Bundles: refuse entries that share their data (#1303)

Bundle entries are stored, so the files read out of one can only
outgrow the bundle when several entries point at the same bytes. A
small bundle naming one payload thousands of times was copied out once
per name, up to the 1 GiB total limit, in the browser. Check the
archive for overlapping entry data before reading any entry.

crates/engine/src/io.rs

@@ -734,6 +734,12 @@ fn read_bundle(bytes: &[u8], limits: BundleLimits) -> Result<(String, std::colle
     if z.len() > limits.entries {
         return Err(EngineError::Io("bundle: too many files".into()));
     }
+    // Entries are stored, so the files we keep can only outgrow the bundle when several entries
+    // point at the same bytes: a small bundle naming one payload thousands of times would be
+    // copied out once per name.
+    if z.has_overlapping_files().map_err(zerr)? {
+        return Err(EngineError::Io("bundle: files share data".into()));
+    }
     let (mut text, mut files, mut total) = (None, std::collections::HashMap::new(), 0u64);
     for i in 0..z.len() {
         let mut f = z.by_index(i).map_err(zerr)?;

crates/engine/src/io/bundle.rs

@@ -194,6 +194,23 @@ mod bundle_tests {
         }
     }
 
+    #[test]
+    fn entries_sharing_one_payload_are_refused() {
+        use std::io::Write as _;
+        let mut w = zip::ZipWriter::new(std::io::Cursor::new(Vec::new()));
+        let opts = zip::write::SimpleFileOptions::default().compression_method(zip::CompressionMethod::Stored);
+        w.start_file("Song.scraft", opts).unwrap();
+        w.write_all(Session::default().to_json().unwrap().as_bytes()).unwrap();
+        w.start_file("Audio Files/a.wav", opts).unwrap();
+        w.write_all(&[7u8; 4096]).unwrap();
+        for i in 0..100 {
+            w.shallow_copy_file("Audio Files/a.wav", &format!("Audio Files/copy{i}.wav")).unwrap();
+        }
+        let bundle = w.finish().unwrap().into_inner();
+        let err = read_bundle(&bundle, BUNDLE_LIMITS).unwrap_err();
+        assert!(err.to_string().contains("share data"), "{err}");
+    }
+
     #[test]
     fn a_bundle_is_saved_only_if_it_opens_again() {
         // #433: the writer keeps to the reader's limits.