quinn-proto: make cids_exhausted overflow-safe on 32-bit

Author: KevinRoebert

quinn-proto/src/endpoint.rs

@@ -913,11 +913,18 @@ impl Endpoint {
     /// We leave some space unused so that `new_cid` can be relied upon to finish quickly. We don't
     /// bother to check when CID longer than 4 bytes are used because 2^40 connections is a lot.
     fn cids_exhausted(&self) -> bool {
-        self.local_cid_generator.cid_len() <= 4
-            && self.local_cid_generator.cid_len() != 0
-            && (2usize.pow(self.local_cid_generator.cid_len() as u32 * 8)
-                - self.index.connection_ids.len())
-                < 2usize.pow(self.local_cid_generator.cid_len() as u32 * 8 - 2)
+        let cid_len = self.local_cid_generator.cid_len();
+        if cid_len == 0 || cid_len > 4 {
+            return false;
+        }
+
+        // Keep this architecture-independent: on 32-bit targets, 2usize.pow(32) overflows.
+        let bits = (cid_len * 8) as u32;
+        let space = 1u64 << bits;
+        let reserve = 1u64 << (bits - 2);
+        let len = self.index.connection_ids.len() as u64;
+
+        len > (space - reserve)
     }
 }