Do not overwrite existing files.

See exercism/exercism#1039

assignment.go

@@ -24,10 +24,14 @@ func SaveAssignment(dir string, a Assignment) (err error) {
 			err = fmt.Errorf("Error making directory %v: [%v]", dir, err)
 			return
 		}
-		err = ioutil.WriteFile(file, []byte(text), 0644)
-		if err != nil {
-			err = fmt.Errorf("Error writing file %v: [%v]", name, err)
-			return
+		if _, err = os.Stat(file); err != nil {
+			if os.IsNotExist(err) {
+				err = ioutil.WriteFile(file, []byte(text), 0644)
+				if err != nil {
+					err = fmt.Errorf("Error writing file %v: [%v]", name, err)
+					return
+				}
+			}
 		}
 	}
 

assignment_test.go

@@ -1,22 +1,28 @@
 package main
 
 import (
+	"fmt"
 	"github.com/stretchr/testify/assert"
 	"io/ioutil"
+	"os"
+	"path/filepath"
 	"testing"
 )
 
 func TestSavingAssignment(t *testing.T) {
 	tmpDir, err := ioutil.TempDir("", "")
 	assert.NoError(t, err)
 
+	prepareFixture(t, fmt.Sprintf("%s/ruby/bob/stub.rb", tmpDir), "Existing stub")
+
 	assignment := Assignment{
 		Track: "ruby",
 		Slug:  "bob",
 		Files: map[string]string{
 			"bob_test.rb":     "Tests text",
 			"README.md":       "Readme text",
 			"path/to/file.rb": "File text",
+			"stub.rb":         "New version of stub",
 		},
 	}
 
@@ -34,4 +40,21 @@ func TestSavingAssignment(t *testing.T) {
 	fileInDir, err := ioutil.ReadFile(tmpDir + "/ruby/bob/path/to/file.rb")
 	assert.NoError(t, err)
 	assert.Equal(t, string(fileInDir), "File text")
+
+	stubFile, err := ioutil.ReadFile(tmpDir + "/ruby/bob/stub.rb")
+	assert.NoError(t, err)
+	assert.Equal(t, string(stubFile), "Existing stub")
+}
+
+func prepareFixture(t *testing.T, fixture, s string) {
+	err := os.MkdirAll(filepath.Dir(fixture), 0755)
+	assert.NoError(t, err)
+
+	err = ioutil.WriteFile(fixture, []byte(s), 0644)
+	assert.NoError(t, err)
+
+	// ensure fixture is set up correctly
+	fixtureContents, err := ioutil.ReadFile(fixture)
+	assert.NoError(t, err)
+	assert.Equal(t, string(fixtureContents), s)
 }