fix: file edit operation by adding bounds check (#20)

* fix:handle invalid file edit requests gracefully

* chore: improve slice logic to handle edge cases

Author: hashemix

src/fs_service.rs

@@ -512,7 +512,21 @@ impl FileSystemService {
 
             let mut match_found = false;
 
-            for i in 0..=content_lines.len() - old_lines.len() {
+            // skip when the match is impossible:
+            if old_lines.len() > content_lines.len() {
+                let error_message = format!(
+                    "Cannot apply edit: the original text spans more lines ({}) than the file content ({}).",
+                    old_lines.len(),
+                    content_lines.len()
+                );
+
+                return Err(RpcError::internal_error()
+                    .with_message(error_message)
+                    .into());
+            }
+
+            let max_start = content_lines.len().saturating_sub(old_lines.len());
+            for i in 0..=max_start {
                 let potential_match = &content_lines[i..i + old_lines.len()];
 
                 // Compare lines with normalized whitespace

tests/test_fs_service.rs

@@ -896,3 +896,30 @@ async fn test_preserve_unix_line_endings() {
     let updated = std::fs::read_to_string(&file).unwrap();
     assert_eq!(updated, "updated1\nupdated2\n"); // Still uses \n endings
 }
+
+#[tokio::test]
+// Issue #19: https://github.com/rust-mcp-stack/rust-mcp-filesystem/issues/19
+async fn test_panic_on_out_of_bounds_edit() {
+    let (temp_dir, service) = setup_service(vec!["dir1".to_string()]);
+
+    // Set up an edit that expects to match 5 lines
+    let edit = EditOperation {
+        old_text: "line e\n".repeat(41).to_string(),
+        new_text: "replaced content".to_string(),
+    };
+
+    // Set up your file content with only 2 lines
+    let file_content = "line A\nline B\n";
+    let test_path = create_temp_file(
+        &temp_dir.as_path().join("dir1"),
+        "test_input.txt",
+        file_content,
+    );
+
+    let result = service
+        .apply_file_edits(&test_path, vec![edit], Some(true), None)
+        .await;
+
+    // It should panic without the fix, or return an error after applying the fix
+    assert!(result.is_err());
+}