SMB performance optimization: pread/pwrite, tokio::sync::Mutex, direct response, fast-path

- VfsFile trait: add read_at()/write_at() with seek+read default impl
- LocalFs: override with real pread/pwrite (FileExt::read_at/write_at) — 1 syscall vs 2
- smb_server_backend: use read_at/write_at + tokio::sync::Mutex (non-blocking async)
- read handler: build response directly, avoid Bytes→Vec<u8> copy + intermediate struct
- oplock break: fast-path skip when ≤1 open entry (single-user scenario)

vendor/smb-server/src/oplock.rs

@@ -112,17 +112,23 @@ impl OplockManager {
         new_share_access: u32,
         new_granted_access: Access,
     ) -> Vec<OplockBreakNotification> {
+        // Fast-path: no entries or single entry can't conflict with itself
+        let entry_count = {
+            let file_opens = self.file_opens.read().await;
+            file_opens.get(path).map_or(0, |e| e.len())
+        };
+        if entry_count <= 1 {
+            return Vec::new();
+        }
+
         let mut notifications = Vec::new();
         let mut file_opens = self.file_opens.write().await;
 
         if let Some(entries) = file_opens.get_mut(path) {
             for entry in entries.iter_mut() {
-                // Check if new open conflicts with existing oplock
                 if !share_access_compatible(entry.share_access, new_share_access) {
-                    // Need to break the oplock
-                    let new_level = OplockLevel::Ii as u8; // Downgrade to Level II
+                    let new_level = OplockLevel::Ii as u8;
 
-                    // Build notification (MS-SMB2 §2.2.23.1)
                     notifications.push(OplockBreakNotification {
                         structure_size: 24,
                         oplock_level: new_level,
@@ -131,7 +137,6 @@ impl OplockManager {
                         file_id: entry.file_id,
                     });
 
-                    // Update entry's oplock level
                     entry.oplock_level = new_level;
                 }
             }
@@ -266,6 +271,11 @@ impl LeaseManager {
     
     /// Break lease when conflicting access occurs (MS-SMB2 §3.3.5.10).
     pub async fn break_lease(&self, requested_state: u32) -> Vec<LeaseBreakNotification> {
+        // Fast-path: no leases to break
+        if self.leases.read().await.is_empty() {
+            return Vec::new();
+        }
+
         let mut leases = self.leases.write().await;
         let mut notifications = Vec::new();