fix(forms): deleting a field removes ancestors it leaves without kids (#6997)

* forms: deleting a field removes ancestors it leaves without kids

* forms test: full-save check, widget only in the sibling case

crates/forms/src/author/lifecycle.rs

@@ -262,10 +262,26 @@ pub fn delete_widget(doc: &mut Document, name: &str, widget: usize) -> Result<()
 fn delete_found_field(doc: &mut Document, f: Field) -> Result<(), FormError> {
     let widgets: std::collections::HashSet<ObjRef> = f.widgets.iter().map(|w| w.obj).collect();
     drop_from_pages(doc, &widgets)?;
+    let mut parent = doc.get(f.obj).as_dict().and_then(|d| d.reference(b"Parent"));
     detach(doc, f.obj)?;
     // Out of the calculation order too: a `/CO` entry would keep the field, its value and its
     // widgets reachable for a full save.
-    drop_from_calculation_order(doc, |r| r == f.obj || widgets.contains(&r))
+    drop_from_calculation_order(doc, |r| r == f.obj || widgets.contains(&r))?;
+    // An ancestor left with no kids would become a field of its own, and keep the `/V` and `/DV`
+    // the deleted field inherited from it: it goes too, up the chain.
+    let mut gone = std::collections::HashSet::new();
+    while let Some(p) = parent.filter(|p| gone.len() < 64 && !gone.contains(p)) {
+        let Some(d) = doc.get(p).as_dict().cloned() else { break };
+        let empty = d.get(b"Kids").map(|o| doc.resolve(o)).and_then(|o| o.as_array().map(Vec::is_empty)) == Some(true);
+        if !empty {
+            break;
+        }
+        parent = d.reference(b"Parent");
+        gone.insert(p);
+        drop_from_pages(doc, &std::iter::once(p).collect())?;
+        detach(doc, p)?;
+    }
+    drop_from_calculation_order(doc, |r| gone.contains(&r))
 }
 
 /// Take the annotations `widgets` off every page's `/Annots`, held directly or by reference.

crates/forms/src/tests.rs

@@ -1664,6 +1664,38 @@ fn properties_rename_and_delete() {
     assert_eq!(annots.as_array().unwrap().len(), 13 - 3, "the deleted widgets left the page");
 }
 
+/// A deleted field's ancestor left with no kids stays in `/Fields` as a field of its own, with the
+/// `/V` the deleted field inherited from it: it leaves too. An ancestor a sibling still has stays.
+#[test]
+fn deleting_the_last_kid_removes_the_emptied_ancestors_and_their_values() {
+    let form = |kids: &str, annots: &str| {
+        document(&[
+            "<< /Type /Catalog /Pages 2 0 R /AcroForm 4 0 R >>".into(),
+            "<< /Type /Pages /Kids [3 0 R] /Count 1 /MediaBox [0 0 600 800] >>".into(),
+            format!("<< /Type /Page /Parent 2 0 R /Annots [{annots}] >>"),
+            "<< /Fields [5 0 R] >>".into(),
+            "<< /FT /Tx /T (outer) /V (SECRET) /DV (SECRET) /Kids [6 0 R] >>".into(),
+            format!("<< /T (inner) /Parent 5 0 R /Kids [{kids}] >>"),
+            "<< /Type /Annot /Subtype /Widget /T (a) /Parent 6 0 R /Rect [50 700 250 720] /P 3 0 R >>".into(),
+            "<< /Type /Annot /Subtype /Widget /T (b) /Parent 6 0 R /Rect [50 650 250 670] /P 3 0 R >>".into(),
+        ])
+    };
+    let top = |doc: &Document| {
+        let af = doc.get(doc.root().unwrap()).as_dict().unwrap().get(b"AcroForm").map(|o| doc.resolve(o)).unwrap().as_dict().unwrap().clone();
+        doc.resolve(af.get(b"Fields").unwrap()).as_array().unwrap().len()
+    };
+    let mut doc = form("7 0 R", "7 0 R");
+    delete_field(&mut doc, "outer.inner.a").unwrap();
+    assert_eq!(top(&doc), 0, "the emptied ancestors left /Fields");
+    let saved = pdfcraft_cos::write_full(&doc, &SaveOptions::default()).unwrap();
+    assert!(!saved.windows(6).any(|w| w == b"SECRET"), "the inherited value is gone from a full save");
+
+    let mut doc = form("7 0 R 8 0 R", "7 0 R 8 0 R");
+    delete_field(&mut doc, "outer.inner.a").unwrap();
+    assert_eq!(top(&doc), 1, "an ancestor with a surviving kid stays");
+    assert!(fields(&doc).iter().any(|f| f.name == "outer.inner.b"));
+}
+
 /// A parent `MASTER` with kids `a` and `b` (each a field merged with its widget) and `e` (a field
 /// with two widget kids), and the top-level fields `c` and `d`.
 fn dotted_form() -> Document {