Changed siganture to return a ReadCloser

container.go

@@ -52,7 +52,7 @@ type Container interface {
 	Exec(ctx context.Context, cmd []string) (int, error)
 	ContainerIP(context.Context) (string, error) // get container ip
 	CopyFileToContainer(ctx context.Context, hostFilePath string, containerFilePath string, fileMode int64) error
-	CopyFileFromContainer(ctx context.Context, filePath string) ([]byte, error)
+	CopyFileFromContainer(ctx context.Context, filePath string) (io.ReadCloser, error)
 }
 
 // ImageBuildInfo defines what is needed to build an image

docker.go

@@ -334,28 +334,39 @@ func (c *DockerContainer) Exec(ctx context.Context, cmd []string) (int, error) {
 	return exitCode, nil
 }
 
-func (c *DockerContainer) CopyFileFromContainer(ctx context.Context, filePath string) ([]byte, error) {
+type FileFromContainer struct {
+	underlying *io.ReadCloser
+	tarreader *tar.Reader
+}
+
+func (fc *FileFromContainer) Read(b []byte) (int, error) {
+	return (*fc.tarreader).Read(b)
+}
+
+func (fc *FileFromContainer) Close() error {
+	return (*fc.underlying).Close()
+}
+
+func (c *DockerContainer) CopyFileFromContainer(ctx context.Context, filePath string) (io.ReadCloser, error) {
 	r, _, err := c.provider.client.CopyFromContainer(ctx, c.ID, filePath)
 	if err != nil {
 		return nil, err
 	}
-	defer r.Close()
+	tarReader := tar.NewReader(r)
 
-	tr := tar.NewReader(r)
-	for {
-		_, err := tr.Next()
-		if err == io.EOF {
-			// end of tar archive
-			return []byte{}, nil
-		}
-		if err != nil {
-			return nil, err
-		}
-		buf := new(bytes.Buffer)
-		buf.ReadFrom(tr)
+	//if we got here we have exactly one file in the TAR-stream
+	//so we advance the index by one so the next call to Read will start reading it
+	_, err = tarReader.Next()
+	if err != nil {
+		return nil, err
+	}
 
-		return buf.Bytes(), nil
+	ret := &FileFromContainer{
+		underlying: &r,
+		tarreader:  tarReader,
 	}
+
+	return ret, nil
 }
 
 func (c *DockerContainer) CopyFileToContainer(ctx context.Context, hostFilePath string, containerFilePath string, fileMode int64) error {

docker_test.go

@@ -1482,13 +1482,53 @@ func TestDockerContainerCopyFileFromContainer(t *testing.T) {
 		t.Fatalf("File %s should exist, expected return code 0, got %v", copiedFileName, c)
 	}
 
-	fileContentFromContainer, err := nginxC.CopyFileFromContainer(ctx, "/"+copiedFileName)
+	reader, err := nginxC.CopyFileFromContainer(ctx, "/"+copiedFileName)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	fileContentFromContainer, err := ioutil.ReadAll(reader)
 	if err != nil {
 		t.Fatal(err)
 	}
 	assert.Equal(t, fileContent, fileContentFromContainer)
 }
 
+func TestDockerContainerCopyEmptyFileFromContainer(t *testing.T) {
+	ctx := context.Background()
+
+	nginxC, err := GenericContainer(ctx, GenericContainerRequest{
+		ContainerRequest: ContainerRequest{
+			Image:        "nginx:1.17.6",
+			ExposedPorts: []string{"80/tcp"},
+			WaitingFor:   wait.ForListeningPort("80/tcp"),
+		},
+		Started: true,
+	})
+	defer nginxC.Terminate(ctx)
+
+	copiedFileName := "hello_copy.sh"
+	nginxC.CopyFileToContainer(ctx, "./testresources/empty.sh", "/"+copiedFileName, 700)
+	c, err := nginxC.Exec(ctx, []string{"bash", copiedFileName})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if c != 0 {
+		t.Fatalf("File %s should exist, expected return code 0, got %v", copiedFileName, c)
+	}
+
+	reader, err := nginxC.CopyFileFromContainer(ctx, "/"+copiedFileName)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	fileContentFromContainer, err := ioutil.ReadAll(reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	assert.Empty(t, fileContentFromContainer)
+}
+
 func TestContainerWithReaperNetwork(t *testing.T) {
 	ctx := context.Background()
 	networks := []string{