rust: Add new protover::UnknownProtocol type.

 * ADD new type, protover::UnknownProtocol, so that we have greater type safety
   and our protover functionality which works with unsanitised protocol names is
   more clearly demarcated.
 * REFACTOR protover::Proto, renaming it protover::Protocol to mirror the new
   protover::UnknownProtocol type name.
 * ADD a utility conversion of `impl From<Protocol> for UnknownProtocol` so that
   we can easily with known protocols and unknown protocols simultaneously
   (e.g. doing comparisons, checking their version numbers), while not allowing
   UnknownProtocols to be accidentally used in functions which should only take
   Protocols.
 * FIXES part of #24031: https://bugs.torproject.org/24031

2 files changed, 55 insertions, 28 deletions

diff --git a/src/rust/protover/ffi.rs b/src/rust/protover/ffi.rs
index 2ee0286ecf..ce28378326 100644
--- a/src/rust/protover/ffi.rs
+++ b/src/rust/protover/ffi.rs
@@ -9,30 +9,32 @@ use libc::{c_char, c_int, uint32_t};
 use std::ffi::CStr;
 use std::ffi::CString;
 
-use protover::*;
 use smartlist::*;
 use tor_allocate::allocate_and_copy_string;
 use tor_util::strings::byte_slice_is_c_like;
 use tor_util::strings::empty_static_cstr;
 
 
+use errors::ProtoverError;
+use protover::*;
+
 /// Translate C enums to Rust Proto enums, using the integer value of the C
-/// enum to map to its associated Rust enum
+/// enum to map to its associated Rust enum.
 ///
 /// C_RUST_COUPLED: src/or/protover.h `protocol_type_t`
-fn translate_to_rust(c_proto: uint32_t) -> Result<Proto, &'static str> {
+fn translate_to_rust(c_proto: uint32_t) -> Result<Protocol, ProtoverError> {
     match c_proto {
-        0 => Ok(Proto::Link),
-        1 => Ok(Proto::LinkAuth),
-        2 => Ok(Proto::Relay),
-        3 => Ok(Proto::DirCache),
-        4 => Ok(Proto::HSDir),
-        5 => Ok(Proto::HSIntro),
-        6 => Ok(Proto::HSRend),
-        7 => Ok(Proto::Desc),
-        8 => Ok(Proto::Microdesc),
-        9 => Ok(Proto::Cons),
-        _ => Err("Invalid protocol type"),
+        0 => Ok(Protocol::Link),
+        1 => Ok(Protocol::LinkAuth),
+        2 => Ok(Protocol::Relay),
+        3 => Ok(Protocol::DirCache),
+        4 => Ok(Protocol::HSDir),
+        5 => Ok(Protocol::HSIntro),
+        6 => Ok(Protocol::HSRend),
+        7 => Ok(Protocol::Desc),
+        8 => Ok(Protocol::Microdesc),
+        9 => Ok(Protocol::Cons),
+        _ => Err(ProtoverError::UnknownProtocol),
     }
 }
 
diff --git a/src/rust/protover/protover.rs b/src/rust/protover/protover.rs
index d74ca00c17..1ccad4af4a 100644
--- a/src/rust/protover/protover.rs
+++ b/src/rust/protover/protover.rs
@@ -56,8 +56,8 @@ pub(crate) const SUPPORTED_PROTOCOLS: &'static [u8] =
 /// Known subprotocols in Tor. Indicates which subprotocol a relay supports.
 ///
 /// C_RUST_COUPLED: src/or/protover.h `protocol_type_t`
-#[derive(Hash, Eq, PartialEq, Debug)]
-pub enum Proto {
+#[derive(Clone, Hash, Eq, PartialEq, Debug)]
+pub enum Protocol {
     Cons,
     Desc,
     DirCache,
@@ -70,7 +70,7 @@ pub enum Proto {
     Relay,
 }
 
-impl fmt::Display for Proto {
+impl fmt::Display for Protocol {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         write!(f, "{:?}", self)
     }
@@ -80,26 +80,51 @@ impl fmt::Display for Proto {
 /// Error if the string is an unrecognized protocol name.
 ///
 /// C_RUST_COUPLED: src/or/protover.c `PROTOCOL_NAMES`
-impl FromStr for Proto {
+impl FromStr for Protocol {
     type Err = ProtoverError;
 
     fn from_str(s: &str) -> Result<Self, Self::Err> {
         match s {
-            "Cons" => Ok(Proto::Cons),
-            "Desc" => Ok(Proto::Desc),
-            "DirCache" => Ok(Proto::DirCache),
-            "HSDir" => Ok(Proto::HSDir),
-            "HSIntro" => Ok(Proto::HSIntro),
-            "HSRend" => Ok(Proto::HSRend),
-            "Link" => Ok(Proto::Link),
-            "LinkAuth" => Ok(Proto::LinkAuth),
-            "Microdesc" => Ok(Proto::Microdesc),
-            "Relay" => Ok(Proto::Relay),
+            "Cons" => Ok(Protocol::Cons),
+            "Desc" => Ok(Protocol::Desc),
+            "DirCache" => Ok(Protocol::DirCache),
+            "HSDir" => Ok(Protocol::HSDir),
+            "HSIntro" => Ok(Protocol::HSIntro),
+            "HSRend" => Ok(Protocol::HSRend),
+            "Link" => Ok(Protocol::Link),
+            "LinkAuth" => Ok(Protocol::LinkAuth),
+            "Microdesc" => Ok(Protocol::Microdesc),
+            "Relay" => Ok(Protocol::Relay),
             _ => Err(ProtoverError::UnknownProtocol),
         }
     }
 }
 
+/// A protocol string which is not one of the `Protocols` we currently know
+/// about.
+#[derive(Clone, Debug, Hash, Eq, PartialEq)]
+pub struct UnknownProtocol(String);
+
+impl fmt::Display for UnknownProtocol {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+impl FromStr for UnknownProtocol {
+    type Err = ProtoverError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        Ok(UnknownProtocol(s.to_string()))
+    }
+}
+
+impl From<Protocol> for UnknownProtocol {
+    fn from(p: Protocol) -> UnknownProtocol {
+        UnknownProtocol(p.to_string())
+    }
+}
+
 /// Get the string representation of current supported protocols
 ///
 /// # Returns