fix: stop iterAsync reliably and never lose a failure of its action - #38

Proposed Changes

Part of splitting #22 into small pull requests that can be reviewed one at a time. It is stacked on #37 (fix/iter-async-overflow), so it shows only its own change; merge #37 first.

Composing R3's SelectAwait with ForEachAsync left four gaps in both flavours of iterAsync:

• Over a synchronous source, R3 attaches the terminal to the mapped stage only after Subscribe returns, so the action kept running for every element after it had thrown.
• With an already cancelled token the action still ran for every element of a synchronous source, although the iteration was cancelled.
• R3 drops a failure of the action that happens after the source completed, so the iteration completed successfully.
• SelectAwait swallows an OperationCanceledException: a timeout such as HttpClient's TaskCanceledException stopped the sequential worker for good and iterAsync never completed.

The action's exceptions no longer travel through R3 at all. An internal IterationGuard records the first one, skips the remaining actions and cancels an internal token that completes the iteration through TakeUntil, which also unsubscribes from the source at once; the iteration then fails with the recorded exception. A cancellation that R3 itself caused for an invocation (AwaitSwitch, CancelOnCompleted, disposal) is not a failure, and a cancelled token cancels the iteration without subscribing.

Types of changes

What types of changes does your code introduce to FSharp.Control.R3?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• Build and tests pass locally – 0 warnings, Fantomas is clean, and the tests that exist at this commit pass
• I have added tests that prove my fix is effective or that my feature works (if appropriate) – in test: cover the library with integration tests #22, the top of this stack
• I have added necessary documentation (if appropriate)

Further comments

The iterAsync docs now describe which elements reach the action, when the iteration completes, also with CancelOnCompleted, and how it fails.

Tests (in #22): the iterAsync tests of AsyncObservableTests.fs and TaskObservableTests.fs, one for each gap above.

Stack – every pull request is based on the one before it, so each shows only its own change. Merge them in this order:

1. refactor!: prefix AwaitOperationConfiguration cases with Await #23 refactor!: prefix AwaitOperationConfiguration cases with Await
2. fix: validate the concurrency limit of mapAsync options eagerly #24 fix: validate the concurrency limit of mapAsync options eagerly
3. fix(rxquery): emit yield/zero synchronously to keep element order #25 fix(rxquery): emit yield/zero synchronously to keep element order
4. fix(rxquery)!: stop seeding sumBy with Unchecked.defaultof #26 fix(rxquery)!: stop seeding sumBy with Unchecked.defaultof
5. feat(rxquery): add rxqueryWith to cancel the terminal query operators #27 feat(rxquery): add rxqueryWith to cancel the terminal query operators
6. docs(rxquery): document the query builder and every query operator #28 docs(rxquery): document the query builder and every query operator
7. feat(observable)!: make ofSeq reachable as Observable.ofSeq #29 feat(observable)!: make ofSeq reachable as Observable.ofSeq
8. feat(observable)!: make Observable.choose take a voption chooser, add ObservableOption #30 feat(observable)!: make Observable.choose take a voption chooser, add ObservableOption
9. feat(observable): add chunkByBoundaries for boundaries of any element type #31 feat(observable): add chunkByBoundaries for boundaries of any element type
10. fix(observable): validate every chunk length eagerly #32 fix(observable): validate every chunk length eagerly
11. docs: document the processing and chunk configuration types #33 docs: document the processing and chunk configuration types
12. docs(observable): correct misleading XML docs and document every function #34 docs(observable): correct misleading XML docs and document every function
13. fix(async)!: remove the ignored cancellationToken from toLookup #35 fix(async)!: remove the ignored cancellationToken from toLookup
14. fix(async)!: surface the original exception and cancellation from Async wrappers #36 fix(async)!: surface the original exception and cancellation from Async wrappers
15. fix: wait for iterAsync through ForEachAsync instead of CountAsync #37 fix: wait for iterAsync through ForEachAsync instead of CountAsync
16. fix: stop iterAsync reliably and never lose a failure of its action #38 fix: stop iterAsync reliably and never lose a failure of its action ← this pull request
17. fix(task)!: default ofTask configureAwait to true #39 fix(task)!: default ofTask configureAwait to true
18. feat(task)!: move toArray and toList into the Task.Observable module #40 feat(task)!: move toArray and toList into the Task.Observable module
19. docs: correct the Async and Task module docs and document every member #41 docs: correct the Async and Task module docs and document every member
20. docs: document the R3 1.3.1 limitations of bind, catch and mapAsync #42 docs: document the R3 1.3.1 limitations of bind, catch and mapAsync
21. test: categorize the tests per tested component #43 test: categorize the tests per tested component
22. test: cover the library with integration tests #22 test: cover the library with integration tests

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