protover: Fix memleak in Rust impl of protover_compute_for_old_tor.

 * FIXES #25127: https://bugs.torproject.org/25127
 * ADDS a new module to the Rust tor_util crate for small utilities
   for working with static strings between languages.
 * CHANGES the return type of protover_compute_for_old_tor to point to
   immutable data.
 * CHANGES the code from the previous commit to use the new static
   string utilities.

1 files changed, 14 insertions, 19 deletions

diff --git a/src/rust/protover/protover.rs b/src/rust/protover/protover.rs
index 00fe084083..1680d3394e 100644
--- a/src/rust/protover/protover.rs
+++ b/src/rust/protover/protover.rs
@@ -10,6 +10,7 @@ use std::collections::{HashMap, HashSet};
 use std::ops::Range;
 use std::string::String;
 
+use tor_util::strings::NUL_BYTE;
 
 /// The first version of Tor that included "proto" entries in its descriptors.
 /// Authorities should use this to decide whether to guess proto lines.
@@ -724,11 +725,11 @@ pub fn is_supported_here(proto: Proto, vers: u32) -> bool {
 ///
 /// # Inputs
 ///
-/// * `version`, a string comprised of "[0-9,-]"
+/// * `version`, a string comprised of "[0-9a-z.-]"
 ///
 /// # Returns
 ///
-/// A `String` whose value is series of pairs, comprising of the protocol name
+/// A `&'static [u8]` whose value is series of pairs, comprising of the protocol name
 /// and versions that it supports. The string takes the following format:
 ///
 /// "HSDir=1-1 LinkAuth=1"
@@ -737,33 +738,27 @@ pub fn is_supported_here(proto: Proto, vers: u32) -> bool {
 /// only for tor versions older than FIRST_TOR_VERSION_TO_ADVERTISE_PROTOCOLS.
 ///
 /// C_RUST_COUPLED: src/rust/protover.c `compute_for_old_tor`
-pub fn compute_for_old_tor(version: &str) -> String {
-    if c_tor_version_as_new_as(
-        version,
-        FIRST_TOR_VERSION_TO_ADVERTISE_PROTOCOLS,
-    )
-    {
-        return String::new();
+pub fn compute_for_old_tor(version: &str) -> &'static [u8] {
+    if c_tor_version_as_new_as(version, FIRST_TOR_VERSION_TO_ADVERTISE_PROTOCOLS) {
+        return NUL_BYTE;
     }
 
     if c_tor_version_as_new_as(version, "0.2.9.1-alpha") {
-        let ret = "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";
-        return String::from(ret);
+        return b"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\0";
     }
 
     if c_tor_version_as_new_as(version, "0.2.7.5") {
-        let ret = "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";
-        return String::from(ret);
+        return b"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\0";
     }
 
     if c_tor_version_as_new_as(version, "0.2.4.19") {
-        let ret = "Cons=1 Desc=1 DirCache=1 HSDir=1 HSIntro=3 HSRend=1 \
-                   Link=1-4 LinkAuth=1 Microdesc=1 Relay=1-2";
-        return String::from(ret);
+        return b"Cons=1 Desc=1 DirCache=1 HSDir=1 HSIntro=3 HSRend=1 \
+                 Link=1-4 LinkAuth=1 Microdesc=1 Relay=1-2\0";
     }
-    String::new()
+
+    NUL_BYTE
 }
 
 #[cfg(test)]