Swift 6.4 tools version, async/await download stack, and WASI guards

[leogdion]

Why

Package.swift declared // swift-tools-version: 5.8 — the lowest of any BrightDigit package (every other one is on 6.4; SyndiKit is on 5.10). But CI here only ever builds and tests against Swift 6.4 nightly, so the advertised 5.8 support has never actually been verified by anything. Rather than add a second CI matrix to back a claim nobody relies on, raise the floor to 6.4 so Contribute matches the rest of the stack and keeps the single 6.4 matrix.

At tools-version 5.8 the package built in Swift 5 language mode. At 6.4 it builds in Swift 6 language mode with complete strict concurrency checking, which surfaced a handful of real issues. All of them are fixed properly — no swiftLanguageMode(.v5), no relaxed -strict-concurrency, no suppressed diagnostics, no @unchecked Sendable.

Strict-concurrency fixes

MarkdownContentBuilderOptions → Sendable
Its internal static let shouldOverwriteExisting / includeMissingPrevious were hard errors (static property … is not concurrency-safe because non-'Sendable' type … may have shared mutable state). The type is an OptionSet over a single let rawValue: Int, so it just needed the conformance spelled out — public types don't get it inferred.

ImportError payloads
In Swift 6 Error refines Sendable, so every associated value of an Error-conforming enum is checked. The four nested field enums (ResponseComponent, VideoField, EpisodeField, NewsletterField) now declare : Sendable, and the Any payloads on invalidPodcastEpisodeFromRSSItem, missingFieldForVideo, missingVideoForEpisode, missingFieldFromPodcastEpisode and duplicateTitle(_:forVideos:) became any Sendable.

Note this is a signature change on a public, already @available(*, deprecated) enum. Nothing outside this package uses those cases (brightdigit.com only throws ImportError.newsletterMissingField, which is unchanged), and any Sendable accepts everything a caller would realistically pass, so the practical break is nil — but flagging it since this is a released v1.0.0 package.

Sendable propagation through the download stack — the substantive one.

URLSessionable.download(fromURL:completion:) already took an @escaping @Sendable completion (it has to: URLSession.downloadTask(with:completionHandler:) demands one). FileURLDownloader.downloadFromNetwork passes a closure to it that captures both self and its own completion, which in Swift 6 mode is two hard errors:

error: capture of 'self' with non-Sendable type 'FileURLDownloader' in a '@Sendable' closure
error: capture of 'completion' with non-Sendable type '((any Error)?) -> Void' in a '@Sendable' closure

There's no way to restructure that away — the closure genuinely needs the file manager to finish the local copy, and it genuinely needs the caller's completion handler. So the dependency abstractions became Sendable:

• FileManagerProtocol: Sendable
• URLSessionable: Sendable
• URLDownloader: Sendable, with _ completion: @escaping @Sendable (Error?) -> Void
• the same @Sendable on FileURLDownloader's three download* methods

This is a correctness improvement, not a workaround: both standard conformers (FileManager, URLSession) are already Sendable, and FileURLDownloader is a struct over two lets, so it satisfies URLDownloader: Sendable as-is.

FileManagerSpy (tests) follows from the above — a Sendable protocol needs a Sendable conformer, and the spy's four IsCalled flags are mutable. They moved into a private CallRecord struct guarded by an NSLock, exposed through computed properties. Synchronization.Mutex would encode that in the type system, but it requires macOS 15 / iOS 18 and this package supports macOS 12 / iOS 13, so the lock is external and the storage is nonisolated(unsafe) (targeted, and every access goes through the two locked accessors — not the blanket @unchecked Sendable the SwiftLint no_unchecked_sendable rule bans).

XCTestCase+Extension — the two runFileURLDownloader* helpers forward their completion into the now-@Sendable API, so their parameters are @escaping @Sendable too.

Platform floors

Unchanged: .macOS(.v12), .iOS(.v13), .tvOS(.v13), .watchOS(.v6). Nothing in the 6.4 toolchain forced a bump, and this is a released v1.0.0 package. README's Requirements section is updated to Swift 6.4+ with those same floors.

One cosmetic side effect: at tools-version 6.4 the manifest now emits 'v13' is deprecated: iOS 15.0 is the oldest supported version (and the tvOS/watchOS equivalents) — warnings from PackageDescription, not errors. Left as-is deliberately; narrowing platform support is a separate decision.

Verification (macOS, Swift 6.4 / swiftlang-6.4.0.27.1)

• swift build — clean
• swift build -c release — clean (the SwiftSoup fork's inline-crash workaround still holds)
• swift test — 48 tests pass, 0 failures (29 XCTest + 19 swift-testing across 2 suites)
• LINT_MODE=STRICT ./Scripts/lint.sh — Found 0 violations, 0 serious in 55 files, periphery reports no unused code, "Linting completed successfully"

The only remaining build warning is the pre-existing found 1 file(s) which are unhandled … Documentation.docc, which is present on main too (verified by reverting just the tools-version line).

Downstream impact — read before merging

Making URLDownloader's completion @Sendable is not optional (see above), and it does break ContributeWordPress, which is the only package in the stack that touches these APIs. I verified this by building ContributeWordPress against this branch (swift package edit Contribute --path …). Three sites need follow-up there:

1. Sources/ContributeWordPress/Images/AssetDownloader.swift:97 — error: capture of 'asset' with non-Sendable type 'AssetImport' in a '@Sendable' closure
2. Sources/ContributeWordPress/Images/AssetDownloader.swift:97 — error: mutation of captured var 'errors' in concurrently-executing code
3. Tests/ContributeWordPressTests/Helpers/Spy/FileDownloaderSpy.swift:6 — error: stored property 'downloadIsCalled' of 'Sendable'-conforming class 'FileDownloaderSpy' is mutable

(2) is worth calling out: AssetDownloader.downloadUsingGroupDispatch fans downloads out over a DispatchGroup and every callback writes into the same var errors = [URL: Error]() with no synchronization. With the real FileURLDownloader those callbacks arrive on URLSession's delegate queue, so that is an actual pre-existing data race that Swift 5 mode was hiding — this change surfaces it rather than causing it.

Nothing else is affected: ContributeMailchimp / ContributeYouTube / ContributeRSS only use MarkdownContentBuilderOptions (unchanged apart from gaining Sendable), and brightdigit.com itself only throws ImportError.newsletterMissingField.

So this should land together with a matching ContributeWordPress PR.

🤖 Generated with Claude Code

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Run ID: 12f40914-361a-4e39-ba6a-5d27aa511166

📥 Commits

Reviewing files that changed from the base of the PR and between 5dc9049 and 165dfc4.

📒 Files selected for processing (14)

• Package.swift
• README.md
• Sources/Contribute/FileURLDownloader.swift
• Sources/Contribute/ImportError.swift
• Sources/Contribute/MarkdownContentBuilderOptions.swift
• Sources/Contribute/Protocols/FileManagerProtocol.swift
• Sources/Contribute/Protocols/URLDownloader.swift
• Sources/Contribute/Protocols/URLSessionable.swift
• Sources/Contribute/URLDownloaderError.swift
• Tests/ContributeTests/FileURLDownloaderLocalFileTests.swift
• Tests/ContributeTests/FileURLDownloaderRemoteFileTests.swift
• Tests/ContributeTests/Spies/FileManagerSpy.swift
• Tests/ContributeTests/Spies/NetworkManagerSpy.swift
• Tests/ContributeTests/XCTestCase+Extension.swift

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• Create PR with unit tests
• Commit unit tests in branch chore/raise-swift-tools-version-6.4

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