fix(mp4): preserve hdlr atom when writing new tags to metadata containers with missing ilst

lofty/src/mp4/ilst/write.rs

@@ -230,17 +230,7 @@ fn save_to_existing(
 		ParseOptions::DEFAULT_PARSING_MODE,
 	)?;
 
-	if tree.is_empty() {
-		// Nothing to do
-		if remove_tag {
-			return Ok(());
-		}
-
-		let meta_end = (meta.start + meta.len) as usize;
-
-		replacement = ilst;
-		range = meta_end..meta_end;
-	} else {
+	if let Some(ilst_idx) = ilst_idx {
 		let existing_ilst = &tree[ilst_idx];
 		let existing_ilst_size = existing_ilst.len;
 
@@ -315,6 +305,16 @@ fn save_to_existing(
 			replacement = ilst;
 			range = range_start as usize..range_end as usize;
 		}
+	} else {
+		// Nothing to do
+		if remove_tag {
+			return Ok(());
+		}
+
+		let meta_end = (meta.start + meta.len) as usize;
+
+		replacement = ilst;
+		range = meta_end..meta_end;
 	}
 
 	drop(write_handle);

lofty/src/mp4/read/mod.rs

@@ -156,11 +156,11 @@ pub(super) fn atom_tree<R>(
 	mut len: u64,
 	up_to: &[u8],
 	parse_mode: ParsingMode,
-) -> Result<(usize, Vec<AtomInfo>)>
+) -> Result<(Option<usize>, Vec<AtomInfo>)>
 where
 	R: Read + Seek,
 {
-	let mut found_idx: usize = 0;
+	let mut found_idx: Option<usize> = None;
 	let mut buf = Vec::new();
 
 	let mut i = 0;
@@ -174,18 +174,15 @@ where
 		len = len.saturating_sub(atom.len);
 
 		if let AtomIdent::Fourcc(ref fourcc) = atom.ident {
-			i += 1;
-
 			if fourcc == up_to {
-				found_idx = i;
+				found_idx = Some(i);
 			}
+			i += 1;
 
 			buf.push(atom);
 		}
 	}
 
-	found_idx = found_idx.saturating_sub(1);
-
 	Ok((found_idx, buf))
 }
 

lofty/tests/files/mp4.rs

@@ -73,3 +73,33 @@ fn read_no_properties() {
 fn read_no_tags() {
 	crate::util::no_tag_test("tests/files/assets/minimal/m4a_codec_aac.m4a", None);
 }
+
+#[test_log::test]
+fn hdlr_preservation_on_tag_recreation() {
+	use lofty::tag::Tag;
+	use std::fs;
+
+	let src = "tests/files/assets/minimal/m4a_codec_aac.m4a";
+	let temp_path = "tests/files/assets/minimal/temp_hdlr_test.m4a";
+	let _ = fs::remove_file(temp_path);
+	fs::copy(src, temp_path).unwrap();
+
+	// 1. Remove all tags
+	TagType::Mp4Ilst.remove_from_path(temp_path).unwrap();
+
+	// 2. Open the file and write a brand new tag
+	let mut tagged_file = Probe::open(temp_path).unwrap().read().unwrap();
+	let tag = Tag::new(TagType::Mp4Ilst);
+	tagged_file.insert_tag(tag);
+	tagged_file
+		.save_to_path(temp_path, lofty::config::WriteOptions::default())
+		.unwrap();
+
+	// 3. Read the file bytes and verify that "hdlr" is present
+	let bytes = fs::read(temp_path).unwrap();
+	let has_hdlr = bytes.windows(4).any(|w| w == b"hdlr");
+
+	let _ = fs::remove_file(temp_path);
+
+	assert!(has_hdlr, "Output M4A is missing the hdlr atom!");
+}