Optimize: a removed external action is not replaced by a null in its /Next

When an action to another document was removed and its /Next list
started with a null or a reference to something other than an action,
that entry became the head of the chain, and the actions after it, such
as a web link, were dropped with it. Leading entries that are not
action dictionaries are now skipped. The Settings::discard_external_refs
doc comment lists every action type the option removes.

crates/optimize/src/external_refs.rs

@@ -265,6 +265,9 @@ fn strip_action(doc: &mut Document, a: &Object, owner: ObjRef) -> Result<Option<
     if walk.removed == 0 && !walk.cut {
         return Ok(None);
     }
+    // A null or a reference to something other than an action cannot lead the chain.
+    let head = v.iter().position(|o| doc.resolve(o).as_dict().is_some()).unwrap_or(v.len());
+    v.drain(..head);
     if v.is_empty() {
         return Ok(Some(None));
     }

crates/optimize/src/lib.rs

@@ -168,8 +168,9 @@ pub struct Settings {
     pub remove_invalid_links: bool,
     /// Clean Up: remove named destinations nothing refers to.
     pub remove_unreferenced_dests: bool,
-    /// Discard User Data: take `GoToR`, `GoToE` and `Launch` actions out of links and bookmarks;
-    /// a link left with nothing to do is removed. Links within the document and web links stay.
+    /// Discard User Data: take `GoToR`, `GoToE`, `Launch` and `ImportData` actions, and `Thread`
+    /// actions with an `/F`, out of links and bookmarks; a link left with nothing to do is
+    /// removed. Links within the document and web links stay.
     pub discard_external_refs: bool,
 }
 

crates/optimize/tests/optimize.rs

@@ -623,6 +623,31 @@ fn an_action_to_another_document_hands_its_place_to_the_actions_after_it() {
     );
 }
 
+#[test]
+fn entries_that_are_not_actions_do_not_take_the_place_of_a_removed_action() {
+    let (doc, p, items) = linked_doc(
+        |doc, _| {
+            // A null, then a reference to an empty object, before the web link.
+            let empty = doc.add(Object::Null);
+            let next = Object::Array(vec![Object::Null, Object::Ref(empty), action("URI", None), Object::Null]);
+            vec![link_with(Some(action("GoToR", Some(next))), None)]
+        },
+        &[
+            action("Launch", Some(Object::Array(vec![Object::Null, action("URI", None)]))),
+            action("GoToR", Some(Object::Array(vec![Object::Null, Object::Null]))),
+        ],
+    );
+    let mut doc = doc;
+    let report = optimize(&mut doc, &Settings { discard_external_refs: true, ..Settings::default() }).unwrap();
+    assert_eq!((report.external_links, report.external_bookmarks), (1, 2), "{report:?}");
+    let links = annots_of(&doc, p);
+    assert_eq!(links.len(), 1, "the link keeps its web link, so it stays on the page");
+    assert_eq!(chain(&doc, links[0].get(b"A").unwrap()), ["URI"]);
+    let bookmark = |r: ObjRef| doc.get(r).as_dict().unwrap().clone();
+    assert_eq!(chain(&doc, bookmark(items[0]).get(b"A").unwrap()), ["URI"]);
+    assert!(bookmark(items[1]).get(b"A").is_none(), "a chain of nulls alone leaves no action");
+}
+
 #[test]
 fn a_chain_deeper_than_the_limit_loses_its_tail_and_one_without_a_remote_action_stays() {
     let deep = |n: usize, last: &str| (0..n).fold(action(last, None), |next, _| action("URI", Some(next)));