CHANGELOG.md

@@ -40,8 +40,10 @@
     (`Error::Unsupported`, file untouched) when any part is not supported:
     new chunks in a version-2 B-tree index (two or more unlimited
     dimensions) or an implicit index, shrinking, variable-length and
-    reference data, attributes in dense storage, past an object's compact
+    reference data, chunks through a filter this build cannot encode
-    limit or with tracked creation order, files with a metadata cache
+    (scale-offset, N-Bit, SZIP), attributes in dense storage, past an
+    object's compact limit or with tracked creation order, files with a
+    metadata cache
     image, paged or persistent free space, or marked open by another
     writer. New error variants `Error::Unsupported`,
     `Error::InvalidArgument`, `Error::Locked`, and `clawhdf5::Error` is now

crates/clawhdf5-tools/tests/edit_interop.rs

@@ -1402,3 +1402,36 @@ fn attribute_count_in_version_2_headers() {
         }
     }
 }
+
+/// A dataset whose filters this build cannot encode (scale-offset)
+/// is `Error::Unsupported`, as the editor documents, not a format error,
+/// and the file is left byte for byte as it was.
+#[test]
+fn unencodable_filters_are_unsupported() {
+    if !tools_ok() {
+        return;
+    }
+    let dir = tmpdir();
+    let path = dir.path().join("unencodable.h5");
+    py(&format!(
+        "import h5py, numpy as np\n\
+         with h5py.File({p:?}, 'w') as f:\n\
+         \x20   f.create_dataset('so', data=np.arange(16, dtype='<i4'), chunks=(8,), maxshape=(None,), scaleoffset=0)\n\
+         \x20   f.create_dataset('so_new', shape=(16,), dtype='<i4', chunks=(8,), maxshape=(None,), scaleoffset=0)\n\
+         \x20   f.create_dataset('sof', data=np.arange(16, dtype='<f8'), chunks=(8,), scaleoffset=2)\n",
+        p = path.to_str().unwrap()
+    ));
+    let before = std::fs::read(&path).unwrap();
+    let mut ed = FileEditor::open(&path).unwrap();
+    for (ds, es) in [("so", 4), ("so_new", 4), ("sof", 8)] {
+        match ed.write_all(ds, &vec![0u8; 16 * es]) {
+            Err(Error::Unsupported(msg)) => assert!(msg.contains("filter"), "{ds}: {msg}"),
+            other => panic!("{ds}: {other:?}"),
+        }
+    }
+    drop(ed);
+    assert!(
+        std::fs::read(&path).unwrap() == before,
+        "a refused edit changed the file"
+    );
+}

crates/clawhdf5/src/edit/mod.rs

@@ -622,7 +622,7 @@ impl FileEditor {
         check_editable(&f)?;
         let sb = f.superblock().clone();
         let mut img = Image::new(f.as_bytes(), sb.offset_size, sb.length_size);
-        let r = op(&f, &mut img)?;
+        let r = op(&f, &mut img).map_err(unsupported_filter)?;
         if img.is_dirty() {
             if img.eoa() != img.old_eoa() {
                 set_superblock_eof(&mut img, &sb)?;
@@ -840,6 +840,20 @@ impl FileEditor {
     }
 }
 
+/// A filter this build cannot run (scale-offset, N-Bit and SZIP have no
+/// encoder; a plugin filter may be missing) is something the editor does not
+/// support, not a malformed file.
+fn unsupported_filter(e: Error) -> Error {
+    match e {
+        Error::Format(clawhdf5_format::error::FormatError::UnsupportedFilter(id)) => {
+            Error::Unsupported(format!(
+                "datasets with filter {id}, which this build cannot run"
+            ))
+        }
+        e => e,
+    }
+}
+
 /// libhdf5's checks before it opens a file for writing, and what this
 /// editor cannot keep consistent.
 fn check_editable(f: &File) -> Result<(), Error> {

docs/known-issues.md

@@ -13,11 +13,16 @@ deleting it.
 with `Error::Unsupported` and without writing anything:
 - new, moved or resized chunks in a **version-2 B-tree** chunk index (what
   libhdf5 uses for two or more unlimited dimensions) — existing unfiltered
-  chunks, and filtered ones that re-encode to the same size, are
+  chunks, and filtered ones that re-encode to the same size and filter
+  mask, are
   overwritten in place; `resize` works — and new chunks in an **implicit**
   index (it has all of its chunks from the start);
 - **shrinking** a dataset;
 - variable-length and reference data;
+- chunks through a filter this build cannot encode (scale-offset, N-Bit,
+  SZIP, or a plugin filter it lacks), even an optional one: libhdf5 skips
+  an optional filter only when its own build lacks it, which none does for
+  these;
 - attributes of an object in **dense storage**, past its compact limit (8
   by default) or with tracked **creation order**;
 - partial edge chunks stored unfiltered (`H5Pset_chunk_opts`), external