors)
+
+                        self._ReloadManifest(None, manifest)
+                        project_list = self.GetProjects(
+                            args,
+                            missing_ok=True,
+                            submodules_ok=opt.fetch_submodules,
+                            manifest=manifest,
+                            all_manifests=not opt.this_manifest_only,
+                        )
+                finally:
+                    sync_event.set()
+                    sync_progress_thread.join()
+
+        pm.end()
+
+        err_update_projects, err_update_linkfiles = self._UpdateManifestLists(
+            opt, err_event, errors
+        )
+        if not self.outer_client.manifest.IsArchive:
+            self._GCProjects(project_list, opt, err_event)
+
+        self._PrintManifestNotices(opt)
+        if err_event.is_set():
+            self._ReportErrors(
+                errors,
+                err_network_sync=self._interleaved_err_network,
+                failing_network_repos=self._interleaved_err_network_results,
+                err_checkout=self._interleaved_err_checkout,
+                failing_checkout_repos=self._interleaved_err_checkout_results,
+                err_update_projects=err_update_projects,
+                err_update_linkfiles=err_update_linkfiles,
+            )
 
 
 def _PostRepoUpgrade(manifest, quiet=False):

subcmds/upload.py

@@ -627,9 +627,12 @@ Gerrit Code Review:  https://www.gerritcodereview.com/
         # If using superproject, add the root repo as a push option.
         manifest = branch.project.manifest
         push_options = list(opt.push_options)
-        sp = manifest.superproject
-        if sp and sp.repo_id and manifest.manifestProject.use_superproject:
-            push_options.append(f"custom-keyed-value=rootRepo:{sp.repo_id}")
+        if manifest.manifestProject.use_superproject:
+            sp = manifest.superproject
+            if sp:
+                r_id = sp.repo_id
+                if r_id:
+                    push_options.append(f"custom-keyed-value=rootRepo:{r_id}")
 
         branch.UploadForReview(
             people,

tests/test_subcmds_sync.py

@@ -305,8 +305,20 @@ class LocalSyncState(unittest.TestCase):
 
 
 class FakeProject:
-    def __init__(self, relpath):
+    def __init__(self, relpath, name=None, objdir=None):
         self.relpath = relpath
+        self.name = name or relpath
+        self.objdir = objdir or relpath
+
+        self.use_git_worktrees = False
+        self.UseAlternates = False
+        self.manifest = mock.MagicMock()
+        self.manifest.GetProjectsWithName.return_value = [self]
+        self.config = mock.MagicMock()
+        self.EnableRepositoryExtension = mock.MagicMock()
+
+    def RelPath(self, local=None):
+        return self.relpath
 
     def __str__(self):
         return f"project: {self.relpath}"
@@ -513,3 +525,394 @@ class SyncCommand(unittest.TestCase):
             self.cmd.Execute(self.opt, [])
             self.assertIn(self.sync_local_half_error, e.aggregate_errors)
             self.assertIn(self.sync_network_half_error, e.aggregate_errors)
+
+
+class SyncUpdateRepoProject(unittest.TestCase):
+    """Tests for Sync._UpdateRepoProject."""
+
+    def setUp(self):
+        """Common setup."""
+        self.repodir = tempfile.mkdtemp(".repo")
+        self.manifest = manifest = mock.MagicMock(repodir=self.repodir)
+        # Create a repoProject with a mock Sync_NetworkHalf.
