Add `withMVarMasked` (re #8818)

Like `withMVar`, but the @IO@ action in the second argument is executed
with asynchronous exceptions masked. This completes the `MVar` API,
which already contained `modifyMVarMasked` and `modifyMVarMasked_`.
Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>

libraries/base/Control/Concurrent/MVar.hs

@@ -139,6 +139,7 @@ module Control.Concurrent.MVar
         , tryPutMVar
         , isEmptyMVar
         , withMVar
+        , withMVarMasked
         , modifyMVar_
         , modifyMVar
         , modifyMVarMasked_
@@ -188,6 +189,21 @@ withMVar m io =
     putMVar m a
     return b
 
+{-|
+  Like 'withMVar', but the @IO@ action in the second argument is executed
+  with asynchronous exceptions masked.
+
+  /Since: 4.7.0.0/
+-}
+{-# INLINE withMVarMasked #-}
+withMVarMasked :: MVar a -> (a -> IO b) -> IO b
+withMVarMasked m io =
+  mask_ $ do
+    a <- takeMVar m
+    b <- io a `onException` putMVar m a
+    putMVar m a
+    return b
+
 {-|
   An exception-safe wrapper for modifying the contents of an 'MVar'.
   Like 'withMVar', 'modifyMVar' will replace the original contents of

libraries/base/changelog.md

@@ -51,6 +51,10 @@
     `putMVar`.  There is also a new `tryReadMVar` which is a
     non-blocking version.
 
+  * New `Control.Concurrent.MVar.withMVarMasked` which executes
+    `IO` action with asynchronous exceptions masked in the same style
+    as the existing `modifyMVarMasked` and `modifyMVarMasked_`.
+
   * New `threadWait{Read,Write}STM :: Fd -> IO (STM (), IO ())`
     functions added to `Control.Concurrent` for waiting on FD
     readiness with STM actions.