Form controls: a button's file command names its file in the approval question

Pressing a button whose command reads or writes a file asked "Run
file.exportCsv from 'Button 1'? It writes a file." without saying which
file, so the person could not tell what they were approving. The
question now ends with the command's `path` in parentheses; control
characters show as spaces and a path longer than 200 characters is
shortened in the middle.

crates/engine/src/cmd/controls.rs

@@ -484,7 +484,7 @@ fn press(s: &mut Session, p: &Json) -> Result<Json> {
                 let key = (doc, approval_key(&id, &params));
                 if !s.macro_approvals.contains(&key) {
                     if bool_param(p, "approve") != Some(true) {
-                        return Err(EngineError::Approve(format!("Run {id} from '{name}'? It {why}.")));
+                        return Err(EngineError::Approve(format!("Run {id} from '{name}'? It {why}{}.", path_note(&params))));
                     }
                     s.macro_approvals.insert(key);
                 }
@@ -497,6 +497,27 @@ fn press(s: &mut Session, p: &Json) -> Result<Json> {
     }
 }
 
+/// The file a button's command names in its `path`, as ` (path)` for its approval question, so
+/// the person sees what it reads or writes before saying yes. Empty without a `path`; a long one
+/// is shortened in the middle.
+fn path_note(params: &Json) -> String {
+    let text = match params.get("path") {
+        None | Some(Json::Null) => return String::new(),
+        Some(Json::String(t)) => t.clone(),
+        Some(other) => other.to_string(),
+    };
+    let text: String = text.chars().map(|c| if c.is_control() { ' ' } else { c }).collect();
+    let n = text.chars().count();
+    let shown = if n > 200 {
+        let head: String = text.chars().take(100).collect();
+        let tail: String = text.chars().skip(n - 99).collect();
+        format!("{head}…{tail}")
+    } else {
+        text
+    };
+    format!(" ({shown})")
+}
+
 /// The approval key of a button's command: its id and parameters, with a prefix that keeps it
 /// apart from a macro's fingerprint (canonical JSON, which starts with `{`).
 fn approval_key(id: &str, params: &Json) -> String {

crates/engine/src/tests/controls.rs

@@ -246,6 +246,35 @@ fn a_command_button_goes_through_the_policy_with_the_documents_trust() {
     assert_eq!(v(&s, "B1"), Value::Number(1.0));
 }
 
+#[test]
+fn a_file_commands_approval_question_names_the_file() {
+    let mut s = s();
+    let b = id(&s.execute("insert.button", json!({"command": "file.exportCsv", "params": {"path": "/tmp/reports/q3 totals.csv"}})).unwrap());
+    match s.execute("control.press", json!({"id": b})) {
+        Err(EngineError::Approve(q)) => assert!(q.contains("writes a file (/tmp/reports/q3 totals.csv)"), "{q}"),
+        other => panic!("expected a question, got {other:?}"),
+    }
+    // A long path is shortened in the middle, and line breaks can't hide part of it.
+    let long = format!("/tmp/{}\n{}.csv", "a".repeat(300), "b".repeat(300));
+    let sh = std::sync::Arc::make_mut(&mut s.doc_mut().unwrap().wb).sheet_mut(0).unwrap();
+    if let Some(c) = sh.controls.iter_mut().find(|c| c.id as u64 == b) {
+        c.action = Some(ControlAction::Command { id: "file.exportCsv".into(), params: json!({"path": long}) });
+    }
+    match s.execute("control.press", json!({"id": b})) {
+        Err(EngineError::Approve(q)) => {
+            assert!(q.contains("(/tmp/aaa") && q.contains("…") && q.contains("bbb.csv)") && !q.contains('\n') && q.len() < 600, "{q}")
+        }
+        other => panic!("expected a question, got {other:?}"),
+    }
+    // Without a path the question is unchanged.
+    let plain = id(&s.execute("insert.button", json!({"command": "cell.set", "params": {"cell": "A1", "input": "1"}})).unwrap());
+    s.doc_mut().unwrap().trust = Trust::FromFile;
+    match s.execute("control.press", json!({"id": plain})) {
+        Err(EngineError::Approve(q)) => assert!(q.ends_with("came with a workbook you opened."), "{q}"),
+        other => panic!("expected a question, got {other:?}"),
+    }
+}
+
 #[test]
 fn hostile_params_are_refused() {
     let mut s = s();