a method to reverse the syncer

Differential Revision: D54965426

fbshipit-source-id: b7f4527566ee1775041bb50619faf39fcff70e1a

eden/mononoke/commit_rewriting/cross_repo_sync/src/commit_syncer.rs

@@ -164,6 +164,18 @@ where
         }
     }
 
+    // Builds the syncer that can be used for opposite sync direction.
+    // Note: doesn't support large-to-small as input right now
+    pub fn reverse(&self) -> Result<CommitSyncer<M, R>, Error> {
+        Ok(Self {
+            mapping: self.mapping.clone(),
+            repos: self.repos.reverse()?,
+            live_commit_sync_config: self.live_commit_sync_config.clone(),
+            scuba_sample: self.scuba_sample.clone(),
+            x_repo_sync_lease: self.x_repo_sync_lease.clone(),
+        })
+    }
+
     // ------------------------------------------------------------------------
     // Sync methods
 

eden/mononoke/commit_rewriting/cross_repo_sync/src/commit_syncers_lib.rs

@@ -574,6 +574,24 @@ impl<R: Repo> CommitSyncRepos<R> {
         }
     }
 
+    // Builds the repos that can be used for opposite sync direction.
+    // Note: doesn't support large-to-small as input right now
+    pub fn reverse(&self) -> Result<Self> {
+        match self {
+            CommitSyncRepos::LargeToSmall { .. } => Err(anyhow!(
+                "reversing sync direction is only supported for small to large sync (because of submodule dependencies)"
+            )),
+            CommitSyncRepos::SmallToLarge {
+                large_repo,
+                small_repo,
+                ..
+            } => Ok(CommitSyncRepos::LargeToSmall {
+                large_repo: large_repo.clone(),
+                small_repo: small_repo.clone(),
+            }),
+        }
+    }
+
     pub fn get_submodule_deps(&self) -> &SubmoduleDeps<R> {
         match self {
             CommitSyncRepos::LargeToSmall { .. } => &SubmoduleDeps::NotNeeded,