Fix e.File(), c.Attachment(), being picky with relative paths after 4.7.0 introduced echo.Fs support

Author: aldas

echo_fs_go1.16.go

@@ -94,10 +94,12 @@ func StaticFileHandler(file string, filesystem fs.FS) HandlerFunc {
 	}
 }
 
-// defaultFS emulates os.Open behaviour with filesystem opened by `os.DirFs`. Difference between `os.Open` and `fs.Open`
-// is that FS does not allow to open path that start with `..` or `/` etc. For example previously you could have `../images`
-// in your application but `fs := os.DirFS("./")` would not allow you to use `fs.Open("../images")` and this would break
-// all old applications that rely on being able to traverse up from current executable run path.
+// defaultFS exists to preserve pre v4.7.0 behaviour where files were open by `os.Open`.
+// v4.7 introduced `echo.Filesystem` field which is Go1.16+ `fs.Fs` interface.
+// Difference between `os.Open` and `fs.Open` is that FS does not allow opening path that start with `.`, `..` or `/`
+// etc. For example previously you could have `../images` in your application but `fs := os.DirFS("./")` would not
+// allow you to use `fs.Open("../images")` and this would break all old applications that rely on being able to
+// traverse up from current executable run path.
 // NB: private because you really should use fs.FS implementation instances
 type defaultFS struct {
 	prefix string
@@ -108,20 +110,26 @@ func newDefaultFS() *defaultFS {
 	dir, _ := os.Getwd()
 	return &defaultFS{
 		prefix: dir,
-		fs:     os.DirFS(dir),
+		fs:     nil,
 	}
 }
 
 func (fs defaultFS) Open(name string) (fs.File, error) {
-	return fs.fs.Open(filepath.ToSlash(filepath.Clean(name)))
+	if fs.fs == nil {
+		return os.Open(name)
+	}
+	return fs.fs.Open(name)
 }
 
 func subFS(currentFs fs.FS, root string) (fs.FS, error) {
 	root = filepath.ToSlash(filepath.Clean(root)) // note: fs.FS operates only with slashes. `ToSlash` is necessary for Windows
 	if dFS, ok := currentFs.(*defaultFS); ok {
-		// we need to make exception for `defaultFS` instances as it interprets root prefix differently from fs.FS to
-		// allow cases when root is given as `../somepath` which is not valid for fs.FS
-		root = filepath.Join(dFS.prefix, root)
+		// we need to make exception for `defaultFS` instances as it interprets root prefix differently from fs.FS.
+		// fs.Fs.Open does not like relative paths ("./", "../") and absolute paths at all but prior echo.Filesystem we
+		// were able to use paths like `./myfile.log`, `/etc/hosts` and these would work fine with `os.Open` but not with fs.Fs
+		if len(root) > 0 && root[0] != '/' {
+			root = filepath.Join(dFS.prefix, root)
+		}
 		return &defaultFS{
 			prefix: root,
 			fs:     os.DirFS(root),