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Fix WriteOnceBlock race condition where ReceiveAsync returns null#123435

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copilot/fix-writeonceblock-null-receiveasync
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Fix WriteOnceBlock race condition where ReceiveAsync returns null#123435
Copilot wants to merge 6 commits intomainfrom
copilot/fix-writeonceblock-null-receiveasync

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Copilot AI commented Jan 21, 2026 •
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Description

WriteOnceBlock<T>.ReceiveAsync intermittently returns null when called concurrently with Post.

Root cause: In OfferMessage, _header was set before _value:

_header = Common.SingleMessageHeader;  // HasValue becomes true
_value = messageValue;                  // Value set AFTER

LinkTo checks HasValue (_header.IsValid) inside a lock but reads _value outside it. A concurrent reader can observe HasValue == true while _value is still default(T).

Fix: Assign _value before _header, with a memory barrier to enforce the ordering and prevent CPU/compiler reordering.

Changes

  • WriteOnceBlock.cs: Swap assignment order so _value is set before _header.IsValid becomes true, and add Interlocked.MemoryBarrier() between the writes to enforce the ordering
  • WriteOnceBlockTests.cs: Add TestConcurrentPostAndReceiveAsync regression test (10K iterations), marked as [OuterLoop] since it's a stress test
Original prompt

This section details on the original issue you should resolve

<issue_title>The WriteOnceBlock method ReceiveAsnyc returns null instead of value passed in Post method</issue_title>
<issue_description>### Description

I discovered that the instance of WriteOnceBlock<T> when used to pass a value between two tasks with Post and ReceiveAsnyc methods doesn't work as expected. Sometimes when a value is written to WriteOnceBlock<T> with Post method, ReceiveAsync method reads null instead of value used in Post.

After Post(someObject) is called i expect that await writeOnceBlock.ReceiveAsync() is evaluated to someObject.

Instead i get null.

Reproduction Steps

using System.Threading.Tasks.Dataflow;

Console.WriteLine("Start");
int i = 0;
while (true)
{
    if (i == 100_000)
        break;
    var wo = new WriteOnceBlock<object>(a => a);
    var t1 = Task.Run(() =>
    {
        wo.Post(new object());
    });
    var t2 = Task.Run(async () =>
    {
        var o = await wo.ReceiveAsync();
        if (o == null)
        {
            Console.WriteLine("object is null 1");

            o = await wo.ReceiveAsync();
            if (o == null)
                Console.WriteLine("object is null 2");
        }
    });

    await t1;
    await t2;
    i++;
}
Console.WriteLine("End");

Expected behavior

Start
End

Actual behavior

Start
object is null 1
object is null 1
object is null 1
object is null 1
End

Regression?

/

Known Workarounds

If null is returned, calling ReceiveAsync again will get a value instead of null.

Configuration

.NET 8.0
Windows 11
64-bit operating system, x64-based processor

Other information

I think it might be some kind of synchronization issue.</issue_description>

Comments on the Issue (you are @copilot in this section)

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@stephentoub
Fix race condition in WriteOnceBlock where ReceiveAsync could return …
802c1c4 
…null

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
Copilot AI changed the title [WIP] Fix WriteOnceBlock<T> method to correctly receive posted values Fix WriteOnceBlock race condition where ReceiveAsync returns null Jan 21, 2026
Copilot AI requested a review from stephentoub January 21, 2026 15:55
@stephentoub
Add memory barrier to enforce ordering between _value and _header writes
c8f6101 
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
Copilot AI requested a review from stephentoub January 21, 2026 16:37
@stephentoub stephentoub marked this pull request as ready for review January 21, 2026 16:41
Copilot AI review requested due to automatic review settings January 21, 2026 16:41
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dotnet-policy-service bot commented Jan 21, 2026

Tagging subscribers to this area: @dotnet/area-system-threading-tasks
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@stephentoub
Mark TestConcurrentPostAndReceiveAsync as OuterLoop stress test
dc47710 
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
Copilot AI requested a review from stephentoub January 21, 2026 16:47
Copilot AI reviewed Jan 21, 2026
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Pull request overview

This PR fixes a critical race condition in WriteOnceBlock<T> where ReceiveAsync could intermittently return null instead of the posted value when called concurrently with Post. The root cause was that _header was being set before _value, allowing readers to observe HasValue == true while _value was still default.

Changes:

  • Reordered field assignments in OfferMessage to set _value before _header
  • Added Interlocked.MemoryBarrier() between the writes to enforce ordering and prevent CPU/compiler reordering
  • Added comprehensive regression test with 10,000 iterations to verify the fix

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File Description
WriteOnceBlock.cs Fixed race condition by swapping assignment order and adding memory barrier to ensure _value is visible before _header.IsValid becomes true
WriteOnceBlockTests.cs Added regression test that reproduces the concurrent Post/ReceiveAsync race condition scenario

@build-analysis build-analysis bot mentioned this pull request Jan 21, 2026
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@stephentoub stephentoub enabled auto-merge (squash) January 22, 2026 03:19
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stephentoub commented Jan 22, 2026

/azp list

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azure-pipelines bot commented Jan 22, 2026

CI/CD Pipelines for this repository:

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stephentoub commented Jan 22, 2026

/azp run runtime-libraries-coreclr outerloop-windows

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Azure Pipelines successfully started running 1 pipeline(s).

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stephentoub commented Jan 29, 2026

/azp run runtime-libraries-coreclr outerloop-windows

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Azure Pipelines successfully started running 1 pipeline(s).

This was referenced Jan 29, 2026
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The WriteOnceBlock<T> method ReceiveAsnyc returns null instead of value passed in Post method

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