Disallow decoding BSON to nil pointer.

GODRIVER-1005

Change-Id: I52ea2e1c2b200abfae82baa58b6dc954228e9e2a

Author: Divjot Arora

bson/decoder.go

@@ -16,6 +16,9 @@ import (
 	"go.mongodb.org/mongo-driver/bson/bsonrw"
 )
 
+// ErrDecodeToNil is the error returned when trying to decode to a nil value
+var ErrDecodeToNil = errors.New("cannot Decode to nil value")
+
 // This pool is used to keep the allocations of Decoders down. This is only used for the Marshal*
 // methods and is not consumable from outside of this package. The Decoders retrieved from this pool
 // must have both Reset and SetRegistry called on them.
@@ -78,6 +81,9 @@ func (d *Decoder) Decode(val interface{}) error {
 	if rval.Kind() != reflect.Ptr {
 		return fmt.Errorf("argument to Decode must be a pointer to a type, but got %v", rval)
 	}
+	if rval.IsNil() {
+		return ErrDecodeToNil
+	}
 	rval = rval.Elem()
 	decoder, err := d.dc.LookupDecoder(rval.Type())
 	if err != nil {

bson/decoder_test.go

@@ -235,6 +235,18 @@ func TestDecoderv2(t *testing.T) {
 			t.Errorf("Decoder should use the Registry provided. got %v; want %v", dec.dc, dc2)
 		}
 	})
+	t.Run("DecodeToNil", func(t *testing.T) {
+		data := docToBytes(D{{"item", "canvas"}, {"qty", 4}})
+		vr := bsonrw.NewBSONDocumentReader(data)
+		dec, err := NewDecoder(vr)
+		noerr(t, err)
+
+		var got *D
+		err = dec.Decode(got)
+		if err != ErrDecodeToNil {
+			t.Fatalf("Decode error mismatch; expected %v, got %v", ErrDecodeToNil, err)
+		}
+	})
 }
 
 type testDecoderCodec struct {

mongo/cursor.go

@@ -118,7 +118,7 @@ func (c *Cursor) Next(ctx context.Context) bool {
 	}
 }
 
-// Decode will decode the current document into val.
+// Decode will decode the current document into val. If val is nil or is a typed nil, an error will be returned.
 func (c *Cursor) Decode(val interface{}) error {
 	return bson.UnmarshalWithRegistry(c.registry, c.Current, val)
 }

mongo/single_result.go

@@ -31,7 +31,7 @@ type SingleResult struct {
 // Decode will attempt to decode the first document into v. If there was an
 // error from the operation that created this SingleResult then the error
 // will be returned. If there were no returned documents, ErrNoDocuments is
-// returned.
+// returned. If v is nil or is a typed nil, an error will be returned.
 func (sr *SingleResult) Decode(v interface{}) error {
 	if sr.err != nil {
 		return sr.err