Added a testcase for openeing an existing file for writing

Author: gow

dyconf.go

@@ -142,7 +142,7 @@ func (c *configManager) create_new(fileName string) error {
 
 	// We now seek to the end of the file and write an empty byte. This is to bloat the file upto the
 	// size we expect to mmap. If we don't do this mmap fails with the error "unexpected fault address"
-	seekOffset, err := c.file.Seek(int64(defaultTotalSize), 0)
+	seekOffset, err := c.file.Seek(int64(defaultTotalSize-1), 0)
 	if err != nil {
 		return stackerr.Newf(
 			"dyconf: failed to initialize for writing. Unexpected error occured while seeking to the "+
@@ -152,7 +152,7 @@ func (c *configManager) create_new(fileName string) error {
 			err.Error(),
 		)
 	}
-	if seekOffset != defaultTotalSize {
+	if seekOffset != defaultTotalSize-1 {
 		return stackerr.Newf(
 			"dyconf: failed to initialize for writing. Could not seek the file [%s] till the "+
 				"required number of bytes [%#v]. Current seek offset: [%#v]",

dyconf_test.go

@@ -298,10 +298,7 @@ func TestDyconfInitErrors(t *testing.T) {
 
 func TestDyconfWriteInitNewFile(t *testing.T) {
 	// Create the file first.
-	tmpFile, err := ioutil.TempFile("", "TestDyconfWriteInitNewFile")
-	ensure.Nil(t, err)
-	tmpFileName := tmpFile.Name()
-	tmpFile.Close()
+	tmpFileName := setupTempFile(t, "TestDyconfWriteInitNewFile-")
 	os.Remove(tmpFileName)
 
 	// Initialize the writer.
@@ -315,6 +312,26 @@ func TestDyconfWriteInitNewFile(t *testing.T) {
 	ensure.Nil(t, os.Remove(tmpFileName))
 }
 
+// TestDyconfWriteInitExistingFile tests the initialization of existing config file for writing.
+func TestDyconfWriteInitExistingFile(t *testing.T) {
+	// Create the file first.
+	tmpFileName := setupTempFile(t, "TestDyconfWriteInitExistingFile-")
+	os.Remove(tmpFileName)
+
+	// Initialize the writer and create a new config file.
+	m, err := NewManager(tmpFileName)
+	ensure.Nil(t, err)
+	ensure.Nil(t, m.Close())
+	// Make sure the file is created.
+	_, err = os.Stat(tmpFileName)
+	ensure.Nil(t, err)
+
+	// Initialize the writer with the existing config file.
+	m, err = NewManager(tmpFileName)
+	ensure.Nil(t, err)
+	ensure.Nil(t, m.Close())
+}
+
 func setupTempFile(t *testing.T, prefix string) string {
 	tmpFile, err := ioutil.TempFile("", prefix)
 	ensure.Nil(t, err)