Add the failed queued requests back to available permits

Author: shreyaverma165

src/libraries/System.Threading.RateLimiting/src/System/Threading/RateLimiting/SlidingWindowRateLimiter.cs

@@ -241,8 +241,8 @@ private void ReplenishInternal(long nowTicks)
 
                 _lastReplenishmentTick = nowTicks;
 
-                // Increament the current segment index while move the window
-                // We need to know the no. of requests that were acquired in a segment perviously to ensure that we don't acquire more than the permit limit.
+                // Increment the current segment index while move the window
+                // We need to know the no. of requests that were acquired in a segment previously to ensure that we don't acquire more than the permit limit.
                 _currentSegmentIndex = (_currentSegmentIndex + 1) % _options.SegmentsPerWindow;
                 int oldSegmentRequestCount = _requestsPerSegment[_currentSegmentIndex];
                 _requestsPerSegment[_currentSegmentIndex] = 0;
@@ -280,7 +280,7 @@ private void ReplenishInternal(long nowTicks)
                         if (!nextPendingRequest.Tcs.TrySetResult(SuccessfulLease))
                         {
                             // Queued item was canceled so add count back
-                            _requestCount -= nextPendingRequest.Count;
+                            _requestCount += nextPendingRequest.Count;
                             _requestsPerSegment[_currentSegmentIndex] -= nextPendingRequest.Count;
                             // Updating queue count is handled by the cancellation code
                             _queueCount += nextPendingRequest.Count;

src/libraries/System.Threading.RateLimiting/tests/SlidingWindowRateLimiterTests.cs

@@ -63,8 +63,11 @@ public override async Task CanAcquireResourceAsync()
         }
 
         [Fact]
-        public async Task CanAcquireMulitpleRequestsAsync()
+        public async Task CanAcquireMultipleRequestsAsync()
         {
+            // This test verifies the following behavior
+            // 1. when we have available permits after replenish to serve the queued requests
+            // 2. when the oldest item from queue is remove to accomodate new requests (QueueProcessingOrder: NewestFirst)
             var limiter = new SlidingWindowRateLimiter(new SlidingWindowRateLimiterOptions(4, QueueProcessingOrder.NewestFirst, 4,
                 TimeSpan.Zero, 3, autoReplenishment: false));
 
@@ -456,14 +459,15 @@ public override async Task CancelUpdatesQueueLimit()
             var ex = await Assert.ThrowsAsync<TaskCanceledException>(() => wait.AsTask());
             Assert.Equal(cts.Token, ex.CancellationToken);
 
-            wait = limiter.WaitAsync();
+            wait = limiter.WaitAsync(1);
             Assert.False(wait.IsCompleted);
 
             limiter.TryReplenish();
             limiter.TryReplenish();
 
             lease = await wait;
             Assert.True(lease.IsAcquired);
+            Assert.Equal(1, limiter.GetAvailablePermits());
         }
 
         [Fact]