Expose the underlying behavior through a ReadSeekStater interface

js/modules/k6/experimental/csv/module.go

@@ -99,7 +99,7 @@ func (mi *ModuleInstance) NewParser(call goja.ConstructorCall) *goja.Object {
 	}
 
 	// Instantiate and configure csv reader
-	r := csv.NewReader(&file.Impl)
+	r := csv.NewReader(file.ReadSeekStater)
 	r.ReuseRecord = true        // evaluate if this is needed, and if it leads to unforeseen issues
 	r.Comma = options.Delimiter // default delimiter, should be modifiable by the user
 

js/modules/k6/experimental/fs/file.go

@@ -24,11 +24,14 @@ type file struct {
 }
 
 // Stat returns a FileInfo describing the named file.
-func (f *file) stat() *FileInfo {
+func (f *file) Stat() *FileInfo {
 	filename := filepath.Base(f.path)
 	return &FileInfo{Name: filename, Size: f.size()}
 }
 
+// Ensure that `file` implements the Stater interface.
+var _ Stater = (*file)(nil)
+
 // FileInfo holds information about a file.
 type FileInfo struct {
 	// Name holds the base name of the file.

js/modules/k6/experimental/fs/module.go

@@ -7,6 +7,7 @@ package fs
 import (
 	"errors"
 	"fmt"
+	"io"
 	"reflect"
 
 	"github.com/grafana/sobek"
@@ -136,7 +137,7 @@ func (mi *ModuleInstance) openImpl(path string) (*File, error) {
 
 	file := &File{
 		Path: path,
-		Impl: file{
+		ReadSeekStater: &file{
 			path: path,
 			data: data,
 		},
@@ -147,6 +148,26 @@ func (mi *ModuleInstance) openImpl(path string) (*File, error) {
 	return file, nil
 }
 
+// Stater is an interface that provides information about a file.
+//
+// Although in the context of this module we have a single implementation
+// of this interface, it is defined to allow exposing the `file`'s behavior
+// to other module through the `ReadSeekStater` interface without having to
+// leak our internal abstraction.
+type Stater interface {
+	// Stat returns a FileInfo describing the named file.
+	Stat() *FileInfo
+}
+
+// ReadSeekStater is an interface that combines the io.ReadSeeker and Stater
+// interfaces and ensure that structs implementing it have the necessary
+// methods to interact with files.
+type ReadSeekStater interface {
+	io.Reader
+	io.Seeker
+	Stater
+}
+
 // File represents a file and exposes methods to interact with it.
 //
 // It is a wrapper around the [file] struct, which is meant to be directly
@@ -162,7 +183,7 @@ type File struct {
 	// implementation details, but keep it public so that we can access it
 	// from other modules that would want to leverage its implementation of
 	// io.Reader and io.Seeker.
-	Impl file `js:"-"`
+	ReadSeekStater ReadSeekStater `js:"-"`
 
 	// vu holds a reference to the VU this file is associated with.
 	//
@@ -182,7 +203,7 @@ func (f *File) Stat() *sobek.Promise {
 	promise, resolve, _ := promises.New(f.vu)
 
 	go func() {
-		resolve(f.Impl.stat())
+		resolve(f.ReadSeekStater.Stat())
 	}()
 
 	return promise
@@ -227,7 +248,7 @@ func (f *File) Read(into sobek.Value) *sobek.Promise {
 	// occurs on the main thread, during the promise's resolution.
 	callback := f.vu.RegisterCallback()
 	go func() {
-		n, readErr := f.Impl.Read(buffer)
+		n, readErr := f.ReadSeekStater.Read(buffer)
 		callback(func() error {
 			_ = copy(intoBytes[0:n], buffer)
 
@@ -290,7 +311,7 @@ func (f *File) Seek(offset sobek.Value, whence sobek.Value) *sobek.Promise {
 
 	callback := f.vu.RegisterCallback()
 	go func() {
-		newOffset, err := f.Impl.Seek(intOffset, seekMode)
+		newOffset, err := f.ReadSeekStater.Seek(intOffset, seekMode)
 		callback(func() error {
 			if err != nil {
 				reject(err)

js/modules/k6/experimental/fs/module_test.go

@@ -95,7 +95,7 @@ func TestOpen(t *testing.T) {
 
 		_, err = runtime.RunOnEventLoop(wrapInAsyncLambda(`
 			try {
-				const file = await fs.open('bonjour.txt')	
+				const file = await fs.open('bonjour.txt')
 				throw 'unexpected promise resolution with result: ' + file;
 			} catch (err) {
 				if (err.name !== 'ForbiddenError') {
@@ -199,7 +199,7 @@ func TestOpen(t *testing.T) {
 func TestFile(t *testing.T) {
 	t.Parallel()
 
-	t.Run("stat method should succeed", func(t *testing.T) {
+	t.Run("Stat method should succeed", func(t *testing.T) {
 		t.Parallel()
 
 		runtime, err := newConfiguredRuntime(t)
@@ -213,7 +213,7 @@ func TestFile(t *testing.T) {
 
 		_, err = runtime.RunOnEventLoop(wrapInAsyncLambda(fmt.Sprintf(`
 			const file = await fs.open(%q)
-			const info = await file.stat()
+			const info = await file.Stat()
 
 			if (info.name !== 'bonjour.txt') {
 				throw 'unexpected file name ' + info.name + '; expected \'bonjour.txt\'';
@@ -338,7 +338,7 @@ func TestFile(t *testing.T) {
 		runtime.VU.InitEnvField.FileSystems["file"] = fs
 
 		_, err = runtime.RunOnEventLoop(wrapInAsyncLambda(fmt.Sprintf(`
-			const file = await fs.open(%q);	
+			const file = await fs.open(%q);
 			let bytesRead;
 
 			// No argument should fail with TypeError.
@@ -406,11 +406,11 @@ func TestFile(t *testing.T) {
 		runtime.VU.InitEnvField.FileSystems["file"] = fs
 
 		_, err = runtime.RunOnEventLoop(wrapInAsyncLambda(fmt.Sprintf(`
-			// file size is 3 
+			// file size is 3
 			const file = await fs.open(%q);
 
 			// Create a buffer of size fileSize + 1
-			let buffer = new Uint8Array(4); 
+			let buffer = new Uint8Array(4);
 			let n = await file.read(buffer)
 			if (n !== 3) {
 				throw 'expected read to return 10, got ' + n + ' instead';
@@ -555,7 +555,7 @@ func TestFile(t *testing.T) {
 			// Invalid type offset should fail with TypeError.
 			try {
 				newOffset = await file.seek('abc')
-				throw "file.seek('abc') promise unexpectedly resolved with result: " + newOffset 
+				throw "file.seek('abc') promise unexpectedly resolved with result: " + newOffset
 			} catch (err) {
 				if (err.name !== 'TypeError') {
 					throw "file.seek('1') rejected with unexpected error: " + err
@@ -585,7 +585,7 @@ func TestFile(t *testing.T) {
 			// Invalid whence should fail with TypeError.
 			try {
 				newOffset = await file.seek(1, -1)
-				throw "file.seek(1, -1) promise unexpectedly resolved with result: " + newOffset 
+				throw "file.seek(1, -1) promise unexpectedly resolved with result: " + newOffset
 			} catch (err) {
 				if (err.name !== 'TypeError') {
 					throw "file.seek(1, -1) rejected with unexpected error: " + err