fix(format): refuse to write datatypes the reader refuses

The reader now refuses a compound with a repeated field name or no
fields and an enum member with an empty name, as libhdf5 does, but the
writer still wrote them: CompoundTypeBuilder and EnumTypeBuilder build
them without complaint, so clawhdf5 wrote files it could not read back.
They were never valid HDF5; h5py refuses them.

Datatype::check_encodable, which FileWriter::finish runs on every
dataset and attribute type, now parses the type's own encoding back and
refuses one the reader refuses, with the reader's reason. That keeps the
writer in step with every reader check, not only these three.

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

crates/clawhdf5-format/src/datatype.rs

@@ -1141,9 +1141,23 @@ impl Datatype {
     }
 
     /// Check that this datatype can be written: every part of it has an
-    /// on-disk encoding. [`Self::serialize`] cannot report errors, so the
-    /// writer calls this first.
+    /// on-disk encoding, and the encoding is one the reader (and libhdf5)
+    /// accepts. [`Self::serialize`] cannot report errors, so the writer calls
+    /// this first. A compound with no fields or a repeated field name, or an
+    /// enum member with an empty name, is refused here: libhdf5 and h5py
+    /// refuse such types, and so does [`Self::parse`], so writing one made a
+    /// file that could not be read back.
     pub fn check_encodable(&self) -> Result<(), FormatError> {
+        self.check_encodable_parts()?;
+        Self::parse(&self.serialize()).map_err(|e| {
+            FormatError::SerializationError(format!(
+                "datatype cannot be written: HDF5 readers refuse it ({e})"
+            ))
+        })?;
+        Ok(())
+    }
+
+    fn check_encodable_parts(&self) -> Result<(), FormatError> {
         match self {
             Datatype::Opaque { tag, .. } if opaque_tag_text(tag).len() > MAX_OPAQUE_TAG_LEN => {
                 Err(FormatError::SerializationError(format!(
@@ -1156,10 +1170,10 @@ impl Datatype {
             )),
             Datatype::Compound { members, .. } => members
                 .iter()
-                .try_for_each(|m| m.datatype.check_encodable()),
+                .try_for_each(|m| m.datatype.check_encodable_parts()),
             Datatype::Enumeration { base_type, .. }
             | Datatype::VariableLength { base_type, .. }
-            | Datatype::Array { base_type, .. } => base_type.check_encodable(),
+            | Datatype::Array { base_type, .. } => base_type.check_encodable_parts(),
             _ => Ok(()),
         }
     }

crates/clawhdf5/tests/legacy_writer_files.rs

@@ -81,3 +81,61 @@ fn every_object_of_a_v2_7_0_file_reads() {
     let paged = File::open(fixture("written_by_v2_7_0_paged.h5")).unwrap();
     assert_eq!(paged.dataset("d").unwrap().read_f32().unwrap(), [1.0, 2.0]);
 }
+
+/// Datatypes libhdf5 (and this reader) refuse — a compound with a repeated
+/// field name or no fields, an enum member with an empty name — must not be
+/// written: they made files that did not read back. Valid neighbours of each
+/// still write and read back.
+#[test]
+fn datatypes_the_reader_refuses_are_not_written() {
+    use clawhdf5::{CompoundTypeBuilder, EnumTypeBuilder, FileBuilder};
+
+    let write_compound = |dt, raw: Vec<u8>| {
+        let mut b = FileBuilder::new();
+        b.create_dataset("d").with_compound_data(dt, raw, 1);
+        b.finish()
+    };
+    let write_enum = |dt| {
+        let mut b = FileBuilder::new();
+        b.create_dataset("d").with_enum_u8_data(dt, &[0, 1]);
+        b.finish()
+    };
+    let refused = |r: Result<Vec<u8>, clawhdf5::Error>, what: &str| {
+        let err = r.expect_err("written");
+        let msg = err.to_string();
+        assert!(msg.contains("datatype cannot be written"), "{msg}");
+        assert!(msg.contains(what), "{msg}");
+    };
+
+    let dup = CompoundTypeBuilder::new()
+        .f64_field("x")
+        .f64_field("x")
+        .build();
+    refused(
+        write_compound(dup, vec![0; 16]),
+        "duplicated compound field name 'x'",
+    );
+    refused(
+        write_compound(CompoundTypeBuilder::new().build(), vec![]),
+        "invalid",
+    );
+    let empty_name = EnumTypeBuilder::u8_based()
+        .u8_value("A", 0)
+        .u8_value("", 1)
+        .build();
+    refused(write_enum(empty_name), "0 length enum name");
+
+    let ok = CompoundTypeBuilder::new()
+        .f64_field("x")
+        .f64_field("y")
+        .build();
+    let bytes = write_compound(ok, vec![0; 16]).unwrap();
+    let file = File::from_bytes(bytes).unwrap();
+    file.dataset("d").unwrap().dtype().unwrap();
+    let ok = EnumTypeBuilder::u8_based()
+        .u8_value("A", 0)
+        .u8_value("B", 1)
+        .build();
+    let file = File::from_bytes(write_enum(ok).unwrap()).unwrap();
+    file.dataset("d").unwrap().dtype().unwrap();
+}