feat(webidl): write BufferSource rows without changing padding

Extracted from wpt-misc-fix.

Source-commit: 4fb35a340e

moli-webidl/src/buffer_source.rs

@@ -115,7 +115,20 @@ impl AllowSharedBufferSource<'_> {
 
     /// Copy into the native view's byte range, preserving surrounding bytes.
     pub fn write_bytes(&self, scope: &mut v8::PinScope<'_, '_>, bytes: &[u8]) -> bool {
-        if bytes.len() > self.byte_length() {
+        self.write_bytes_at(scope, 0, bytes)
+    }
+
+    /// Copy a row without touching plane padding or other shared-memory bytes.
+    pub fn write_bytes_at(
+        &self,
+        scope: &mut v8::PinScope<'_, '_>,
+        start: usize,
+        bytes: &[u8],
+    ) -> bool {
+        if start
+            .checked_add(bytes.len())
+            .is_none_or(|end| end > self.byte_length())
+        {
             return false;
         }
         if bytes.is_empty() {
@@ -124,7 +137,7 @@ impl AllowSharedBufferSource<'_> {
         let backing = self
             .backing_store(scope)
             .expect("BufferSource has native backing");
-        let offset = self.byte_offset();
+        let offset = self.byte_offset() + start;
         if backing.is_shared() {
             // SAFETY: fixed shared storage is retained and V8 validated the
             // view range; byte stores are unordered as required for BufferSource.

moli-webidl/tests/buffer_source_rows.rs

@@ -0,0 +1,33 @@
+use moli_webidl::Context;
+
+#[test]
+fn buffer_source_row_writes_preserve_padding_and_reject_out_of_bounds() {
+    moli_v8_test_util::ensure_v8();
+    let mut isolate = v8::Isolate::new(v8::CreateParams::default());
+    v8::scope!(let scope, &mut isolate);
+    let context = v8::Context::new(scope, Default::default());
+    let scope = &mut v8::ContextScope::new(scope, context);
+    for source in [
+        "new DataView(new ArrayBuffer(8),2,4)",
+        "new DataView(new SharedArrayBuffer(8),2,4)",
+    ] {
+        let source = v8::String::new(scope, source).unwrap();
+        let value = v8::Script::compile(scope, source, None)
+            .unwrap()
+            .run(scope)
+            .unwrap();
+        let buffer = moli_webidl::convert::<moli_webidl::AllowSharedBufferSource>(
+            scope,
+            value,
+            Context::argument("Test", 1),
+        )
+        .unwrap();
+        assert!(buffer.write_bytes(scope, &[7, 7, 7, 7]));
+        assert!(buffer.write_bytes_at(scope, 1, &[1, 2]));
+        assert_eq!(buffer.to_vec(scope), vec![7, 1, 2, 7]);
+        assert!(!buffer.write_bytes_at(scope, 3, &[1, 2]));
+        assert!(!buffer.write_bytes_at(scope, usize::MAX, &[1]));
+        assert!(buffer.write_bytes_at(scope, 4, &[]));
+        assert_eq!(buffer.to_vec(scope), vec![7, 1, 2, 7]);
+    }
+}