add urandom-channel to the benchmarks

bench/dune

@@ -2,7 +2,7 @@
  (names speed)
  (modules speed)
  (libraries mirage-crypto mirage-crypto-rng mirage-crypto-rng.unix
-   mirage-crypto-pk mirage-crypto-ec mirage-crypto-rng-miou-unix))
+   mirage-crypto-pk mirage-crypto-ec mirage-crypto-rng-miou-unix threads.posix))
 
 ; marking as "(optional)" leads to OCaml-CI failures
 ; marking with "(package mirage-crypto-rng-miou-unix)" only has an effect with a "public_name"

bench/speed.ml

@@ -502,6 +502,15 @@ let benchmarks = [
     throughput name (fun buf ->
         let buf = Bytes.unsafe_of_string buf in
         Mirage_crypto_rng_unix.getrandom_into buf ~off:0 ~len:(Bytes.length buf))) ;
+
+  bm "urandom-channel" (fun name ->
+    In_channel.with_open_bin "/dev/urandom" @@ fun ic ->
+    let m = Mutex.create () in
+    let finally () = Mutex.unlock m in
+    throughput name (fun buf ->
+        let buf = Bytes.unsafe_of_string buf in
+        Mutex.lock m;
+        Fun.protect ~finally (fun () -> really_input ic buf 0 (Bytes.length buf))));
 ]
 
 let help () =