writer: track attribute creation order with track_order

h5py's track_order=True orders attributes as well as links; the writer
tracked links only. A tracking object's header now sets the attribute
creation order tracked/indexed flags and carries per-message creation
orders, an Attribute Info message holds the next order (inline too),
and dense storage gets a type-9 creation-order index. The file default
applies to datasets, with DatasetBuilder::track_order per dataset; more
than 65 535 attributes on a tracking object is an error (libhdf5's
counter is 2 bytes). The reader lists such attributes in creation
order.

h5py lists them in order (inline, dense, 20 000 on one dataset) and
keeps numbering in r+ mode, including its inline-to-dense move.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/clawhdf5-format/src/attribute.rs

@@ -450,6 +450,8 @@ fn extract_attributes_with(
     on_error: &mut dyn FnMut(FormatError) -> Result<(), FormatError>,
 ) -> Result<Vec<AttributeMessage>, FormatError> {
     let mut attrs = Vec::new();
+    // Each attribute's creation order, where the file records one.
+    let mut orders: Vec<u32> = Vec::new();
 
     // Collect compact attributes (inline in OH)
     for msg in &header.messages {
@@ -479,7 +481,10 @@ fn extract_attributes_with(
             };
             let attr = attr.and_then(|a| check_in_header(a, header));
             match attr {
-                Ok(attr) => attrs.push(attr),
+                Ok(attr) => {
+                    attrs.push(attr);
+                    orders.push(msg.creation_order.map_or(0, u32::from));
+                }
                 Err(e) => on_error(e)?,
             }
         }
@@ -487,20 +492,30 @@ fn extract_attributes_with(
 
     // Check for dense attributes via AttributeInfo message
     let attr_info = find_attribute_info(header, offset_size)?;
-    if let Some(info) = attr_info
+    if let Some(info) = &attr_info
         && let Some(fh_addr) = info.fractal_heap_address
     {
         extract_dense_attributes(
             file_data,
-            &info,
+            info,
             fh_addr,
             offset_size,
             length_size,
             &mut attrs,
+            &mut orders,
             on_error,
         )?;
     }
 
+    // An object that tracks attribute creation order lists its attributes
+    // in that order (h5py's `track_order=True`), as libhdf5 does; otherwise
+    // they come in storage order.
+    if attr_info.is_some_and(|i| i.max_creation_index.is_some()) {
+        let mut paired: Vec<(u32, AttributeMessage)> = orders.into_iter().zip(attrs).collect();
+        paired.sort_by_key(|(o, _)| *o);
+        attrs = paired.into_iter().map(|(_, a)| a).collect();
+    }
+
     Ok(attrs)
 }
 
@@ -518,7 +533,9 @@ fn find_attribute_info(
     Ok(None)
 }
 
-/// Extract attributes from dense storage (fractal heap + B-tree v2).
+/// Extract attributes from dense storage (fractal heap + B-tree v2), and
+/// each one's creation order into `orders`.
+#[allow(clippy::too_many_arguments)]
 fn extract_dense_attributes(
     file_data: &[u8],
     attr_info: &AttributeInfoMessage,
@@ -526,6 +543,7 @@ fn extract_dense_attributes(
     offset_size: u8,
     length_size: u8,
     attrs: &mut Vec<AttributeMessage>,
+    orders: &mut Vec<u32>,
     on_error: &mut dyn FnMut(FormatError) -> Result<(), FormatError>,
 ) -> Result<(), FormatError> {
     // Parse fractal heap
@@ -561,7 +579,14 @@ fn extract_dense_attributes(
                 AttributeMessage::parse_in_file(&attr_data, file_data, offset_size, length_size)
             });
         match attr {
-            Ok(attr) => attrs.push(attr),
+            Ok(attr) => {
+                attrs.push(attr);
+                let order = record
+                    .data
+                    .get(id_len + 1..id_len + 5)
+                    .map_or(0, |b| u32::from_le_bytes([b[0], b[1], b[2], b[3]]));
+                orders.push(order);
+            }
             Err(e) => on_error(e)?,
         }
     }