Links: links inside symbols, patterns and masks are saved with relative paths

A save wrote the path relative to the document only for linked images and
placed documents in the layers. Links inside symbols, graph designs,
patterns and opacity masks kept only their absolute path, so after the
document and its files moved to another folder together, those files were
not found.

- The save sets the relative path of every link the document holds.
- On open, a link inside that art whose file was found away from its path
  takes the new path, as a link in the layers does, so the next save
  writes the right relative path.
- Document::update_links_where changes only the links it picks and copies
  only the nodes on the way to them.

Docs: docs/mcp.md (Linked images), docs/target-app-parity.md.

crates/engine/src/tests/links.rs

@@ -244,6 +244,98 @@ fn the_relative_path_finds_the_file_after_the_folder_moves() {
     assert!(near(centre_colour(&s.doc().unwrap().doc), RED));
 }
 
+/// Links in symbols, graph designs, patterns and opacity masks are saved with their relative
+/// paths, as links in the layers are: after the folder moves, each file is found and its link takes
+/// the new path, which the next save writes relative to the document again.
+#[test]
+fn links_outside_the_layers_find_their_files_after_the_folder_moves() {
+    let a = Folder::new("nested-move-a");
+    let b = Folder::new("nested-move-b");
+    let c = Folder::new("nested-move-c");
+    let names = ["layer", "symbol", "design", "pattern", "mask"];
+    for (i, name) in (0u8..).zip(names) {
+        write(&a.file(&format!("art/{name}.png")), &png(20, 10, [20 + 40 * i, 90, 160]));
+    }
+    let mut s = session();
+    place(&mut s, &a.file("art/layer.png"));
+    let id = place(&mut s, &a.file("art/symbol.png"));
+    s.execute("symbol.new", &json!({"name": "Mark", "ids": [id.0]})).unwrap();
+    let id = place(&mut s, &a.file("art/design.png"));
+    s.execute("graph.design", &json!({"new": "Bar"})).unwrap();
+    s.execute("edit.clear", &json!({"ids": [id.0]})).unwrap();
+    let id = place(&mut s, &a.file("art/pattern.png"));
+    s.execute("object.pattern.make", &json!({"ids": [id.0], "edit": false})).unwrap();
+    s.execute("edit.clear", &json!({"ids": [id.0]})).unwrap();
+    let masked = super::rect(&mut s, 10.0, 10.0, 60.0, 60.0);
+    let mask = place(&mut s, &a.file("art/mask.png"));
+    s.execute("transparency.makeOpacityMask", &json!({"ids": [masked.0, mask.0]})).unwrap();
+    let mut in_layers = vec![];
+    s.doc().unwrap().doc.walk(|n| {
+        if let NodeKind::Image(im) = &n.kind {
+            in_layers.extend(im.link.as_ref().map(|l| l.name().to_string()));
+        }
+    });
+    assert_eq!(in_layers, ["layer.png"], "only one file is shown by an object in the layers");
+    // The links' paths, each image checked to show its file's pixels rather than the saved preview.
+    let paths = |s: &Session| {
+        let d = &s.doc().unwrap().doc;
+        let mut out = vec![];
+        d.visit_images(|_, im| {
+            let l = im.link.as_ref().unwrap();
+            assert!(!d.images[&im.key].is_proxy(), "{} shows its preview", l.name());
+            out.push(PathBuf::from(&l.path));
+        });
+        out.sort();
+        out
+    };
+    let files = |dir: &Folder| {
+        let mut out: Vec<PathBuf> = names.iter().map(|n| PathBuf::from(dir.file(&format!("art/{n}.png")))).collect();
+        out.sort();
+        out
+    };
+    assert_eq!(paths(&s), files(&a));
+    save(&mut s, &a.file("poster.vectorcraft"));
+    // Move the document with its links: the old folder is gone.
+    std::fs::remove_dir_all(&b.0).unwrap();
+    std::fs::rename(&a.0, &b.0).unwrap();
+    let r = open(&mut s, &b.file("poster.vectorcraft"));
+    assert_eq!((r["missingLinks"].clone(), r["modifiedLinks"].clone()), (json!([]), json!([])), "{r}");
+    assert_eq!(paths(&s), files(&b), "the links follow the files");
+    // Saved again and moved again.
+    save(&mut s, &b.file("poster.vectorcraft"));
+    std::fs::remove_dir_all(&c.0).unwrap();
+    std::fs::rename(&b.0, &c.0).unwrap();
+    let r = open(&mut s, &c.file("poster.vectorcraft"));
+    assert_eq!((r["missingLinks"].clone(), r["modifiedLinks"].clone()), (json!([]), json!([])), "{r}");
+    assert_eq!(paths(&s), files(&c));
+}
+
+/// With Update Links: Automatically, a file in a symbol and in the layers that changed and moved
+/// with the document is read again for the layers' image, and the symbol's link takes its new path
+/// too.
+#[test]
+fn a_symbols_link_to_a_moved_and_modified_file_takes_its_new_path() {
+    let a = Folder::new("nested-modified-a");
+    let b = Folder::new("nested-modified-b");
+    let pic = a.file("art/photo.png");
+    write(&pic, &png(20, 10, RED));
+    let mut s = session();
+    s.execute("prefs.set", &json!({"key": "updateLinks", "value": "automatically"})).unwrap();
+    let id = place(&mut s, &pic);
+    s.execute("symbol.new", &json!({"name": "Mark", "ids": [id.0]})).unwrap();
+    let id = place(&mut s, &pic);
+    save(&mut s, &a.file("poster.vectorcraft"));
+    write(&pic, &png(30, 30, BLUE));
+    std::fs::remove_dir_all(&b.0).unwrap();
+    std::fs::rename(&a.0, &b.0).unwrap();
+    let r = open(&mut s, &b.file("poster.vectorcraft"));
+    assert_eq!((r["modifiedLinks"].clone(), r["updatedLinks"][0]["ids"].clone()), (json!([]), json!([id.0])), "{r}");
+    let moved = PathBuf::from(b.file("art/photo.png"));
+    let mut paths = vec![];
+    s.doc().unwrap().doc.visit_images(|_, im| paths.extend(im.link.as_ref().map(|l| PathBuf::from(&l.path))));
+    assert_eq!(paths, [moved.clone(), moved], "the layers' link, then the symbol's");
+}
+
 #[test]
 fn a_missing_link_renders_its_preview_and_relinks() {
     let dir = Folder::new("missing");