Feature: Add NewFileRefreshableWithReaderFunc for custom file processing (#414)

Add NewFileRefreshableWithReaderFunc for custom file processing

Author: tabboud

refreshable/refreshable_file.go

@@ -23,15 +23,27 @@ func NewFileRefreshable(ctx context.Context, filePath string) Validated[[]byte]
 // NewFileRefreshableWithTicker returns a Validated refreshable whose current value is the bytes of the file at the provided path.
 // This function reads the file once then starts a goroutine which re-reads the file on each tick until the provided context is cancelled.
 // If reading the file fails, the Current() value will be unchanged. The error is present in v.Validation().
+// It is equivalent to calling NewFileRefreshableWithReaderFunc with os.ReadFile.
 func NewFileRefreshableWithTicker(ctx context.Context, filePath string, updateTicker <-chan time.Time) Validated[[]byte] {
+	return NewFileRefreshableWithReaderFunc(ctx, filePath, updateTicker, os.ReadFile)
+}
+
+// NewFileRefreshableWithReaderFunc returns a [Validated] refreshable whose current value is the bytes read using the provided readerFunc.
+// This function is similar to [NewFileRefreshableWithTicker] but allows callers to provide a custom file reading function
+// instead of using os.ReadFile directly. This is useful for scenarios where custom file processing is needed
+// (e.g., decompression, decryption, or other transformations).
+//
+// The readerFunc is called once initially and then on each tick until the context is cancelled.
+// If reading fails, the Current() value will be unchanged. The error is present in v.Validation().
+func NewFileRefreshableWithReaderFunc(ctx context.Context, filePath string, updateTicker <-chan time.Time, readerFunc func(string) ([]byte, error)) Validated[[]byte] {
 	v := newValidRefreshable[[]byte]()
-	updateValidRefreshable[string, []byte](v, filePath, os.ReadFile)
+	updateValidRefreshable(v, filePath, readerFunc)
 	go func() {
 		for {
 			select {
 			case <-updateTicker:
-				// Read file and update refreshable. If ReadFile fails, the error is present in v.Validation().
-				updateValidRefreshable[string, []byte](v, filePath, os.ReadFile)
+				// Read file and update refreshable. If readerFunc fails, the error is present in v.Validation().
+				updateValidRefreshable(v, filePath, readerFunc)
 			case <-ctx.Done():
 				return
 			}

refreshable/refreshable_file_test.go

@@ -72,6 +72,69 @@ func TestNewFileRefreshable(t *testing.T) {
 	}, time.Second, 10*time.Millisecond)
 }
 
+func TestNewFileRefreshableWithReaderFunc(t *testing.T) {
+	ctx := t.Context()
+
+	dir := t.TempDir()
+	filename := filepath.Join(dir, "file.txt")
+	ticker := make(chan time.Time, 1)
+
+	// Custom reader that uppercases the content
+	uppercaseReader := func(path string) ([]byte, error) {
+		content, err := os.ReadFile(path)
+		if err != nil {
+			return nil, err
+		}
+		// Convert to uppercase
+		for i := range content {
+			if content[i] >= 'a' && content[i] <= 'z' {
+				content[i] = content[i] - 'a' + 'A'
+			}
+		}
+		return content, nil
+	}
+
+	refreshableFile := NewFileRefreshableWithReaderFunc(ctx, filename, ticker, uppercaseReader)
+
+	// Assert we start with IsNotExist error.
+	curr, err := refreshableFile.Validation()
+	require.Error(t, err)
+	require.True(t, os.IsNotExist(err))
+	require.Empty(t, curr)
+	require.Empty(t, refreshableFile.Current())
+
+	// Create file with lowercase content.
+	require.NoError(t, os.WriteFile(filename, []byte("test"), 0644))
+	ticker <- time.Now()
+	require.EventuallyWithT(t, func(t *assert.CollectT) {
+		curr, err := refreshableFile.Validation()
+		require.NoError(t, err)
+		require.Equal(t, "TEST", string(curr)) // Should be uppercased
+		require.Equal(t, "TEST", string(refreshableFile.Current()))
+	}, time.Second, 10*time.Millisecond)
+
+	// Update file.
+	require.NoError(t, os.WriteFile(filename, []byte("hello world"), 0644))
+	ticker <- time.Now()
+	require.EventuallyWithT(t, func(t *assert.CollectT) {
+		curr, err := refreshableFile.Validation()
+		require.NoError(t, err)
+		require.Equal(t, "HELLO WORLD", string(curr)) // Should be uppercased
+		require.Equal(t, "HELLO WORLD", string(refreshableFile.Current()))
+	}, time.Second, 10*time.Millisecond)
+
+	// Delete file - Current() should retain last valid value.
+	require.NoError(t, os.Remove(filename))
+	ticker <- time.Now()
+	require.EventuallyWithT(t, func(t *assert.CollectT) {
+		curr, err := refreshableFile.Validation()
+		require.Error(t, err)
+		require.True(t, os.IsNotExist(err))
+		require.Empty(t, curr)
+		require.Equal(t, "HELLO WORLD", string(refreshableFile.Current()))
+	}, time.Second, 10*time.Millisecond)
+}
+
 func TestNewMultiFileRefreshable(t *testing.T) {
 	ctx := t.Context()
 	dir := t.TempDir()