Fix empty traefik/provider.yaml (#62)

Fixes canonical/microk8s#4197

pkg/proxy/internal/util.go

@@ -46,8 +46,37 @@ func writeYaml(file string, data interface{}) error {
 	if err != nil {
 		return fmt.Errorf("failed to marshal yaml: %w", err)
 	}
-	if err := os.WriteFile(file, b, 0664); err != nil {
-		return fmt.Errorf("failed to write file: %w", err)
+
+	// NOTE(Hue): This is technically not a good thing to do here,
+	// because for whatever reason we might *want* to write an empty file.
+	// And we're also making this function context aware (read coupled)
+	// which is not a good thing.
+	// However, since this function is **only** used to write `provider.yaml` file and
+	// we know we don't want that file to be empty, we can safely return an error here.
+	// Make sure to remove this check if the above statement is no longer true.
+	if data == nil || len(b) == 0 {
+		return fmt.Errorf("empty yaml data")
+	}
+
+	dir := filepath.Dir(file)
+	tmpFile, err := os.CreateTemp(dir, "tmp-*")
+	if err != nil {
+		return fmt.Errorf("failed to create temp file: %w", err)
+	}
+	defer os.Remove(tmpFile.Name())
+
+	if _, err := tmpFile.Write(b); err != nil {
+		tmpFile.Close()
+		return fmt.Errorf("failed to write to temp file: %w", err)
 	}
+
+	if err := tmpFile.Close(); err != nil {
+		return fmt.Errorf("failed to close temp file: %w", err)
+	}
+
+	if err := os.Rename(tmpFile.Name(), file); err != nil {
+		return fmt.Errorf("failed to rename temp file to target file: %w", err)
+	}
+
 	return nil
 }

pkg/proxy/internal/util_test.go

@@ -2,7 +2,10 @@ package internal
 
 import (
 	"os"
+	"sync"
 	"testing"
+
+	. "github.com/onsi/gomega"
 )
 
 func TestGetDefaultProviderFile(t *testing.T) {
@@ -35,3 +38,53 @@ func TestGetDefaultProviderFile(t *testing.T) {
 		})
 	}
 }
+
+func TestWriteFile(t *testing.T) {
+	t.Run("EmptyData", func(t *testing.T) {
+		g := NewWithT(t)
+
+		file, err := os.CreateTemp("", "test-*.yaml")
+		defer os.Remove(file.Name())
+
+		g.Expect(err).ToNot(HaveOccurred())
+		g.Expect(writeYaml(file.Name(), nil)).ToNot(Succeed())
+	})
+
+	t.Run("ValidData", func(t *testing.T) {
+		g := NewWithT(t)
+
+		file, err := os.CreateTemp("", "test-*.yaml")
+		defer os.Remove(file.Name())
+
+		g.Expect(err).ToNot(HaveOccurred())
+
+		const (
+			numWriters    = 200
+			numIterations = 200
+		)
+
+		var wg sync.WaitGroup
+		wg.Add(numWriters)
+
+		// The data to write to the file
+		testData := map[string]interface{}{
+			"key": "value",
+		}
+
+		for i := 0; i < numWriters; i++ {
+			go func(writerID int) {
+				defer wg.Done()
+
+				for j := 0; j < numIterations; j++ {
+					g.Expect(writeYaml(file.Name(), testData)).To(Succeed())
+
+					content, err := os.ReadFile(file.Name())
+					g.Expect(err).ToNot(HaveOccurred())
+					g.Expect(string(content)).To(Equal("key: value\n"))
+				}
+			}(i)
+		}
+
+		wg.Wait()
+	})
+}