#602 Invoke APM callbacks on the threadpool.

src/Microsoft.AspNetCore.Http/Internal/ReferenceReadStream.cs

@@ -138,6 +138,9 @@ namespace Microsoft.AspNetCore.Http.Internal
             }
 
             if (callback != null)
+            {
+                // Offload callbacks to avoid stack dives on sync completions.
+                var ignored = Task.Run(() =>
                 {
                     try
                     {
@@ -147,11 +150,17 @@ namespace Microsoft.AspNetCore.Http.Internal
                     {
                         // Suppress exceptions on background threads.
                     }
+                });
             }
         }
 
         public override int EndRead(IAsyncResult asyncResult)
         {
+            if (asyncResult == null)
+            {
+                throw new ArgumentNullException(nameof(asyncResult));
+            }
+
             var task = (Task<int>)asyncResult;
             return task.GetAwaiter().GetResult();
         }

src/Microsoft.AspNetCore.WebUtilities/BufferedReadStream.cs

@@ -228,9 +228,20 @@ namespace Microsoft.AspNetCore.WebUtilities
                 TaskCompletionSource<int> tcs = new TaskCompletionSource<int>(state);
                 tcs.TrySetResult(toCopy);
                 if (callback != null)
+                {
+                    // Offload callbacks to avoid stack dives on sync completions.
+                    var ignored = Task.Run(() =>
+                    {
+                        try
                         {
                             callback(tcs.Task);
                         }
+                        catch (Exception)
+                        {
+                            // Suppress exceptions on background threads.
+                        }
+                    });
+                }
                 return tcs.Task;
             }
 
@@ -239,6 +250,11 @@ namespace Microsoft.AspNetCore.WebUtilities
 
         public override int EndRead(IAsyncResult asyncResult)
         {
+            if (asyncResult == null)
+            {
+                throw new ArgumentNullException(nameof(asyncResult));
+            }
+
             Task<int> task = asyncResult as Task<int>;
             if (task != null)
             {

src/Microsoft.AspNetCore.WebUtilities/FileBufferingReadStream.cs

@@ -231,6 +231,9 @@ namespace Microsoft.AspNetCore.WebUtilities
             }
 
             if (callback != null)
+            {
+                // Offload callbacks to avoid stack dives on sync completions.
+                var ignored = Task.Run(() =>
                 {
                     try
                     {
@@ -240,11 +243,17 @@ namespace Microsoft.AspNetCore.WebUtilities
                     {
                         // Suppress exceptions on background threads.
                     }
+                });
             }
         }
 
         public override int EndRead(IAsyncResult asyncResult)
         {
+            if (asyncResult == null)
+            {
+                throw new ArgumentNullException(nameof(asyncResult));
+            }
+
             var task = (Task<int>)asyncResult;
             return task.GetAwaiter().GetResult();
         }

src/Microsoft.AspNetCore.WebUtilities/MultipartReaderStream.cs

@@ -183,6 +183,9 @@ namespace Microsoft.AspNetCore.WebUtilities
             }
 
             if (callback != null)
+            {
+                // Offload callbacks to avoid stack dives on sync completions.
+                var ignored = Task.Run(() =>
                 {
                     try
                     {
@@ -192,11 +195,17 @@ namespace Microsoft.AspNetCore.WebUtilities
                     {
                         // Suppress exceptions on background threads.
                     }
+                });
             }
         }
 
         public override int EndRead(IAsyncResult asyncResult)
         {
+            if (asyncResult == null)
+            {
+                throw new ArgumentNullException(nameof(asyncResult));
+            }
+
             var task = (Task<int>)asyncResult;
             return task.GetAwaiter().GetResult();
         }