Implement attribute overwrite functionality

Author: kojix2

lib/hdf5/attribute.rb

@@ -71,6 +71,14 @@ def write(attr_name, value)
       values = normalize_data(value)
       datatype_id = datatype_id_for(values)
 
+      exists = HDF5::FFI.H5Aexists(@dataset_id, attr_name)
+      raise HDF5::Error, "Failed to check attribute existence: #{attr_name}" if exists.negative?
+
+      if exists.positive?
+        status = HDF5::FFI.H5Adelete(@dataset_id, attr_name)
+        raise HDF5::Error, "Failed to replace attribute: #{attr_name}" if status < 0
+      end
+
       dims = ::FFI::MemoryPointer.new(:ulong_long, 1)
       dims.write_array_of_ulong_long([values.length])
       dataspace_id = HDF5::FFI.H5Screate_simple(1, dims, nil)

test/hdf5_test.rb

@@ -105,6 +105,22 @@ class HDF5Test < Test::Unit::TestCase
     end
   end
 
+  test 'dataset attrs overwrite existing value' do
+    Dir.mktmpdir do |dir|
+      path = File.join(dir, 'attribute-overwrite.h5')
+
+      HDF5::File.create(path) do |file|
+        dataset = file.create_dataset('values', [1, 2, 3])
+        dataset.attrs['scale'] = 42
+        dataset.attrs['scale'] = 100
+      end
+
+      HDF5::File.open(path) do |file|
+        assert_equal([100], file['values'].attrs['scale'])
+      end
+    end
+  end
+
   test 'group list_datasets returns only datasets' do
     Dir.mktmpdir do |dir|
       path = File.join(dir, 'group-list.h5')