Fix to_parquet append behavior with global metadata file (#17198)

Closes #17177

When appending to a parquet dataset with Dask cuDF, the original metadata must be converted from `pq.FileMetaData` to `bytes` before it can be passed down to `cudf.io.merge_parquet_filemetadata`.

Authors:
  - Richard (Rick) Zamora (https://github.com/rjzamora)

Approvers:
  - Mads R. B. Kristensen (https://github.com/madsbk)

URL: #17198

python/dask_cudf/dask_cudf/io/parquet.py

@@ -383,6 +383,12 @@ def write_metadata(parts, fmd, fs, path, append=False, **kwargs):
             metadata_path = fs.sep.join([path, "_metadata"])
             _meta = []
             if append and fmd is not None:
+                # Convert to bytes: <https://github.com/rapidsai/cudf/issues/17177>
+                if isinstance(fmd, pq.FileMetaData):
+                    with BytesIO() as myio:
+                        fmd.write_metadata_file(myio)
+                        myio.seek(0)
+                        fmd = np.frombuffer(myio.read(), dtype="uint8")
                 _meta = [fmd]
             _meta.extend([parts[i][0]["meta"] for i in range(len(parts))])
             _meta = (

python/dask_cudf/dask_cudf/io/tests/test_parquet.py

@@ -644,3 +644,23 @@ def test_read_parquet_arrow_filesystem(tmpdir, min_part_size):
         dd.assert_eq(df, ddf, check_index=False)
         assert isinstance(ddf._meta, cudf.DataFrame)
         assert isinstance(ddf.compute(), cudf.DataFrame)
+
+
+@pytest.mark.parametrize("write_metadata_file", [True, False])
+def test_to_parquet_append(tmpdir, write_metadata_file):
+    df = cudf.DataFrame({"a": [1, 2, 3]})
+    ddf = dask_cudf.from_cudf(df, npartitions=1)
+    ddf.to_parquet(
+        tmpdir,
+        append=True,
+        write_metadata_file=write_metadata_file,
+        write_index=False,
+    )
+    ddf.to_parquet(
+        tmpdir,
+        append=True,
+        write_metadata_file=write_metadata_file,
+        write_index=False,
+    )
+    ddf2 = dask_cudf.read_parquet(tmpdir)
+    dd.assert_eq(cudf.concat([df, df]), ddf2)