Fix defaultFs relative path check for windows

Author: aldas

echo_fs_go1.16.go

@@ -10,6 +10,7 @@ import (
 	"net/url"
 	"os"
 	"path/filepath"
+	"runtime"
 	"strings"
 )
 
@@ -127,7 +128,7 @@ func subFS(currentFs fs.FS, root string) (fs.FS, error) {
 		// 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] != '/' {
+		if isRelativePath(root) {
 			root = filepath.Join(dFS.prefix, root)
 		}
 		return &defaultFS{
@@ -138,6 +139,21 @@ func subFS(currentFs fs.FS, root string) (fs.FS, error) {
 	return fs.Sub(currentFs, root)
 }
 
+func isRelativePath(path string) bool {
+	if path == "" {
+		return true
+	}
+	if path[0] == '/' {
+		return false
+	}
+	if runtime.GOOS == "windows" && strings.IndexByte(path, ':') != -1 {
+		// https://docs.microsoft.com/en-us/windows/win32/fileio/naming-a-file?redirectedfrom=MSDN#file_and_directory_names
+		// https://docs.microsoft.com/en-us/dotnet/standard/io/file-path-formats
+		return false
+	}
+	return true
+}
+
 // MustSubFS creates sub FS from current filesystem or panic on failure.
 // Panic happens when `fsRoot` contains invalid path according to `fs.ValidPath` rules.
 //