rust: Refactor protover::compute_for_old_tor().

During code review and discussion with Chelsea Komlo, she pointed out
that protover::compute_for_old_tor() was a public function whose
return type was `&'static CStr`.  We both agree that C-like parts of
APIs should:

1. not be exposed publicly (to other Rust crates),
2. only be called in the appropriate FFI code,
3. not expose types which are meant for FFI code (e.g. `*mut char`,
   `CString`, `*const c_int`, etc.) to the pure-Rust code of other
   crates.
4. FFI code (e.g. things in `ffi.rs` modules) should _never_ be called
   from pure-Rust, not even from other modules in its own crate
   (i.e. do not call `protover::ffi::*` from anywhere in
   `protover::protoset::*`, etc).

With that in mind, this commit makes the following changes:

 * CHANGE `protover::compute_for_old_tor()` to be
   visible only at the `pub(crate)` level.
 * RENAME `protover::compute_for_old_tor()` to
   `protover::compute_for_old_tor_cstr()` to reflect the last change.
 * ADD a new `protover::compute_for_old_tor()` function wrapper which
   is public and intended for other Rust code to use, which returns a
   `&str`.

src/rust/protover/ffi.rs

@@ -231,6 +231,6 @@ pub extern "C" fn protover_compute_for_old_tor(version: *const c_char) -> *const
         Err(_) => return empty.as_ptr(),
     };
 
-    supported = compute_for_old_tor(&version);
+    supported = compute_for_old_tor_cstr(&version);
     supported.as_ptr()
 }

src/rust/protover/protover.rs

@@ -600,8 +600,8 @@ pub fn is_supported_here(proto: &Protocol, vers: &Version) -> bool {
     supported_versions.contains(vers)
 }
 
-/// Older versions of Tor cannot infer their own subprotocols
-/// Used to determine which subprotocols are supported by older Tor versions.
+/// Since older versions of Tor cannot infer their own subprotocols,
+/// determine which subprotocols are supported by older Tor versions.
 ///
 /// # Inputs
 ///
@@ -615,34 +615,70 @@ pub fn is_supported_here(proto: &Protocol, vers: &Version) -> bool {
 /// "HSDir=1-1 LinkAuth=1"
 ///
 /// This function returns the protocols that are supported by the version input,
-/// only for tor versions older than FIRST_TOR_VERSION_TO_ADVERTISE_PROTOCOLS.
+/// only for tor versions older than `FIRST_TOR_VERSION_TO_ADVERTISE_PROTOCOLS`
+/// (but not older than 0.2.4.19).  For newer tors (or older than 0.2.4.19), it
+/// returns an empty string.
 ///
-/// C_RUST_COUPLED: src/rust/protover.c `compute_for_old_tor`
-pub fn compute_for_old_tor(version: &str) -> &'static CStr {
+/// # Note
+///
+/// This function is meant to be called for/within FFI code.  If you'd
+/// like to use this code in Rust, please see `compute_for_old_tor()`.
+//
+// C_RUST_COUPLED: src/rust/protover.c `compute_for_old_tor`
+pub(crate) fn compute_for_old_tor_cstr(version: &str) -> &'static CStr {
     let empty: &'static CStr = cstr!("");
 
     if c_tor_version_as_new_as(version, FIRST_TOR_VERSION_TO_ADVERTISE_PROTOCOLS) {
         return empty;
     }
-
     if c_tor_version_as_new_as(version, "0.2.9.1-alpha") {
         return cstr!("Cons=1-2 Desc=1-2 DirCache=1 HSDir=1 HSIntro=3 HSRend=1-2 \
                       Link=1-4 LinkAuth=1 Microdesc=1-2 Relay=1-2");
     }
-
     if c_tor_version_as_new_as(version, "0.2.7.5") {
         return cstr!("Cons=1-2 Desc=1-2 DirCache=1 HSDir=1 HSIntro=3 HSRend=1 \
                       Link=1-4 LinkAuth=1 Microdesc=1-2 Relay=1-2");
     }
-
     if c_tor_version_as_new_as(version, "0.2.4.19") {
         return cstr!("Cons=1 Desc=1 DirCache=1 HSDir=1 HSIntro=3 HSRend=1 \
                       Link=1-4 LinkAuth=1 Microdesc=1 Relay=1-2");
     }
-
     empty
 }
 
+/// Since older versions of Tor cannot infer their own subprotocols,
+/// determine which subprotocols are supported by older Tor versions.
+///
+/// # Inputs
+///
+/// * `version`, a string comprised of "[0-9a-z.-]"
+///
+/// # Returns
+///
+/// A `Result` whose `Ok` value is an `&'static str` encoding a list of protocol
+/// names and supported versions. The string takes the following format:
+///
+/// "HSDir=1-1 LinkAuth=1"
+///
+/// This function returns the protocols that are supported by the version input,
+/// only for tor versions older than `FIRST_TOR_VERSION_TO_ADVERTISE_PROTOCOLS`.
+/// (but not older than 0.2.4.19).  For newer tors (or older than 0.2.4.19), its
+/// `Ok` `Result` contains an empty string.
+///
+/// Otherwise, its `Err` contains a `ProtoverError::Unparseable` if the
+/// `version` string was invalid utf-8.
+///
+/// # Note
+///
+/// This function is meant to be called for/within non-FFI Rust code.
+//
+// C_RUST_COUPLED: src/rust/protover.c `compute_for_old_tor`
+pub fn compute_for_old_tor(version: &str) -> Result<&'static str, ProtoverError> {
+    // .to_str() fails with a Utf8Error if it couldn't validate the
+    // utf-8, so convert that here into an Unparseable ProtoverError.
+    compute_for_old_tor_cstr(version).to_str().or(Err(ProtoverError::Unparseable))
+}
+
 #[cfg(test)]
 mod test {
     use std::str::FromStr;