exit DispatchWasiEventLoop without polling if tasks have been canceled

If one or more tasks have been canceled during the call to
`ThreadPoolWorkQueue.Dispatch`, one or more tasks of interest to the application
may have completed, so we return control immediately without polling, allowing
the app to exit if it chooses.

A practical example of this is in the SharedLibrary smoke test.  Without this
patch, that test will take over 100 seconds to complete, whereas with this patch
it completes in under a second.

Signed-off-by: Joel Dice <[email protected]>

src/libraries/System.Private.CoreLib/src/System/Threading/Wasi/WasiEventLoop.cs

@@ -14,6 +14,7 @@ internal static class WasiEventLoop
         // it will be leaked and stay in this list forever.
         // it will also keep the Pollable handle alive and prevent it from being disposed
         private static readonly List<PollableHolder> s_pollables = new();
+        private static bool s_tasksCanceled;
 
         internal static Task RegisterWasiPollableHandle(int handle, CancellationToken cancellationToken)
         {
@@ -35,6 +36,11 @@ internal static Task RegisterWasiPollable(Pollable pollable, CancellationToken c
         internal static void DispatchWasiEventLoop()
         {
             ThreadPoolWorkQueue.Dispatch();
+            if (s_tasksCanceled)
+            {
+                s_tasksCanceled = false;
+                return;
+            }
 
             var holders = new List<PollableHolder>(s_pollables.Count);
             var pending = new List<Pollable>(s_pollables.Count);
@@ -114,6 +120,11 @@ private static void CancelAndDispose(object? s)
                     return;
                 }
 
+                // Tell event loop to exit early, giving the application a
+                // chance to quit if the task(s) it is interested in have
+                // completed.
+                s_tasksCanceled = true;
+
                 // it will be removed from s_pollables on the next run
                 self.isDisposed = true;
                 self.pollable.Dispose();