feat(mpz-garble): add getters for thread in pool (#114)

garble/mpz-garble/src/lib.rs

@@ -52,6 +52,8 @@ pub enum VmError {
     VerifyError(#[from] VerifyError),
     #[error(transparent)]
     DecodeError(#[from] DecodeError),
+    #[error("thread pools must have at least one thread")]
+    ThreadPoolEmpty,
     #[error("thread already exists: {0}")]
     ThreadAlreadyExists(String),
     #[error("vm is shutdown")]
@@ -161,11 +163,17 @@ pub trait Vm {
     async fn new_thread(&mut self, id: &str) -> Result<Self::Thread, VmError>;
 
     /// Creates a new thread pool.
+    ///
+    /// A thread pool must have at least one thread.
     async fn new_thread_pool(
         &mut self,
         id: &str,
         thread_count: usize,
     ) -> Result<ThreadPool<Self::Thread>, VmError> {
+        if thread_count == 0 {
+            return Err(VmError::ThreadPoolEmpty);
+        }
+
         let mut id = NestedId::new(id).append_counter();
         let mut threads = Vec::with_capacity(thread_count);
         for _ in 0..thread_count {

garble/mpz-garble/src/threadpool.rs

@@ -99,10 +99,28 @@ where
     T: Thread + 'static,
 {
     /// Creates a new thread pool.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the number of threads is zero.
     pub(crate) fn new(threads: Vec<T>) -> Self {
+        assert!(
+            !threads.is_empty(),
+            "thread pool must have at least one thread"
+        );
         Self { threads }
     }
 
+    /// Returns a reference to the first thread in the pool.
+    pub fn get(&self) -> &T {
+        &self.threads[0]
+    }
+
+    /// Returns a mutable reference to the first thread in the pool.
+    pub fn get_mut(&mut self) -> &mut T {
+        &mut self.threads[0]
+    }
+
     /// Returns a new thread pool scope.
     pub fn new_scope<R>(&mut self) -> Scope<'_, T, R> {
         Scope {