TUN-3581: Tunnels can be run by name using only --credentials-file, no

origin cert necessary.

cmd/cloudflared/tunnel/filesystem.go

@@ -0,0 +1,27 @@
+package tunnel
+
+import (
+	"io/ioutil"
+	"os"
+)
+
+// Abstract away details of reading files, so that SubcommandContext can read
+// from either the real filesystem, or a mock (when running unit tests).
+type fileSystem interface {
+	readFile(filePath string) ([]byte, error)
+	validFilePath(path string) bool
+}
+
+type realFileSystem struct{}
+
+func (fs realFileSystem) validFilePath(path string) bool {
+	fileStat, err := os.Stat(path)
+	if err != nil {
+		return false
+	}
+	return !fileStat.IsDir()
+}
+
+func (fs realFileSystem) readFile(filePath string) ([]byte, error) {
+	return ioutil.ReadFile(filePath)
+}