Fix possible arithmetic overflow in libp2p-kad. (libp2p#1291)

When the number of active queries exceeds the (internal)
JOBS_MAX_QUERIES limit, which is only supposed to bound
the number of concurrent queries relating to background
jobs, an arithmetic overflow occurs. This is fixed by
using saturating subtraction.

protocols/kad/src/behaviour.rs

@@ -1313,7 +1313,7 @@ where
         let now = Instant::now();
 
         // Calculate the available capacity for queries triggered by background jobs.
-        let mut jobs_query_capacity = JOBS_MAX_QUERIES - self.queries.size();
+        let mut jobs_query_capacity = JOBS_MAX_QUERIES.saturating_sub(self.queries.size());
 
         // Run the periodic provider announcement job.
         if let Some(mut job) = self.add_provider_job.take() {

protocols/kad/src/behaviour/test.rs

@@ -623,3 +623,34 @@ fn add_provider() {
     QuickCheck::new().tests(3).quickcheck(prop as fn(_,_))
 }
 
+/// User code should be able to start queries beyond the internal
+/// query limit for background jobs. Originally this even produced an
+/// arithmetic overflow, see https://github.com/libp2p/rust-libp2p/issues/1290.
+#[test]
+fn exceed_jobs_max_queries() {
+    let (_, mut swarms) = build_nodes(1);
+    let num = JOBS_MAX_QUERIES + 1;
+    for _ in 0 .. num {
+        swarms[0].bootstrap();
+    }
+
+    assert_eq!(swarms[0].queries.size(), num);
+
+    current_thread::run(
+        future::poll_fn(move || {
+            for _ in 0 .. num {
+                // There are no other nodes, so the queries finish instantly.
+                if let Ok(Async::Ready(Some(e))) = swarms[0].poll() {
+                    if let KademliaEvent::BootstrapResult(r) = e {
+                        assert!(r.is_ok(), "Unexpected error")
+                    } else {
+                        panic!("Unexpected event: {:?}", e)
+                    }
+                } else {
+                    panic!("Expected event")
+                }
+            }
+            Ok(Async::Ready(()))
+        }))
+}
+