Write failed snapshots to disk (bradleyjkemp#70)

* added option to write failed snapshots to disk. default is false

* added test

config.go

@@ -85,15 +85,24 @@ func UseStringerMethods(useStringerMethods bool) Configurator {
 	}
 }
 
+// WriteFailedSnapshot writes the failed snapshot to disk with an extra suffix .failed
+// Default: false
+func WriteFailedSnapshot(shouldWriteFailedSnapshot bool) Configurator {
+	return func(c *Config) {
+		c.shouldWriteFailedSnapshot = shouldWriteFailedSnapshot
+	}
+}
+
 // Config provides the same snapshotting functions with additional configuration capabilities.
 type Config struct {
-	shouldUpdate           func() bool
-	subDirName             string
-	failOnUpdate           bool
-	createNewAutomatically bool
-	fatalOnMismatch        bool
-	snapshotFileExtension  string
-	useStringerMethods     bool
+	shouldUpdate              func() bool
+	subDirName                string
+	failOnUpdate              bool
+	createNewAutomatically    bool
+	fatalOnMismatch           bool
+	snapshotFileExtension     string
+	useStringerMethods        bool
+	shouldWriteFailedSnapshot bool
 }
 
 // NewDefaultConfig returns a new Config instance initialised with the same options as
@@ -107,6 +116,7 @@ func NewDefaultConfig() *Config {
 		FatalOnMismatch(false),
 		SnapshotFileExtension(""),
 		UseStringerMethods(true),
+		WriteFailedSnapshot(false),
 	)
 }
 
@@ -115,12 +125,13 @@ var Global = NewDefaultConfig()
 
 func (c *Config) clone() *Config {
 	return &Config{
-		shouldUpdate:           c.shouldUpdate,
-		subDirName:             c.subDirName,
-		failOnUpdate:           c.failOnUpdate,
-		createNewAutomatically: c.createNewAutomatically,
-		fatalOnMismatch:        c.fatalOnMismatch,
-		snapshotFileExtension:  c.snapshotFileExtension,
-		useStringerMethods:     c.useStringerMethods,
+		shouldUpdate:              c.shouldUpdate,
+		subDirName:                c.subDirName,
+		failOnUpdate:              c.failOnUpdate,
+		createNewAutomatically:    c.createNewAutomatically,
+		fatalOnMismatch:           c.fatalOnMismatch,
+		snapshotFileExtension:     c.snapshotFileExtension,
+		useStringerMethods:        c.useStringerMethods,
+		shouldWriteFailedSnapshot: c.shouldWriteFailedSnapshot,
 	}
 }

cupaloy.go

@@ -132,6 +132,11 @@ func (c *Config) snapshot(snapshotName string, i ...interface{}) error {
 		return c.updateSnapshot(snapshotName, prevSnapshot, snapshot)
 	}
 
+	if c.shouldWriteFailedSnapshot {
+		// writes the current (not matching) to a file with an extra suffix: .failed
+		return c.writeFailedSnapshot(snapshotName, prevSnapshot, snapshot)
+	}
+
 	return internal.ErrSnapshotMismatch{
 		Diff: diffSnapshots(prevSnapshot, snapshot),
 	}

examples/.snapshots/TestWriteFailedSnapshot

@@ -0,0 +1 @@
+Hello World!

examples/advanced_test.go

@@ -2,7 +2,9 @@ package examples_test
 
 import (
 	"bytes"
+	"io/fs"
 	"io/ioutil"
+	"os"
 	"strings"
 	"testing"
 
@@ -236,3 +238,29 @@ func TestUseStringerMethods(t *testing.T) {
 		cupaloy.New(cupaloy.UseStringerMethods(false)).SnapshotT(t, s)
 	})
 }
+
+func TestWriteFailedSnapshot(t *testing.T) {
+	dir := "./.snapshots"
+	snapShotName := t.Name()
+	failedSnapshotName := snapShotName + ".failed"
+	fileStat := func() error {
+		_, err := fs.Stat(os.DirFS(dir), failedSnapshotName)
+		return err
+	}
+	err := fileStat()
+	if err == nil {
+		t.Fatalf("File %s should not be there yet.", failedSnapshotName)
+	}
+	defer func() {
+		_ = os.Remove(dir + "/" + failedSnapshotName)
+	}()
+	snapshotter := cupaloy.New(cupaloy.WriteFailedSnapshot(true))
+	err = snapshotter.SnapshotWithName(snapShotName, "Hello World! <changed>")
+	if err == nil {
+		t.Fatal("It should return an error as the snapshots are not equal.")
+	}
+	err = fileStat()
+	if err != nil {
+		t.Fatalf("Could not find %s", failedSnapshotName)
+	}
+}

util.go

@@ -129,6 +129,21 @@ func (c *Config) updateSnapshot(snapshotName string, prevSnapshot string, snapsh
 	}
 }
 
+func (c *Config) writeFailedSnapshot(snapshotName string, prevSnapshot string, snapshot string) error {
+	snapshotFile := c.snapshotFilePath(snapshotName)
+	_, err := os.Stat(snapshotFile)
+	if err != nil {
+		return errors.New("could not find snapshot file")
+	}
+	err = ioutil.WriteFile(snapshotFile+".failed", []byte(snapshot), os.FileMode(0644))
+	if err != nil {
+		return err
+	}
+	return internal.ErrSnapshotMismatch{
+		Diff: diffSnapshots(prevSnapshot, snapshot),
+	}
+}
+
 func diffSnapshots(previous, current string) string {
 	diff, _ := difflib.GetUnifiedDiffString(difflib.UnifiedDiff{
 		A:        difflib.SplitLines(previous),