Runtime-async sequence improvements: allow awaits in try/with and guard pending moves by majocha · Pull Request #20655 · dotnet/fsharp

majocha · 2026-09-28T12:13:44Z

Description

Runtime-async sequence producers can now suspend inside try/with: in the protected body, in when filters, and in
handlers. The source is disposed before the handler runs.

// low level
let source =
    __runtimeAsyncSequence (fun () -> seq {
        try
            yield 1
            Task.Delay 200 |> AsyncHelpers.Await
            failwith "expected"
        with _ ->
            Task.Delay 200 |> AsyncHelpers.Await
            yield 2
    })

// using a builder
let builderSource =
            asyncSeq2 {
                try
                    yield 1
                    do! Task.Delay 10
                    failwith "expected"
                with _ ->
                    do! Task.Delay 10
                    yield 2
            }


runtimeTask {
    for i in source do
        printfn $"step {i}"
}

A second MoveNextAsync or a DisposeAsync while a move is still pending now throws InvalidOperationException instead of corrupting the state machine.
The component tests now use updated async2, runtimeTask, and asyncSeq2 sample builders. asyncSeq2 shows stack-safe yield! and a YieldFromFinal tail handoff built on the compiler feature.

Fixes # (issue, if applicable)

Checklist

Test cases added
Performance benchmarks added in case of performance changes
Release notes entry updated

Lower producer try/with to RuntimeAsyncSequenceHelpers.EnumerateTryWith, a new FSharp.Core compiler-services helper. The source, filter and handler become independent runtime-async iterators driven with the enumeration's cancellation token, so awaits are allowed in the protected body, filters and handlers. The source is disposed before the handler runs. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Generated sequence types no longer re-implement IAsyncEnumerator directly, so interface calls dispatch through GeneratedRuntimeAsyncSequenceBase. The base rejects a second MoveNextAsync, or a DisposeAsync, while a move is pending. The generated MoveNextAsync releases the guard from a finally block through CompleteMoveNext, before its result completes. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Replace the runtime-async test fixtures' sample builders with copies of the reference async2 (cancellable), runtimeTask and asyncSeq2 builders and their test suites. The producer/consumer asyncSeq builder and its tests are removed; asyncSeq2 covers async sequences through __runtimeAsyncSequence, including stack-safe yield! and YieldFromFinal handoff. Tests share the builders through withSampleBuilders. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

github-actions · 2026-09-28T12:13:57Z

✅ Release notes checked

✅ Found changes and release notes in following paths:

Change path Release notes path Description

`src/FSharp.Core` docs/release-notes/.FSharp.Core/11.0.200.md

`src/Compiler` docs/release-notes/.FSharp.Compiler.Service/11.0.200.md

Bring back RuntimeAsyncEnumerableTests (try/with, tail recursion, pull-driven and concurrent MoveNext coverage), now running on asyncSeq2. asyncSeq2 accepts F# Async as a cold source that receives the enumeration token. New builder tests cover cancellation, unmatched exceptions escaping try/with unwrapped, and deep non-tail yield!. The MergeSources overlap test checks start order with gated awaitables instead of elapsed time. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

…ders in their folder Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Replace the FSharp.Core EnumerateTryWith helper with in-machine handling: - MoveNextAsync catches faults and re-enters the jump table, which routes each state to its innermost pending finally or handler, so bodies, guards, handlers and finally blocks can all await. - DisposeAsync sets a disposing flag and runs MoveNextAsync once more to execute pending finally blocks, skipping handlers. - GeneratedRuntimeAsyncSequenceBase implements DisposeAsync; generated types override a sync Close instead. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Remove the Seq.collect fallback. Non-delayed sources are enumerated inside the protected region, and filters that the handler does not already encode run before the handler. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

github-actions · 2026-09-29T02:55:58Z

🔍 Tooling Safety Check — Affects-Compiler-Output
Affects-Compiler-Output: Changes compiler-emitted member signatures.

Generated by PR Tooling Safety Check · gpt56 2.5M · ◷

T-Gro

🤖 🕵️ AI review — verify independently.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

T-Gro

🤖 🕵️ Re-reviewed all recent changes as of 27a4dca; LGTM

T-Gro · 2026-10-06T11:46:07Z

@majocha :

I will want to backport some bug fixes (also w.r.t to runtime async) to net11 GA.
I think this PR does not meet the servicing bar and will have to wait for 11.0.200 (later 2027 release) - unless it fixes a real regression.

Since this touches many relevant lines, I will delay merging this one until the fixes are ported (to make them portable), and only merge after.

WDYT?

majocha · 2026-10-06T12:32:42Z

Yes, I have no objections. The whole thing is very much a preview feature and my main concern is to keep it as self-contained as possible and not to break something unrelated.

T-Gro

🤖 🕵️ Re-reviewed all recent changes as of 807c328; LGTM

T-Gro

🤖 🕵️ Re-reviewed all recent changes as of 0499ce3; LGTM

majocha and others added 4 commits September 28, 2026 14:04

Document runtime-async sequence try/with, move guard and sample builders

1466f7b

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

github-project-automation Bot added this to F# Compiler and Tooling Sep 28, 2026

github-project-automation Bot moved this to New in F# Compiler and Tooling Sep 28, 2026

majocha changed the title ~~Runtime-async improvements: allow awaits in try/with and guard pending moves in async sequences~~ Runtime-async sequence improvements: allow awaits in try/with and guard pending moves Sep 28, 2026

majocha and others added 5 commits September 28, 2026 14:47

Move runtime-async sequence release notes to 11.0.200 with PR link

af07fa2

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Limit runtime-async doc to the compiler feature; describe sample buil…

1ff54f2

…ders in their folder Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

majocha mentioned this pull request Sep 28, 2026

Inline conditional runtime-async callbacks in a separate lowering pass #20647

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majocha marked this pull request as ready for review September 28, 2026 20:11

majocha requested a review from a team as a code owner September 28, 2026 20:11

github-actions Bot added the ⚠️ Affects-Compiler-Output Tooling check: PR touches IL emission or codegen label Sep 28, 2026

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T-Gro reviewed Sep 29, 2026

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Comment thread src/Compiler/Optimize/LowerSequences.fs

Comment thread tests/FSharp.Compiler.ComponentTests/Language/RuntimeAsync/Async2Builder.fs Outdated

T-Gro self-requested a review September 29, 2026 10:00

T-Gro added AI-reviewed PR reviewed by AI review council Needs-Servicing labels Sep 29, 2026

majocha and others added 3 commits October 5, 2026 12:13

Merge branch 'main' into runtime-async-seq-trywith

4856589

preserve EC in async2 trampoline

91c8117

Test async2 sibling execution contexts

27a4dca

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

T-Gro reviewed Oct 5, 2026

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majocha added 2 commits October 5, 2026 15:53

Merge branch 'main' into runtime-async-seq-trywith

1babb8c

Merge branch 'main' into runtime-async-seq-trywith

807c328

T-Gro reviewed Oct 7, 2026

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Merge branch 'main' into runtime-async-seq-trywith

0499ce3

T-Gro removed the Needs-Servicing label Oct 7, 2026

T-Gro reviewed Oct 8, 2026

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Runtime-async sequence improvements: allow awaits in try/with and guard pending moves - #20655

Runtime-async sequence improvements: allow awaits in try/with and guard pending moves#20655
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