commitcloud: use metalog root to test repo changes

Summary:
This works more reliably and fully restores `test-commitcloud-sync-race.t` to
pre-D28595292 (c72cd2333f) state.

Reviewed By: markbt

Differential Revision: D30974286

fbshipit-source-id: 729e20f23cb5d8aacdbcef1c869fc9a73ac4d4d4

eden/scm/edenscm/hgext/commitcloud/sync.py

@@ -200,8 +200,7 @@ def _sync(
         # The line below makes sure that working copy is updated.
         return _maybeupdateworkingcopy(repo, startnode), None
 
-    origheads = _getheads(repo)
-    origbookmarks = _getbookmarks(repo)
+    origmlroot = repo.metalog().root()
 
     remotepath = ccutil.getremotepath(ui)
 
@@ -246,14 +245,6 @@ def _sync(
         {("treemanifest", "prefetchdraftparents"): False}, "cloudsync"
     ), repo.wlock(), repo.lock():
 
-        if origheads != _getheads(repo) or origbookmarks != _getbookmarks(repo):
-            # Another transaction changed the repository while we were backing
-            # up commits. This may have introduced new commits that also need
-            # backing up.  That transaction should have started its own sync
-            # process, so give up on this sync, and let the later one perform
-            # the sync.
-            raise ccerror.SynchronizationError(ui, _("repo changed while backing up"))
-
         synced = False
         attempt = 0
         while not synced:
@@ -266,6 +257,16 @@ def _sync(
 
             with repo.transaction("cloudsync") as tr:
 
+                if repo.metalog().root() != origmlroot:
+                    # Another transaction changed the repository while we were backing
+                    # up commits. This may have introduced new commits that also need
+                    # backing up.  That transaction should have started its own sync
+                    # process, so give up on this sync, and let the later one perform
+                    # the sync.
+                    raise ccerror.SynchronizationError(
+                        ui, _("repo changed while backing up")
+                    )
+
                 # Apply any changes from the cloud to the local repo.
                 if cloudrefs.version != fetchversion:
                     _applycloudchanges(

eden/scm/tests/test-commitcloud-sync-race.t

@@ -124,4 +124,6 @@ Wait for the background backup to finish and check its output.
   $ cat $TESTTMP/bgsync.out
   commitcloud: synchronizing 'testrepo' with 'user/test/default'
   visibility: read 0 heads: 
-  abort: unknown revision '1292cc1f1c1783567cebc137763d8fcf042370c6'!
+  abort: commitcloud: failed to synchronize commits: 'repo changed while backing up'
+  (please retry 'hg cloud sync')
+  (please contact the Source Control Team if this error persists)

eden/scm/tests/test-commitcloud-sync.t

@@ -905,7 +905,7 @@ Run cloud status after failing to synchronize
   Raw Workspace Name: user/test/default
   Automatic Sync (on local changes): OFF
   Automatic Sync via 'Scm Daemon' (on remote changes): OFF
-  Last Sync Version: 18
+  Last Sync Version: 17
   Last Sync Heads: 1 (0 omitted)
   Last Sync Bookmarks: 1 (0 omitted)
   Last Sync Remote Bookmarks: 0