+        repoProject = mock.MagicMock(name="repo")
+        repoProject.Sync_NetworkHalf = mock.Mock(
+            return_value=SyncNetworkHalfResult(True, None)
+        )
+        manifest.repoProject = repoProject
+        manifest.IsArchive = False
+        manifest.CloneFilter = None
+        manifest.PartialCloneExclude = None
+        manifest.CloneFilterForDepth = None
+
+        git_event_log = mock.MagicMock(ErrorEvent=mock.Mock(return_value=None))
+        self.cmd = sync.Sync(manifest=manifest, git_event_log=git_event_log)
+
+        opt, _ = self.cmd.OptionParser.parse_args([])
+        opt.local_only = False
+        opt.repo_verify = False
+        opt.verbose = False
+        opt.quiet = True
+        opt.force_sync = False
+        opt.clone_bundle = False
+        opt.tags = False
+        opt.optimized_fetch = False
+        opt.retry_fetches = 0
+        opt.prune = False
+        self.opt = opt
+        self.errors = []
+
+        mock.patch.object(sync.Sync, "_GetCurrentBranchOnly").start()
+
+    def tearDown(self):
+        shutil.rmtree(self.repodir)
+        mock.patch.stopall()
+
+    def test_fetches_when_stale(self):
+        """Test it fetches when the repo project is stale."""
+        self.manifest.repoProject.LastFetch = time.time() - (
+            sync._ONE_DAY_S + 1
+        )
+
+        with mock.patch.object(sync, "_PostRepoFetch") as mock_post_fetch:
+            self.cmd._UpdateRepoProject(self.opt, self.manifest, self.errors)
+            self.manifest.repoProject.Sync_NetworkHalf.assert_called_once()
+            mock_post_fetch.assert_called_once()
+            self.assertEqual(self.errors, [])
+
+    def test_skips_when_fresh(self):
+        """Test it skips fetch when repo project is fresh."""
+        self.manifest.repoProject.LastFetch = time.time()
+
+        with mock.patch.object(sync, "_PostRepoFetch") as mock_post_fetch:
+            self.cmd._UpdateRepoProject(self.opt, self.manifest, self.errors)
+            self.manifest.repoProject.Sync_NetworkHalf.assert_not_called()
+            mock_post_fetch.assert_not_called()
+
+    def test_skips_local_only(self):
+        """Test it does nothing with --local-only."""
+        self.opt.local_only = True
+        self.manifest.repoProject.LastFetch = time.time() - (
+            sync._ONE_DAY_S + 1
+        )
+
+        with mock.patch.object(sync, "_PostRepoFetch") as mock_post_fetch:
+            self.cmd._UpdateRepoProject(self.opt, self.manifest, self.errors)
+            self.manifest.repoProject.Sync_NetworkHalf.assert_not_called()
+            mock_post_fetch.assert_not_called()
+
+    def test_post_repo_fetch_skipped_on_env_var(self):
+        """Test _PostRepoFetch is skipped when REPO_SKIP_SELF_UPDATE is set."""
+        self.manifest.repoProject.LastFetch = time.time()
+
+        with mock.patch.dict(os.environ, {"REPO_SKIP_SELF_UPDATE": "1"}):
+            with mock.patch.object(sync, "_PostRepoFetch") as mock_post_fetch:
+                self.cmd._UpdateRepoProject(
+                    self.opt, self.manifest, self.errors
+                )
+                mock_post_fetch.assert_not_called()
+
+    def test_fetch_failure_is_handled(self):
+        """Test that a fetch failure is recorded and doesn't crash."""
+        self.manifest.repoProject.LastFetch = time.time() - (
+            sync._ONE_DAY_S + 1
+        )
+        fetch_error = GitError("Fetch failed")
+        self.manifest.repoProject.Sync_NetworkHalf.return_value = (
+            SyncNetworkHalfResult(False, fetch_error)
+        )
+
+        with mock.patch.object(sync, "_PostRepoFetch") as mock_post_fetch:
+            self.cmd._UpdateRepoProject(self.opt, self.manifest, self.errors)
+            self.manifest.repoProject.Sync_NetworkHalf.assert_called_once()
+            mock_post_fetch.assert_not_called()
+            self.assertEqual(self.errors, [fetch_error])
+
+
+class InterleavedSyncTest(unittest.TestCase):
+    """Tests for interleaved sync."""
+
+    def setUp(self):
+        """Set up a sync command with mocks."""
