Use double-checked locking for DuckLake attachment

Move the "is DuckLake attached?" check outside the mutex so that
subsequent connections don't block waiting for the mutex. The mutex
is only acquired when DuckLake actually needs to be attached (first
connection only).

This prevents connection backups when many clients connect simultaneously
after the first connection has already attached DuckLake.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: EDsCODE

server/server.go

@@ -330,19 +330,27 @@ func (s *Server) attachDuckLake(db *sql.DB) error {
 		return nil // DuckLake not configured
 	}
 
-	// Serialize DuckLake attachment to avoid race conditions
-	// Multiple connections trying to attach simultaneously can cause
+	// Fast path: check if DuckLake is already attached (no mutex needed)
+	// This avoids blocking on the mutex for the common case where DuckLake
+	// was already attached by an earlier connection.
+	var count int
+	err := db.QueryRow("SELECT COUNT(*) FROM duckdb_databases() WHERE database_name = 'ducklake'").Scan(&count)
+	if err == nil && count > 0 {
+		// Already attached, just set as default
+		if _, err := db.Exec("USE ducklake"); err != nil {
+			return fmt.Errorf("failed to set DuckLake as default catalog: %w", err)
+		}
+		return nil
+	}
+
+	// Slow path: need to attach DuckLake. Serialize to avoid race conditions
+	// where multiple connections try to attach simultaneously, causing
 	// "database with name '__ducklake_metadata_ducklake' already exists" errors
-	//
-	// TODO: This mutex can block all incoming connections if DuckLake attachment
-	// is slow (e.g., network latency to metadata store). Consider adding a timeout
-	// or moving attachment to happen asynchronously after connection establishment.
 	s.duckLakeMu.Lock()
 	defer s.duckLakeMu.Unlock()
 
-	// Check if DuckLake catalog is already attached
-	var count int
-	err := db.QueryRow("SELECT COUNT(*) FROM duckdb_databases() WHERE database_name = 'ducklake'").Scan(&count)
+	// Double-check after acquiring mutex (another goroutine may have attached)
+	err = db.QueryRow("SELECT COUNT(*) FROM duckdb_databases() WHERE database_name = 'ducklake'").Scan(&count)
 	if err == nil && count > 0 {
 		// Already attached, just set as default
 		if _, err := db.Exec("USE ducklake"); err != nil {