Protect against double Dispose() issues

If the underlying timer were already disposed and we tried to dispose it again,
the subsequent wait would block indefinitely.

Author: mqudsi

SqliteCache/SqliteCache.cs

@@ -71,8 +71,13 @@ public void Dispose()
                 // EventWaitHandle.Set(SafeWaitHandle) method, which is just a wrapper around Kernel32's
                 // SetEvent() -- all of which is to say, we can't use a ManualResetEventSlim here.
                 using var resetEvent = new ManualResetEvent(false);
-                _cleanupTimer.Dispose(resetEvent);
-                resetEvent.WaitOne();
+                // If the timer has already been disposed, it'll return false immediately without setting the
+                // event. Since we don't really protect against our own .Dispose() method being called twice
+                // (maybe in a race), we should handle this eventuality.
+                if (_cleanupTimer.Dispose(resetEvent))
+                {
+                    resetEvent.WaitOne();
+                }
             }
             Commands.Dispose();