+        self.repodir = tempfile.mkdtemp(".repo")
+        self.manifest = mock.MagicMock(repodir=self.repodir)
+        self.manifest.repoProject.LastFetch = time.time()
+        self.manifest.repoProject.worktree = self.repodir
+        self.manifest.manifestProject.worktree = self.repodir
+        self.manifest.IsArchive = False
+        self.manifest.CloneBundle = False
+        self.manifest.default.sync_j = 1
+
+        self.outer_client = mock.MagicMock()
+        self.outer_client.manifest.IsArchive = False
+        self.cmd = sync.Sync(
+            manifest=self.manifest, outer_client=self.outer_client
+        )
+        self.cmd.outer_manifest = self.manifest
+
+        # Mock projects.
+        self.projA = FakeProject("projA", objdir="objA")
+        self.projB = FakeProject("projB", objdir="objB")
+        self.projA_sub = FakeProject(
+            "projA/sub", name="projA_sub", objdir="objA_sub"
+        )
+        self.projC = FakeProject("projC", objdir="objC")
+
+        # Mock methods that are not part of the core interleaved sync logic.
+        mock.patch.object(self.cmd, "_UpdateAllManifestProjects").start()
+        mock.patch.object(self.cmd, "_UpdateProjectsRevisionId").start()
+        mock.patch.object(self.cmd, "_ValidateOptionsWithManifest").start()
+        mock.patch.object(sync, "_PostRepoUpgrade").start()
+        mock.patch.object(sync, "_PostRepoFetch").start()
+
+        # Mock parallel context for worker tests.
+        self.parallel_context_patcher = mock.patch(
+            "subcmds.sync.Sync.get_parallel_context"
+        )
+        self.mock_get_parallel_context = self.parallel_context_patcher.start()
+        self.sync_dict = {}
+        self.mock_context = {
+            "projects": [],
+            "sync_dict": self.sync_dict,
+        }
+        self.mock_get_parallel_context.return_value = self.mock_context
+
+        # Mock _GetCurrentBranchOnly for worker tests.
+        mock.patch.object(sync.Sync, "_GetCurrentBranchOnly").start()
+
+    def tearDown(self):
+        """Clean up resources."""
+        shutil.rmtree(self.repodir)
+        mock.patch.stopall()
+
+    def test_interleaved_fail_fast(self):
+        """Test that --fail-fast is respected in interleaved mode."""
+        opt, args = self.cmd.OptionParser.parse_args(
+            ["--interleaved", "--fail-fast", "-j2"]
+        )
+        opt.quiet = True
+
+        # With projA/sub, _SafeCheckoutOrder creates two batches:
+        # 1. [projA, projB]
+        # 2. [projA/sub]
+        # We want to fail on the first batch and ensure the second isn't run.
+        all_projects = [self.projA, self.projB, self.projA_sub]
+        mock.patch.object(
+            self.cmd, "GetProjects", return_value=all_projects
+        ).start()
+
+        # Mock ExecuteInParallel to simulate a failed run on the first batch of
+        # projects.
+        execute_mock = mock.patch.object(
+            self.cmd, "ExecuteInParallel", return_value=False
+        ).start()
+
+        with self.assertRaises(sync.SyncFailFastError):
+            self.cmd._SyncInterleaved(
+                opt,
+                args,
+                [],
+                self.manifest,
+                self.manifest.manifestProject,
+                all_projects,
+                {},
+            )
+
+        execute_mock.assert_called_once()
+
+    def test_interleaved_shared_objdir_serial(self):
+        """Test that projects with shared objdir are processed serially."""
+        opt, args = self.cmd.OptionParser.parse_args(["--interleaved", "-j4"])
+        opt.quiet = True
+
+        # Setup projects with a shared objdir.
