Refactor string handling in util

The util module provides helper methods to deal with the C strings
returned by libnitrokey.  The current implementation has to problems:
- It causes unnecessary allocations if we only want to look at the
  string, for example in get_serial_number.
- If the conversion from a CStr to a String fails, the string pointer
  is not freed.

Therefore this patch introduces the run_with_str function that executes
a function with the string returned by libnitrokey and then makes sure
that the pointer is freed correctly.

1 files changed, 4 insertions, 3 deletions

diff --git a/src/device/mod.rs b/src/device/mod.rs
index 83ab90d..067fdf6 100644
--- a/src/device/mod.rs
+++ b/src/device/mod.rs
@@ -18,7 +18,7 @@ use crate::error::{CommunicationError, Error, LibraryError};
 use crate::otp::GenerateOtp;
 use crate::pws::GetPasswordSafe;
 use crate::util::{
-    get_command_result, get_cstring, owned_str_from_ptr, result_from_string, result_or_error,
+    get_command_result, get_cstring, owned_str_from_ptr, result_or_error, run_with_string,
 };
 
 pub use pro::Pro;
@@ -375,8 +375,9 @@ pub trait Device<'a>: Authenticate<'a> + GetPasswordSafe<'a> + GenerateOtp + fmt
     /// # }
     /// ```
     fn get_serial_number(&self) -> Result<SerialNumber, Error> {
-        result_from_string(unsafe { nitrokey_sys::NK_device_serial_number() })
-            .and_then(|s| s.parse())
+        run_with_string(unsafe { nitrokey_sys::NK_device_serial_number() }, |s| {
+            s.parse()
+        })
     }
 
     /// Returns the number of remaining authentication attempts for the user.  The total number of