Detect reference cycles when collecting page resources

src/document.rs

@@ -7,7 +7,7 @@ use crate::{Error, Result, Stream};
 use encoding_rs::UTF_16BE;
 use log::info;
 use std::cmp::max;
-use std::collections::{BTreeMap, HashMap};
+use std::collections::{BTreeMap, HashMap, HashSet};
 use std::io::Write;
 use std::str;
 
@@ -421,24 +421,29 @@ impl Document {
 
     /// Get resources used by a page.
     pub fn get_page_resources(&self, page_id: ObjectId) -> (Option<&Dictionary>, Vec<ObjectId>) {
-        fn collect_resources(page_node: &Dictionary, resource_ids: &mut Vec<ObjectId>, doc: &Document) {
-            if let Ok(resources_id) = page_node.get(b"Resources").and_then(Object::as_reference) {
-                resource_ids.push(resources_id);
+        fn collect_resources(
+            page_node: &Dictionary, resource_ids: &mut Vec<ObjectId>, doc: &Document,
+            already_seen: &mut HashSet<ObjectId>,
+        ) -> Result<()> {
+            if let Ok(resource_id) = page_node.get(b"Resources").and_then(Object::as_reference) {
+                resource_ids.push(resource_id);
             }
-            if let Ok(page_tree) = page_node
-                .get(b"Parent")
-                .and_then(Object::as_reference)
-                .and_then(|id| doc.get_dictionary(id))
-            {
-                collect_resources(page_tree, resource_ids, doc);
+            if let Ok(parent_id) = page_node.get(b"Parent").and_then(Object::as_reference) {
+                if already_seen.contains(&parent_id) {
+                    return Err(Error::ReferenceCycle);
+                }
+                already_seen.insert(parent_id);
+                let parent_dict = doc.get_dictionary(parent_id)?;
+                collect_resources(parent_dict, resource_ids, doc, already_seen)?;
             }
+            Ok(())
         }
 
         let mut resource_dict = None;
         let mut resource_ids = Vec::new();
         if let Ok(page) = self.get_dictionary(page_id) {
             resource_dict = page.get(b"Resources").and_then(Object::as_dict).ok();
-            collect_resources(page, &mut resource_ids, self);
+            collect_resources(page, &mut resource_ids, self, &mut HashSet::new()).unwrap();
         }
         (resource_dict, resource_ids)
     }

src/error.rs

@@ -31,6 +31,8 @@ pub enum Error {
     PageNumberNotFound(u32),
     /// Invalid object while parsing at offset.
     Parse { offset: usize },
+    /// Dereferencing object failed due to a reference cycle.
+    ReferenceCycle,
     /// Dereferencing object reached the limit.
     /// This might indicate a reference loop.
     ReferenceLimit,
@@ -67,7 +69,8 @@ impl fmt::Display for Error {
             Error::Offset(o) => write!(f, "Invalid file offset: {}", o),
             Error::PageNumberNotFound(p) => write!(f, "Page number {} could not be found", p),
             Error::Parse { offset, .. } => write!(f, "Invalid object at byte {}", offset),
-            Error::ReferenceLimit => write!(f, "Could not dereference an object; possible reference loop"),
+            Error::ReferenceCycle => write!(f, "Could not dereference an object; reference cycle detected"),
+            Error::ReferenceLimit => write!(f, "Could not dereference an object; possible reference cycle"),
             Error::StringDecode => write!(f, "Could not decode string"),
             Error::Syntax(msg) => write!(f, "Syntax error: {}", msg),
             Error::Trailer => write!(f, "Invalid file trailer"),