+        self.projA.objdir = "common_objdir"
+        self.projC.objdir = "common_objdir"
+
+        all_projects = [self.projA, self.projB, self.projC]
+        mock.patch.object(
+            self.cmd, "GetProjects", return_value=all_projects
+        ).start()
+
+        def execute_side_effect(jobs, target, work_items, **kwargs):
+            # The callback is a partial object. The first arg is the set we
+            # need to update to avoid the stall detection.
+            synced_relpaths_set = kwargs["callback"].args[0]
+            projects_in_pass = self.cmd.get_parallel_context()["projects"]
+            for item in work_items:
+                for project_idx in item:
+                    synced_relpaths_set.add(
+                        projects_in_pass[project_idx].relpath
+                    )
+            return True
+
+        execute_mock = mock.patch.object(
+            self.cmd, "ExecuteInParallel", side_effect=execute_side_effect
+        ).start()
+
+        self.cmd._SyncInterleaved(
+            opt,
+            args,
+            [],
+            self.manifest,
+            self.manifest.manifestProject,
+            all_projects,
+            {},
+        )
+
+        execute_mock.assert_called_once()
+        jobs_arg, _, work_items = execute_mock.call_args.args
+        self.assertEqual(jobs_arg, 2)
+        work_items_sets = {frozenset(item) for item in work_items}
+        expected_sets = {frozenset([0, 2]), frozenset([1])}
+        self.assertEqual(work_items_sets, expected_sets)
+
+    def _get_opts(self, args=None):
+        """Helper to get default options for worker tests."""
+        if args is None:
+            args = ["--interleaved"]
+        opt, _ = self.cmd.OptionParser.parse_args(args)
+        # Set defaults for options used by the worker.
+        opt.quiet = True
+        opt.verbose = False
+        opt.force_sync = False
+        opt.clone_bundle = False
+        opt.tags = False
+        opt.optimized_fetch = False
+        opt.retry_fetches = 0
+        opt.prune = False
+        opt.detach_head = False
+        opt.force_checkout = False
+        opt.rebase = False
+        return opt
+
+    def test_worker_successful_sync(self):
+        """Test _SyncProjectList with a successful fetch and checkout."""
+        opt = self._get_opts()
+        project = self.projA
+        project.Sync_NetworkHalf = mock.Mock(
+            return_value=SyncNetworkHalfResult(error=None, remote_fetched=True)
+        )
+        project.Sync_LocalHalf = mock.Mock()
+        project.manifest.manifestProject.config = mock.MagicMock()
+        self.mock_context["projects"] = [project]
+
+        with mock.patch("subcmds.sync.SyncBuffer") as mock_sync_buffer:
+            mock_sync_buf_instance = mock.MagicMock()
+            mock_sync_buf_instance.Finish.return_value = True
+            mock_sync_buffer.return_value = mock_sync_buf_instance
+
+            result_obj = self.cmd._SyncProjectList(opt, [0])
+
+            self.assertEqual(len(result_obj.results), 1)
+            result = result_obj.results[0]
+            self.assertTrue(result.fetch_success)
+            self.assertTrue(result.checkout_success)
+            self.assertIsNone(result.fetch_error)
+            self.assertIsNone(result.checkout_error)
+            project.Sync_NetworkHalf.assert_called_once()
+            project.Sync_LocalHalf.assert_called_once()
+
+    def test_worker_fetch_fails(self):
+        """Test _SyncProjectList with a failed fetch."""
+        opt = self._get_opts()
+        project = self.projA
+        fetch_error = GitError("Fetch failed")
+        project.Sync_NetworkHalf = mock.Mock(
+            return_value=SyncNetworkHalfResult(
+                error=fetch_error, remote_fetched=False
+            )
+        )
+        project.Sync_LocalHalf = mock.Mock()
+        self.mock_context["projects"] = [project]
+
+        result_obj = self.cmd._SyncProjectList(opt, [0])
+        result = result_obj.results[0]
+
+        self.assertFalse(result.fetch_success)
+        self.assertFalse(result.checkout_success)
+        self.assertEqual(result.fetch_error, fetch_error)
+        self.assertIsNone(result.checkout_error)
+        project.Sync_NetworkHalf.assert_called_once()
+        project.Sync_LocalHalf.assert_not_called()
+
+    def test_worker_fetch_fails_exception(self):
+        """Test _SyncProjectList with an exception during fetch."""
