chore: check against errors of `io/fs` instead of `os` (#6011)

* chore: replace `os.ErrNotExist` with `fs.ErrNotExist`

* check against permission error from `io/fs` package

7 files changed, 23 insertions, 22 deletions

diff --git a/caddy.go b/caddy.go
index 4a7bd8b6d..cd8af5fe2 100644
--- a/caddy.go
+++ b/caddy.go
@@ -22,6 +22,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"log"
 	"net/http"
 	"os"
@@ -828,7 +829,7 @@ func InstanceID() (uuid.UUID, error) {
 	appDataDir := AppDataDir()
 	uuidFilePath := filepath.Join(appDataDir, "instance.uuid")
 	uuidFileBytes, err := os.ReadFile(uuidFilePath)
-	if os.IsNotExist(err) {
+	if errors.Is(err, fs.ErrNotExist) {
 		uuid, err := uuid.NewRandom()
 		if err != nil {
 			return uuid, err
diff --git a/caddytest/caddytest.go b/caddytest/caddytest.go
index 39aca2347..26d3de660 100644
--- a/caddytest/caddytest.go
+++ b/caddytest/caddytest.go
@@ -8,6 +8,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"log"
 	"net"
 	"net/http"
@@ -232,7 +233,7 @@ const initConfig = `{
 func validateTestPrerequisites(t *testing.T) error {
 	// check certificates are found
 	for _, certName := range Default.Certifcates {
-		if _, err := os.Stat(getIntegrationDir() + certName); os.IsNotExist(err) {
+		if _, err := os.Stat(getIntegrationDir() + certName); errors.Is(err, fs.ErrNotExist) {
 			return fmt.Errorf("caddy integration test certificates (%s) not found", certName)
 		}
 	}
diff --git a/cmd/commandfuncs.go b/cmd/commandfuncs.go
index b0c576a3d..746cf3da6 100644
--- a/cmd/commandfuncs.go
+++ b/cmd/commandfuncs.go
@@ -190,7 +190,7 @@ func cmdRun(fl Flags) (int, error) {
 	var config []byte
 	if resumeFlag {
 		config, err = os.ReadFile(caddy.ConfigAutosavePath)
-		if os.IsNotExist(err) {
+		if errors.Is(err, fs.ErrNotExist) {
 			// not a bad error; just can't resume if autosave file doesn't exist
 			caddy.Log().Info("no autosave file exists", zap.String("autosave_file", caddy.ConfigAutosavePath))
 			resumeFlag = false
diff --git a/cmd/main.go b/cmd/main.go
index fa15c0874..73d70f04d 100644
--- a/cmd/main.go
+++ b/cmd/main.go
@@ -21,6 +21,7 @@ import (
 	"flag"
 	"fmt"
 	"io"
+	"io/fs"
 	"log"
 	"net"
 	"os"
@@ -128,7 +129,7 @@ func loadConfigWithLogger(logger *zap.Logger, configFile, adapterName string) ([
 		cfgAdapter = caddyconfig.GetAdapter("caddyfile")
 		if cfgAdapter != nil {
 			config, err = os.ReadFile("Caddyfile")
-			if os.IsNotExist(err) {
+			if errors.Is(err, fs.ErrNotExist) {
 				// okay, no default Caddyfile; pretend like this never happened
 				cfgAdapter = nil
 			} else if err != nil {
diff --git a/modules/caddyhttp/fileserver/browse.go b/modules/caddyhttp/fileserver/browse.go
index 81eb08592..492d52d65 100644
--- a/modules/caddyhttp/fileserver/browse.go
+++ b/modules/caddyhttp/fileserver/browse.go
@@ -19,6 +19,7 @@ import (
 	"context"
 	_ "embed"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"io/fs"
@@ -92,9 +93,9 @@ func (fsrv *FileServer) serveBrowse(root, dirPath string, w http.ResponseWriter,
 	// TODO: not entirely sure if path.Clean() is necessary here but seems like a safe plan (i.e. /%2e%2e%2f) - someone could verify this
 	listing, err := fsrv.loadDirectoryContents(r.Context(), dir.(fs.ReadDirFile), root, path.Clean(r.URL.EscapedPath()), repl)
 	switch {
-	case os.IsPermission(err):
+	case errors.Is(err, fs.ErrPermission):
 		return caddyhttp.Error(http.StatusForbidden, err)
-	case os.IsNotExist(err):
+	case errors.Is(err, fs.ErrNotExist):
 		return fsrv.notFound(w, r, next)
 	case err != nil:
 		return caddyhttp.Error(http.StatusInternalServerError, err)
diff --git a/modules/caddyhttp/fileserver/staticfiles.go b/modules/caddyhttp/fileserver/staticfiles.go
index 0ed558e8b..aa582eac0 100644
--- a/modules/caddyhttp/fileserver/staticfiles.go
+++ b/modules/caddyhttp/fileserver/staticfiles.go
@@ -506,10 +506,10 @@ func (fsrv *FileServer) openFile(filename string, w http.ResponseWriter) (fs.Fil
 	file, err := fsrv.fileSystem.Open(filename)
 	if err != nil {
 		err = fsrv.mapDirOpenError(err, filename)
-		if os.IsNotExist(err) {
+		if errors.Is(err, fs.ErrNotExist) {
 			fsrv.logger.Debug("file not found", zap.String("filename", filename), zap.Error(err))
 			return nil, caddyhttp.Error(http.StatusNotFound, err)
-		} else if os.IsPermission(err) {
+		} else if errors.Is(err, fs.ErrPermission) {
 			fsrv.logger.Debug("permission denied", zap.String("filename", filename), zap.Error(err))
 			return nil, caddyhttp.Error(http.StatusForbidden, err)
 		}
diff --git a/modules/caddyhttp/templates/tplcontext_test.go b/modules/caddyhttp/templates/tplcontext_test.go
index fdf2c1065..67ebbac70 100644
--- a/modules/caddyhttp/templates/tplcontext_test.go
+++ b/modules/caddyhttp/templates/tplcontext_test.go
@@ -17,7 +17,9 @@ package templates
 import (
 	"bytes"
 	"context"
+	"errors"
 	"fmt"
+	"io/fs"
 	"net/http"
 	"os"
 	"path/filepath"
@@ -30,8 +32,7 @@ import (
 	"github.com/caddyserver/caddy/v2/modules/caddyhttp"
 )
 
