refactor: Use fs.FS interface instead of os file access

Replace all IO operations based on the `os` package with the ones from
the abstract `fs` package and pass a `fsys` parameter down to functions
that need it.

Use a custom `osFS` in main() that just wrap around `os.Open`.
Use `embed.FS` in tests.

Signed-off-by: Maximilian Blatt <[email protected]>

Author: MisterMX

fn.go

@@ -5,6 +5,8 @@ import (
 	"context"
 	"encoding/base64"
 	"io"
+	"io/fs"
+	"os"
 
 	"dario.cat/mergo"
 	"google.golang.org/protobuf/encoding/protojson"
@@ -25,11 +27,20 @@ import (
 	"github.com/crossplane-contrib/function-go-templating/input/v1beta1"
 )
 
+// osFS is a dead-simple implementation of [io/fs.FS] that just wraps around
+// [os.Open].
+type osFS struct{}
+
+func (*osFS) Open(name string) (fs.File, error) {
+	return os.Open(name)
+}
+
 // Function uses Go templates to compose resources.
 type Function struct {
 	fnv1beta1.UnimplementedFunctionRunnerServiceServer
 
-	log logging.Logger
+	log  logging.Logger
+	fsys fs.FS
 }
 
 const (
@@ -51,7 +62,7 @@ func (f *Function) RunFunction(_ context.Context, req *fnv1beta1.RunFunctionRequ
 		return rsp, nil
 	}
 
-	tg, err := NewTemplateSourceGetter(in)
+	tg, err := NewTemplateSourceGetter(f.fsys, in)
 	if err != nil {
 		response.Fatal(rsp, errors.Wrap(err, "invalid function input"))
 		return rsp, nil

fn_test.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"embed"
 	"fmt"
 	"testing"
 
@@ -38,6 +39,9 @@ var (
 
 	path      = "testdata/templates"
 	wrongPath = "testdata/wrong"
+
+	//go:embed testdata
+	testdataFS embed.FS
 )
 
 func TestRunFunction(t *testing.T) {
@@ -425,7 +429,7 @@ func TestRunFunction(t *testing.T) {
 					Results: []*fnv1beta1.Result{
 						{
 							Severity: fnv1beta1.Severity_SEVERITY_FATAL,
-							Message:  "invalid function input: cannot read tmpl from the folder {testdata/wrong}: lstat testdata/wrong: no such file or directory",
+							Message:  "invalid function input: cannot read tmpl from the folder {testdata/wrong}: open testdata/wrong: file does not exist",
 						},
 					},
 				},
@@ -587,7 +591,10 @@ func TestRunFunction(t *testing.T) {
 
 	for name, tc := range cases {
 		t.Run(name, func(t *testing.T) {
-			f := &Function{log: logging.NewNopLogger()}
+			f := &Function{
+				log:  logging.NewNopLogger(),
+				fsys: testdataFS,
+			}
 			rsp, err := f.RunFunction(tc.args.ctx, tc.args.req)
 
 			if diff := cmp.Diff(tc.want.rsp, rsp, protocmp.Transform()); diff != "" {

go.mod

@@ -13,7 +13,6 @@ require (
 	github.com/google/go-cmp v0.6.0
 	google.golang.org/protobuf v1.31.0
 	gopkg.in/yaml.v3 v3.0.1
-	k8s.io/api v0.28.4
 	k8s.io/apimachinery v0.28.4
 	sigs.k8s.io/controller-tools v0.13.0
 )
@@ -73,6 +72,7 @@ require (
 	google.golang.org/grpc v1.59.0 // indirect
 	gopkg.in/inf.v0 v0.9.1 // indirect
 	gopkg.in/yaml.v2 v2.4.0 // indirect
+	k8s.io/api v0.28.4 // indirect
 	k8s.io/apiextensions-apiserver v0.28.3 // indirect
 	k8s.io/client-go v0.28.3 // indirect
 	k8s.io/klog/v2 v2.100.1 // indirect

