Fix Condition.await bug when cancelling

lib_eio/condition.ml

@@ -2,6 +2,9 @@ type t = Broadcast.t
 
 let create () = Broadcast.create ()
 
+let lock_protected m =
+  Cancel.protect (fun () -> Eio_mutex.lock m)
+
 let await_generic ?mutex t =
   match
     Suspend.enter_unchecked (fun ctx enqueue ->
@@ -21,10 +24,10 @@ let await_generic ?mutex t =
               )
       )
   with
-  | () -> Option.iter Eio_mutex.lock mutex
+  | () -> Option.iter lock_protected mutex
   | exception ex ->
     let bt = Printexc.get_raw_backtrace () in
-    Option.iter Eio_mutex.lock mutex;
+    Option.iter lock_protected mutex;
     Printexc.raise_with_backtrace ex bt
 
 let await t mutex = await_generic ~mutex t

tests/condition.md

@@ -202,3 +202,38 @@ Cancellation while waiting:
 +x = 0
 - : unit = ()
 ```
+
+### Cancelling while the mutex is held
+
+`await` must always re-acquire the lock, and that lock operation must be non-cancellable:
+
+```ocaml
+# Eio_mock.Backend.run @@ fun () ->
+  Switch.run @@ fun sw ->
+  Fiber.fork ~sw
+    (fun () ->
+       traceln "Forked fiber locking";
+       Eio.Mutex.lock mutex;
+       try
+         Eio.Condition.await cond mutex;
+         assert false;
+       with Eio.Cancel.Cancelled _ as ex ->
+         traceln "Forked fiber unlocking";
+         Eio.Mutex.unlock mutex;
+         raise ex
+    );
+  Eio.Cancel.protect
+    (fun () ->
+       traceln "Main fiber locking";
+       Eio.Mutex.lock mutex;
+       Switch.fail sw (Failure "Simulated error");
+       Fiber.yield ();
+       traceln "Main fiber unlocking";
+       Eio.Mutex.unlock mutex;
+    )
++Forked fiber locking
++Main fiber locking
++Main fiber unlocking
++Forked fiber unlocking
+Exception: Failure "Simulated error".
+```