Allow key value metadata to be set after writing rows

writer.go

@@ -210,6 +210,29 @@ func (w *Writer) ReadRowsFrom(rows RowReader) (written int64, err error) {
 // The returned value will be nil if no schema has yet been configured on w.
 func (w *Writer) Schema() *Schema { return w.schema }
 
+// KeyValueMetadata sets a key/value pair in the Parquet file metadata.
+//
+// Keys are assumed to be unique, if the same key is repeated multiple times the
+// last value is retained. While the parquet format does not require unique keys,
+// this design decision was made to optimize for the most common use case where
+// applications leverage this extension mechanism to associate single values to
+// keys. This may create incompatibilities with other parquet libraries, or may
+// cause some key/value pairs to be lost when open parquet files written with
+// repeated keys. We can revisit this decision if it ever becomes a blocker.
+func (w *Writer) KeyValueMetadata(key, value string) {
+	for i, kv := range w.writer.metadata {
+		if kv.Key == key {
+			kv.Value = value
+			w.writer.metadata[i] = kv
+			return
+		}
+	}
+	w.writer.metadata = append(w.writer.metadata, format.KeyValue{
+		Key:   key,
+		Value: value,
+	})
+}
+
 type writer struct {
 	buffer  *bufio.Writer
 	writer  offsetTrackingWriter

writer_test.go

@@ -711,3 +711,88 @@ func TestWriterMaxRowsPerRowGroup(t *testing.T) {
 		t.Errorf("wrong number of row groups in parquet file: want=10 got=%d", len(rowGroups))
 	}
 }
+
+func TestKeyValueMetadata(t *testing.T) {
+	testKey := "test-key"
+	testValue := "test-value"
+
+	type testStruct struct {
+		A string `parquet:"a,dict"`
+	}
+
+	schema := parquet.SchemaOf(&testStruct{})
+
+	b := bytes.NewBuffer(nil)
+	w := parquet.NewWriter(
+		b,
+		schema,
+	)
+
+	err := w.Write(&testStruct{A: "test"})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	w.KeyValueMetadata(testKey, testValue)
+
+	err = w.Close()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	f, err := parquet.OpenFile(bytes.NewReader(b.Bytes()), int64(b.Len()))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	value, ok := f.Lookup(testKey)
+	if !ok {
+		t.Fatalf("key/value metadata should have included %q", testKey)
+	}
+	if value != testValue {
+		t.Errorf("expected %q, got %q", testValue, value)
+	}
+}
+
+func TestKeyValueMetadataOverwritesExisting(t *testing.T) {
+	testKey := "test-key"
+	testValue := "test-value"
+
+	type testStruct struct {
+		A string `parquet:"a,dict"`
+	}
+
+	schema := parquet.SchemaOf(&testStruct{})
+
+	b := bytes.NewBuffer(nil)
+	w := parquet.NewWriter(
+		b,
+		schema,
+		parquet.KeyValueMetadata(testKey, "original-value"),
+	)
+
+	err := w.Write(&testStruct{A: "test"})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	w.KeyValueMetadata(testKey, testValue)
+
+	err = w.Close()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	f, err := parquet.OpenFile(bytes.NewReader(b.Bytes()), int64(b.Len()))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	value, ok := f.Lookup(testKey)
+	if !ok {
+		t.Fatalf("key/value metadata should have included %q", testKey)
+	}
+	if value != testValue {
+		t.Errorf("expected %q, got %q", testValue, value)
+	}
+}