Bug Unobserved "Collection was modified" exception from HttpClient dispose

We are seeing unobserved exceptions being thrown from use of System.Net.Http.HttpClient. The exception happens upon dispose of HttpClient when a request has not been completed successfully (for example due to a NameResolutionFailure). The callstack looks like this:

Code (CSharp):

1. InvalidOperationException: Collection was modified; enumeration operation may not execute.
2.   at System.Collections.Generic.Dictionary`2+Enumerator[TKey,TValue].MoveNext () [0x00000] in <00000000000000000000000000000000>:0
3.   at System.Net.ServicePointScheduler.RunSchedulerIteration () [0x00000] in <00000000000000000000000000000000>:0
4.   at System.Net.ServicePointScheduler+<StartScheduler>d__32.MoveNext () [0x00000] in <00000000000000000000000000000000>:0
5.   at System.Threading.ContextCallback.Invoke (System.Object state) [0x00000] in <00000000000000000000000000000000>:0
6.   at System.Threading.ExecutionContext.RunInternal (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state, System.Boolean preserveSyncCtx) [0x00000] in <00000000000000000000000000000000>:0
7.   at System.Runtime.CompilerServices.AsyncMethodBuilderCore+MoveNextRunner.Run () [0x00000] in <00000000000000000000000000000000>:0
8.   at System.Threading.ThreadStart.Invoke () [0x00000] in <00000000000000000000000000000000>:0

While it seems that this exception doesn't have direct user-visible impact it creates clutter in our error reporting systems and since it happens within the System.Net implementation we haven't been able to figure out a decent way of gracefully filtering out this exception from reporting tools.

The root cause appears to have been reported to mono and fixed a while ago in https://github.com/mono/mono/pull/7359, but apparently the fix has not been backported to the Unity fork.

Find below a minimal MonoBehaviour script that should trigger the issue (tested with Unity version 2020.3.17f1). Dropping this into a GameObject in an otherwise empty scene and running a standalone player build should generate the InvalidOperationException with call stack shown above.

Code (CSharp):

1. using System.Net.Http;
2. using System.Threading.Tasks;
3. using UnityEngine;
4.  
5. public class Repro : MonoBehaviour
6. {
7.     void Awake()
8.     {
9.         Task.Run(Bug);
10.     }
11.  
12.     async Task Bug()
13.     {
14.         using (var client = new HttpClient()) {
15.             await client.GetAsync("http://nonexistent");
16.         }
17.     }
18. }