Replace os.Rename with io.Copy for cross filesystem transfers (#77)

* Updated selfupdate to use os.Copy(), which allows copying across filesystems

* os.copy doesn't exist, io.copy does.

* Updated to keep file permissions and delete the temporary file.

* Removed permission synchronization.

* Do it right

---------

Co-authored-by: Quinten <67589015+QuintenQVD0@users.noreply.github.com>

Author: Steak-Fish

cmd/selfupdate.go

@@ -118,7 +118,46 @@ func performUpdate(version, binaryName string) error {
 		return fmt.Errorf("failed to locate current executable: %v", err)
 	}
 
-	return os.Rename(binaryPath, currentExecutable)
+	// Try rename first (faster if on same filesystem)
+	err = os.Rename(binaryPath, currentExecutable)
+	if err != nil {
+		// If rename fails (likely due to cross-filesystem), use copy instead
+		fmt.Println("Direct replacement failed, using copy method...")
+		
+		// Open source file
+		src, err := os.Open(binaryPath)
+		if err != nil {
+			return fmt.Errorf("failed to open source file: %v", err)
+		}
+		defer src.Close()
+		
+		// Create a temporary file in the same directory as the executable
+		execDir := filepath.Dir(currentExecutable)
+		tempExec := filepath.Join(execDir, fmt.Sprintf(".%s.new", filepath.Base(currentExecutable)))
+		
+		// Create the new executable file
+		dst, err := os.OpenFile(tempExec, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0755)
+		if err != nil {
+			return fmt.Errorf("failed to create new executable: %v", err)
+		}
+		
+		// Copy the content
+		_, err = io.Copy(dst, src)
+		dst.Close()
+		if err != nil {
+			os.Remove(tempExec) // Clean up on failure
+			return fmt.Errorf("failed to copy new binary: %v", err)
+		}
+		
+		// Replace the old executable with the new one
+		err = os.Rename(tempExec, currentExecutable)
+		if err != nil {
+			os.Remove(tempExec) // Clean up on failure
+			return fmt.Errorf("failed to replace executable: %v", err)
+		}
+	}
+
+	return nil
 }
 
 func downloadWithProgress(url, dest string) error {