-type handle struct {
-}
+type handle struct{}
 
 func (h *handle) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	if r.Header.Get("Accept-Encoding") == "identity" {
@@ -176,11 +177,10 @@ func TestImport(t *testing.T) {
 		} else if !templateWasDefined && actual != "" {
 			// template should be defined, return value should be an empty string
 			t.Errorf("Test %d: Expected template %s to be define but got %s", i, test.expect, tplContext.tpl.DefinedTemplates())
-
 		}
 
 		if absFilePath != "" {
-			if err := os.Remove(absFilePath); err != nil && !os.IsNotExist(err) {
+			if err := os.Remove(absFilePath); err != nil && !errors.Is(err, fs.ErrNotExist) {
 				t.Fatalf("Test %d: Expected no error removing temporary test file, got: %v", i, err)
 			}
 		}
@@ -255,21 +255,20 @@ func TestNestedInclude(t *testing.T) {
 		} else if buf.String() != test.expect {
 			//
 			t.Errorf("Test %d: Expected '%s' but got '%s'", i, test.expect, buf.String())
-
 		}
 
 		if absFilePath != "" {
-			if err := os.Remove(absFilePath); err != nil && !os.IsNotExist(err) {
+			if err := os.Remove(absFilePath); err != nil && !errors.Is(err, fs.ErrNotExist) {
 				t.Fatalf("Test %d: Expected no error removing temporary test file, got: %v", i, err)
 			}
 		}
 		if absFilePath0 != "" {
-			if err := os.Remove(absFilePath0); err != nil && !os.IsNotExist(err) {
+			if err := os.Remove(absFilePath0); err != nil && !errors.Is(err, fs.ErrNotExist) {
 				t.Fatalf("Test %d: Expected no error removing temporary test file, got: %v", i, err)
 			}
 		}
 		if absFilePath1 != "" {
-			if err := os.Remove(absFilePath1); err != nil && !os.IsNotExist(err) {
+			if err := os.Remove(absFilePath1); err != nil && !errors.Is(err, fs.ErrNotExist) {
 				t.Fatalf("Test %d: Expected no error removing temporary test file, got: %v", i, err)
 			}
 		}
@@ -342,11 +341,10 @@ func TestInclude(t *testing.T) {
 			t.Errorf("Test %d: Expected error but had none", i)
 		} else if actual != test.expect {
 			t.Errorf("Test %d: Expected %s but got %s", i, test.expect, actual)
-
 		}
 
 		if absFilePath != "" {
-			if err := os.Remove(absFilePath); err != nil && !os.IsNotExist(err) {
+			if err := os.Remove(absFilePath); err != nil && !errors.Is(err, fs.ErrNotExist) {
 				t.Fatalf("Test %d: Expected no error removing temporary test file, got: %v", i, err)
 			}
 		}
@@ -460,7 +458,7 @@ func TestFileListing(t *testing.T) {
 			fileNames: nil,
 			inputBase: "doesNotExist",
 			shouldErr: true,
-			verifyErr: os.IsNotExist,
+			verifyErr: func(err error) bool { return errors.Is(err, fs.ErrNotExist) },
 		},
 		{
 			// directory and files exist, but path to a file
@@ -476,7 +474,7 @@ func TestFileListing(t *testing.T) {
 			fileNames: nil,
 			inputBase: filepath.Join("..", "..", "..", "..", "..", "etc"),
 			shouldErr: true,
-			verifyErr: os.IsNotExist,
+			verifyErr: func(err error) bool { return errors.Is(err, fs.ErrNotExist) },
 		},
 	} {
 		tplContext := getContextOrFail(t)
@@ -525,7 +523,7 @@ func TestFileListing(t *testing.T) {
 		}
 
 		if dirPath != "" {
-			if err := os.RemoveAll(dirPath); err != nil && !os.IsNotExist(err) {
+			if err := os.RemoveAll(dirPath); err != nil && !errors.Is(err, fs.ErrNotExist) {
 				t.Fatalf("Test %d: Expected no error removing temporary test directory, got: %v", i, err)
 			}
 		}
@@ -602,7 +600,6 @@ title = "Welcome"
 			t.Errorf("Test %d: Expected body %s, found %s. Input was SplitFrontMatter(%s)", i, test.body, result.Body, test.input)
 		}
 	}
-
 }
 
 func TestHumanize(t *testing.T) {