go.sum

@@ -75,8 +75,6 @@ github.com/cncf/udpa/go v0.0.0-20200629203442-efcf912fb354/go.mod h1:WmhPx2Nbnht
 github.com/cncf/udpa/go v0.0.0-20201120205902-5459f2c99403/go.mod h1:WmhPx2Nbnhtbo57+VJT5O0JRkEi1Wbu0z5j0R8u5Hbk=
 github.com/cpuguy83/go-md2man/v2 v2.0.2/go.mod h1:tgQtvFlXSQOSOSIRvRPT7W67SCa46tRHOmNcaadrF8o=
 github.com/creack/pty v1.1.9/go.mod h1:oKZEueFk5CKHvIhNR5MUki03XCEU+Q6VDXinZuGJ33E=
-github.com/crossplane/crossplane-runtime v1.14.2 h1:pV5JMzyzi/kcbeVBVPCat5MHH8zS94MBUapAyGx/Ry0=
-github.com/crossplane/crossplane-runtime v1.14.2/go.mod h1:aOP+5W2wKpvthVs3pFNbVOe1jwrKYbJho0ThGNCVz9o=
 github.com/crossplane/crossplane-runtime v1.14.3 h1:YNGALph/UJTtQO+cJ9KGQ5NfALI5453PeE93Aqy9SWg=
 github.com/crossplane/crossplane-runtime v1.14.3/go.mod h1:aOP+5W2wKpvthVs3pFNbVOe1jwrKYbJho0ThGNCVz9o=
 github.com/crossplane/function-sdk-go v0.1.0 h1:WN3CUSaj6wgDqQPZZP1whMVIkTAZtN3HVCS67pTMzd8=
@@ -724,14 +722,10 @@ honnef.co/go/tools v0.0.0-20190523083050-ea95bdfd59fc/go.mod h1:rf3lG4BRIbNafJWh
 honnef.co/go/tools v0.0.1-2019.2.3/go.mod h1:a3bituU0lyd329TUQxRnasdCoJDkEUEAqEt0JzvZhAg=
 honnef.co/go/tools v0.0.1-2020.1.3/go.mod h1:X/FiERA/W4tHapMX5mGpAtMSVEeEUOyHaw9vFzvIQ3k=
 honnef.co/go/tools v0.0.1-2020.1.4/go.mod h1:X/FiERA/W4tHapMX5mGpAtMSVEeEUOyHaw9vFzvIQ3k=
-k8s.io/api v0.28.3 h1:Gj1HtbSdB4P08C8rs9AR94MfSGpRhJgsS+GF9V26xMM=
-k8s.io/api v0.28.3/go.mod h1:MRCV/jr1dW87/qJnZ57U5Pak65LGmQVkKTzf3AtKFHc=
 k8s.io/api v0.28.4 h1:8ZBrLjwosLl/NYgv1P7EQLqoO8MGQApnbgH8tu3BMzY=
 k8s.io/api v0.28.4/go.mod h1:axWTGrY88s/5YE+JSt4uUi6NMM+gur1en2REMR7IRj0=
 k8s.io/apiextensions-apiserver v0.28.3 h1:Od7DEnhXHnHPZG+W9I97/fSQkVpVPQx2diy+2EtmY08=
 k8s.io/apiextensions-apiserver v0.28.3/go.mod h1:NE1XJZ4On0hS11aWWJUTNkmVB03j9LM7gJSisbRt8Lc=
-k8s.io/apimachinery v0.28.3 h1:B1wYx8txOaCQG0HmYF6nbpU8dg6HvA06x5tEffvOe7A=
-k8s.io/apimachinery v0.28.3/go.mod h1:uQTKmIqs+rAYaq+DFaoD2X7pcjLOqbQX2AOiO0nIpb8=
 k8s.io/apimachinery v0.28.4 h1:zOSJe1mc+GxuMnFzD4Z/U1wst50X28ZNsn5bhgIIao8=
 k8s.io/apimachinery v0.28.4/go.mod h1:wI37ncBvfAoswfq626yPTe6Bz1c22L7uaJ8dho83mgg=
 k8s.io/client-go v0.28.3 h1:2OqNb72ZuTZPKCl+4gTKvqao0AMOl9f3o2ijbAj3LI4=

main.go

@@ -24,7 +24,11 @@ func (c *CLI) Run() error {
 		return err
 	}
 
-	return function.Serve(&Function{log: log},
+	return function.Serve(
+		&Function{
+			log:  log,
+			fsys: &osFS{},
+		},
 		function.Listen(c.Network, c.Address),
 		function.MTLSCertificates(c.TLSCertsDir),
 		function.Insecure(c.Insecure))

template.go

@@ -1,7 +1,7 @@
 package main
 
 import (
-	"os"
+	"io/fs"
 	"path/filepath"
 
 	"github.com/crossplane/function-sdk-go/errors"
@@ -18,12 +18,12 @@ type TemplateGetter interface {
 }
 
 // NewTemplateSourceGetter returns a TemplateGetter based on the cd source
-func NewTemplateSourceGetter(in *v1beta1.GoTemplate) (TemplateGetter, error) {
+func NewTemplateSourceGetter(fsys fs.FS, in *v1beta1.GoTemplate) (TemplateGetter, error) {
 	switch in.Source {
 	case v1beta1.InlineSource:
 		return newInlineSource(in)
 	case v1beta1.FileSystemSource:
-		return newFileSource(in)
+		return newFileSource(fsys, in)
 	case "":
 		return nil, errors.Errorf("source is required")
 	default:
@@ -62,14 +62,14 @@ func (fs *FileSource) GetTemplates() string {
 	return fs.Template
 }
 
-func newFileSource(in *v1beta1.GoTemplate) (*FileSource, error) {
+func newFileSource(fsys fs.FS, in *v1beta1.GoTemplate) (*FileSource, error) {
 	if in.FileSystem == nil || in.FileSystem.DirPath == "" {
 		return nil, errors.New("fileSystem.dirPath should be provided")
 	}
 
 	d := in.FileSystem.DirPath
 
-	tmpl, err := readTemplates(d)
+	tmpl, err := readTemplates(fsys, d)
 	if err != nil {
 		return nil, errors.Errorf("cannot read tmpl from the folder %s: %s", *in.FileSystem, err)
 	}
@@ -80,10 +80,10 @@ func newFileSource(in *v1beta1.GoTemplate) (*FileSource, error) {
 	}, nil
 }
 
-func readTemplates(dir string) (string, error) {
+func readTemplates(fsys fs.FS, dir string) (string, error) {
 	tmpl := ""
 
-	if err := filepath.WalkDir(dir, func(path string, dirEntry os.DirEntry, e error) error {
+	if err := fs.WalkDir(fsys, dir, func(path string, dirEntry fs.DirEntry, e error) error {
 		if e != nil {
 			return e
 		}
@@ -103,7 +103,7 @@ func readTemplates(dir string) (string, error) {
 			return nil
 		}
 
-		data, err := os.ReadFile(path)
+		data, err := fs.ReadFile(fsys, path)
 		if err != nil {
 			return err
 		}