fix(create): refuse an indexed colour space whose base is indexed before following it

Image export parsed an Indexed colour space's base before checking whether
that base was itself Indexed. A base that refers back to the same array
recursed until the stack overflowed. The base is now checked first; since
Indexed is the only colour space that recurses, this also ends any cycle.

crates/create/src/extract.rs

@@ -383,10 +383,17 @@ fn space(doc: &Document, cs: Option<&Object>) -> Result<Space, String> {
             Some(b"CalGray") => Ok(Space::Gray),
             Some(b"CalRGB") => Ok(Space::Rgb),
             Some(b"Indexed" | b"I") => {
-                let base = space(doc, a.get(1))?;
-                if matches!(base, Space::Indexed(..)) {
+                // Checked before recursing: Indexed is the only space that recurses, so refusing
+                // an Indexed base here also ends a cyclic base reference.
+                let nested = match a.get(1).map(|b| doc.resolve(b)).as_deref() {
+                    Some(Object::Array(b)) => matches!(b.first().and_then(Object::as_name), Some(b"Indexed" | b"I")),
+                    Some(Object::Name(n)) => matches!(n.as_slice(), b"Indexed" | b"I"),
+                    _ => false,
+                };
+                if nested {
                     return Err("nested indexed colour spaces aren't valid".into());
                 }
+                let base = space(doc, a.get(1))?;
                 let hival = match a.get(2).map(|value| doc.resolve(value)).as_deref() {
                     Some(Object::Int(value)) if (0..=255).contains(value) => *value as u8,
                     _ => return Err("an indexed colour space with an invalid hival".into()),

crates/create/src/tests.rs

@@ -1181,3 +1181,19 @@ fn a_jpeg_with_an_exif_orientation_becomes_an_upright_page() {
     let img = image_of(&doc, 1);
     assert_eq!((img.int(b"Width"), img.int(b"Height")), (Some(60), Some(20)));
 }
+
+#[test]
+fn a_cyclic_indexed_colour_space_is_refused() {
+    let mut doc = from_images(&[("flat.png".into(), png_bytes(false))]).unwrap();
+    // 5 0 obj [/Indexed 5 0 R 255 <00>]: the base is the indexed space itself.
+    let cs = doc.add(Object::Null);
+    doc.set(cs, Object::Array(vec![Object::name("Indexed"), Object::Ref(cs), Object::Int(255), Object::String(PdfString::literal(vec![0]))]));
+    with_image(&mut doc, |_| {
+        let mut dict = image_dict(1, 1, "DeviceGray", 8);
+        dict.set(b"ColorSpace".to_vec(), Object::Ref(cs));
+        Stream::from_raw(dict, vec![0])
+    });
+    let out = extract_images(&doc, &[0], 0);
+    assert!(out.images.is_empty());
+    assert_eq!(out.skipped.len(), 1, "{:?}", out.skipped);
+}