lighthouse/quorum: avoid split brain and add shrink_only support

proto/torchft.proto

@@ -41,6 +41,7 @@ message QuorumMember {
     string store_address = 3;
     int64 step = 4;
     uint64 world_size = 5;
+    bool shrink_only = 6;
 }
 
 message Quorum {
@@ -72,6 +73,7 @@ message ManagerQuorumRequest {
     int64 rank = 1;
     int64 step = 2;
     string checkpoint_server_addr = 3;
+    bool shrink_only = 4;
 }
 
 message ManagerQuorumResponse {

src/lib.rs

@@ -105,20 +105,22 @@ impl ManagerClient {
         })
     }
 
-    #[pyo3(signature = (rank, step, checkpoint_server_addr, timeout=None))]
+    #[pyo3(signature = (rank, step, checkpoint_server_addr, shrink_only, timeout=None))]
     fn quorum(
         &mut self,
         py: Python<'_>,
         rank: i64,
         step: i64,
         checkpoint_server_addr: String,
+        shrink_only: bool,
         timeout: Option<Duration>,
     ) -> Result<(i64, i64, i64, String, String, i64, Option<i64>, i64, bool), StatusError> {
         py.allow_threads(move || {
             let mut request = tonic::Request::new(ManagerQuorumRequest {
                 rank: rank,
                 step: step,
                 checkpoint_server_addr: checkpoint_server_addr,
+                shrink_only: shrink_only,
             });
             // This notifies the server about the timeout but doesn't affect the
             // endpoint timeout which we set on client creation.