Add truncation check to DecodeValue.

GODRIVER-1233

Change-Id: Ie1b85a73ba7e0eb3b387d596d654eb4da2082cf1

Author: Alice Thum

bson/bsoncodec/struct_codec.go

@@ -195,7 +195,7 @@ func (sc *StructCodec) DecodeValue(r DecodeContext, vr bsonrw.ValueReader, val r
 		}
 		field = field.Addr()
 
-		dctx := DecodeContext{Registry: r.Registry, Truncate: fd.truncate}
+		dctx := DecodeContext{Registry: r.Registry, Truncate: fd.truncate || r.Truncate}
 		if fd.decoder == nil {
 			return ErrNoDecoder{Type: field.Elem().Type()}
 		}

bson/truncation_test.go

@@ -0,0 +1,63 @@
+package bson
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"go.mongodb.org/mongo-driver/bson/bsoncodec"
+)
+
+type inputArgs struct {
+	Name string
+	Val  *float64
+}
+
+type outputArgs struct {
+	Name string
+	Val  *int64
+}
+
+func TestTruncation(t *testing.T) {
+	t.Run("truncation", func(t *testing.T) {
+		inputName := "truncation"
+		inputVal := 4.7892
+
+		input := inputArgs{Name: inputName, Val: &inputVal}
+		ec := bsoncodec.EncodeContext{Registry: DefaultRegistry}
+
+		doc, err := MarshalWithContext(ec, &input)
+		assert.Nil(t, err)
+
+		var output outputArgs
+		dc := bsoncodec.DecodeContext{
+			Registry: DefaultRegistry,
+			Truncate: true,
+		}
+
+		err = UnmarshalWithContext(dc, doc, &output)
+		assert.Nil(t, err)
+
+		assert.Equal(t, inputName, output.Name)
+		assert.Equal(t, int64(inputVal), *output.Val)
+	})
+	t.Run("no truncation", func(t *testing.T) {
+		inputName := "no truncation"
+		inputVal := 7.382
+
+		input := inputArgs{Name: inputName, Val: &inputVal}
+		ec := bsoncodec.EncodeContext{Registry: DefaultRegistry}
+
+		doc, err := MarshalWithContext(ec, &input)
+		assert.Nil(t, err)
+
+		var output outputArgs
+		dc := bsoncodec.DecodeContext{
+			Registry: DefaultRegistry,
+			Truncate: false,
+		}
+
+		// case throws an error when truncation is disabled
+		err = UnmarshalWithContext(dc, doc, &output)
+		assert.NotNil(t, err)
+	})
+}