fix(volume): serve reads from remote after tier upload (Rust) (#10112)

After VolumeTierMoveDatToRemote uploaded the .dat, the volume closed its
local backend but never opened the remote one, leaving both dat_file and
remote_dat_file empty. The needle read path has no lazy reopen, so reads
returned "dat file not open" until the volume reloaded.

Switch to the remote backend right after saving the .vif, the same as the
Go volume server's LoadRemoteFile, so the volume keeps serving from remote
storage immediately after tiering.

seaweed-volume/src/server/grpc_server.rs

@@ -3335,9 +3335,15 @@ impl VolumeServer for VolumeGrpcService {
                             )));
                         }
 
-                        // Close local dat file handle (matches Go's v.LoadRemoteFile
-                        // which closes DataBackend before switching to remote)
-                        vol.close_local_dat_backend();
+                        // Switch the data backend from the local .dat to the remote
+                        // object so reads keep working after upload (matches Go's
+                        // LoadRemoteFile).
+                        if let Err(e) = vol.load_remote_dat_file() {
+                            return Err(Status::internal(format!(
+                                "volume {} failed to load remote file: {}",
+                                vid, e
+                            )));
+                        }
 
                         // Optionally remove local .dat file from disk
                         if !keep_local {

seaweed-volume/src/storage/volume.rs

@@ -949,7 +949,7 @@ impl Volume {
         Ok(())
     }
 
-    fn load_remote_dat_file(&mut self) -> Result<(), VolumeError> {
+    pub(crate) fn load_remote_dat_file(&mut self) -> Result<(), VolumeError> {
         let (storage_name, storage_key) = self.remote_storage_name_key();
         let backend = crate::remote_storage::s3_tier::global_s3_tier_registry()
             .read()
@@ -2225,12 +2225,6 @@ impl Volume {
         }
     }
 
-    /// Close the local .dat file handle (matches Go's v.DataBackend.Close() in LoadRemoteFile).
-    /// Called after tier-upload when the local file is being replaced by remote storage.
-    pub fn close_local_dat_backend(&mut self) {
-        self.dat_file = None;
-    }
-
     /// Close the remote dat file backend (matches Go's v.DataBackend.Close(); v.DataBackend = nil).
     /// Called after tier-download when the remote backend is being replaced by local storage.
     pub fn close_remote_dat_backend(&mut self) {