Add Broker.peek_next_client_id (#1468)

* Add Broker::next_client_id

* rename to peek_

* Undo change, probably not better

libafl_bolts/src/llmp.rs

@@ -1907,6 +1907,8 @@ where
     /// Users of Llmp can add message handlers in the broker.
     /// This allows us to intercept messages right in the broker.
     /// This keeps the out map clean.
+    /// The backing values of `llmp_clients` [`ClientId`]s will always be sorted (but not gapless)
+    /// Make sure to always increase `num_clients_total` when pushing a new [`LlmpReceiver`] to  `llmp_clients`!
     pub llmp_clients: Vec<LlmpReceiver<SP>>,
     /// The own listeners we spawned via `launch_listener` or `crate_attach_to_tcp`.
     /// Listeners will be ignored for `exit_cleanly_after` and they are never considered to have timed out.
@@ -1982,6 +1984,20 @@ where
         })
     }
 
+    /// Gets the [`ClientId`] the next client attaching to this broker will get.
+    /// In its current implememtation, the inner value of the next [`ClientId`]
+    /// is equal to `self.num_clients_total`.
+    /// Calling `peek_next_client_id` mutliple times (without adding a client) will yield the same value.
+    #[must_use]
+    #[inline]
+    pub fn peek_next_client_id(&self) -> ClientId {
+        ClientId(
+            self.num_clients_total
+                .try_into()
+                .expect("More than u32::MAX clients!"),
+        )
+    }
+
     /// Create a new [`LlmpBroker`] attaching to a TCP port
     #[cfg(feature = "std")]
     pub fn create_attach_to_tcp(shmem_provider: SP, port: u16) -> Result<Self, Error> {
@@ -2026,7 +2042,7 @@ where
         // Since we now have a handle to it, it won't be umapped too early (only after we also unmap it)
         client_page.mark_safe_to_unmap();
 
-        let id = ClientId(self.num_clients_total.try_into().unwrap());
+        let id = self.peek_next_client_id();
         self.llmp_clients.push(LlmpReceiver {
             id,
             current_recv_shmem: client_page,
@@ -2037,8 +2053,8 @@ where
             #[cfg(feature = "std")]
             last_msg_time: current_time(),
         });
-
         self.num_clients_total += 1;
+
         id
     }
 
@@ -2090,7 +2106,7 @@ where
         // TODO: handle broker_ids properly/at all.
         let map_description = Self::b2b_thread_on(
             stream,
-            ClientId(self.llmp_clients.len() as u32),
+            self.peek_next_client_id(),
             &self
                 .llmp_out
                 .out_shmems
@@ -2577,7 +2593,7 @@ where
             hostname,
         };
 
-        let llmp_tcp_id = ClientId(self.llmp_clients.len() as u32);
+        let llmp_tcp_id = self.peek_next_client_id();
 
         // Tcp out map sends messages from background thread tcp server to foreground client
         let tcp_out_shmem = LlmpSharedMap::new(
@@ -2675,7 +2691,6 @@ where
         F: FnMut(ClientId, Tag, Flags, &[u8]) -> Result<LlmpMsgHookResult, Error>,
     {
         let mut new_messages = false;
-        let mut next_id = self.llmp_clients.len() as u32;
 
         // TODO: We could memcpy a range of pending messages, instead of one by one.
         loop {
@@ -2683,7 +2698,7 @@ where
                 let pos = if (client_id.0 as usize) < self.llmp_clients.len()
                     && self.llmp_clients[client_id.0 as usize].id == client_id
                 {
-                    // Fast path when no client was removed
+                    // Fast path when no client before this one was removed
                     client_id.0 as usize
                 } else {
                     self.llmp_clients
@@ -2736,11 +2751,11 @@ where
                     ) {
                         Ok(new_shmem) => {
                             let mut new_page = LlmpSharedMap::existing(new_shmem);
-                            let id = next_id;
-                            next_id += 1;
                             new_page.mark_safe_to_unmap();
+
+                            let id = self.peek_next_client_id();
                             self.llmp_clients.push(LlmpReceiver {
-                                id: ClientId(id),
+                                id,
                                 current_recv_shmem: new_page,
                                 last_msg_recvd: ptr::null_mut(),
                                 shmem_provider: self.shmem_provider.clone(),
@@ -2768,7 +2783,7 @@ where
                     let pos = if (client_id.0 as usize) < self.llmp_clients.len()
                         && self.llmp_clients[client_id.0 as usize].id == client_id
                     {
-                        // Fast path when no client was removed
+                        // Fast path when no client before this one was removed
                         client_id.0 as usize
                     } else {
                         self.llmp_clients