+        opt = self._get_opts()
+        project = self.projA
+        fetch_error = GitError("Fetch failed")
+        project.Sync_NetworkHalf = mock.Mock(side_effect=fetch_error)
+        project.Sync_LocalHalf = mock.Mock()
+        self.mock_context["projects"] = [project]
+
+        result_obj = self.cmd._SyncProjectList(opt, [0])
+        result = result_obj.results[0]
+
+        self.assertFalse(result.fetch_success)
+        self.assertFalse(result.checkout_success)
+        self.assertEqual(result.fetch_error, fetch_error)
+        project.Sync_NetworkHalf.assert_called_once()
+        project.Sync_LocalHalf.assert_not_called()
+
+    def test_worker_checkout_fails(self):
+        """Test _SyncProjectList with an exception during checkout."""
+        opt = self._get_opts()
+        project = self.projA
+        project.Sync_NetworkHalf = mock.Mock(
+            return_value=SyncNetworkHalfResult(error=None, remote_fetched=True)
+        )
+        checkout_error = GitError("Checkout failed")
+        project.Sync_LocalHalf = mock.Mock(side_effect=checkout_error)
+        project.manifest.manifestProject.config = mock.MagicMock()
+        self.mock_context["projects"] = [project]
+
+        with mock.patch("subcmds.sync.SyncBuffer"):
+            result_obj = self.cmd._SyncProjectList(opt, [0])
+            result = result_obj.results[0]
+
+            self.assertTrue(result.fetch_success)
+            self.assertFalse(result.checkout_success)
+            self.assertIsNone(result.fetch_error)
+            self.assertEqual(result.checkout_error, checkout_error)
+            project.Sync_NetworkHalf.assert_called_once()
+            project.Sync_LocalHalf.assert_called_once()
+
+    def test_worker_local_only(self):
+        """Test _SyncProjectList with --local-only."""
+        opt = self._get_opts(["--interleaved", "--local-only"])
+        project = self.projA
+        project.Sync_NetworkHalf = mock.Mock()
+        project.Sync_LocalHalf = mock.Mock()
+        project.manifest.manifestProject.config = mock.MagicMock()
+        self.mock_context["projects"] = [project]
+
+        with mock.patch("subcmds.sync.SyncBuffer") as mock_sync_buffer:
+            mock_sync_buf_instance = mock.MagicMock()
+            mock_sync_buf_instance.Finish.return_value = True
+            mock_sync_buffer.return_value = mock_sync_buf_instance
+
+            result_obj = self.cmd._SyncProjectList(opt, [0])
+            result = result_obj.results[0]
+
+            self.assertTrue(result.fetch_success)
+            self.assertTrue(result.checkout_success)
+            project.Sync_NetworkHalf.assert_not_called()
+            project.Sync_LocalHalf.assert_called_once()
+
+    def test_worker_network_only(self):
+        """Test _SyncProjectList with --network-only."""
+        opt = self._get_opts(["--interleaved", "--network-only"])
+        project = self.projA
+        project.Sync_NetworkHalf = mock.Mock(
+            return_value=SyncNetworkHalfResult(error=None, remote_fetched=True)
+        )
+        project.Sync_LocalHalf = mock.Mock()
+        self.mock_context["projects"] = [project]
+
+        result_obj = self.cmd._SyncProjectList(opt, [0])
+        result = result_obj.results[0]
+
+        self.assertTrue(result.fetch_success)
+        self.assertTrue(result.checkout_success)
+        project.Sync_NetworkHalf.assert_called_once()
+        project.Sync_LocalHalf.assert_not_called()