Feature Proposal: Payload Checksum

[o-nnerb]

Overview

Response payload checksum validation (e.g. comparing a Content-MD5 / Digest response header against a locally computed hash) doesn't need new API in this package — it's already achievable today using the existing public RequestTaskModifier protocol, with no changes to the network layer.

Why

TaskResult<Element> (returned by both DataTask.result() and DownloadTask.result()) already exposes:

• head: ResponseHead, with headers: HTTPHeaders (public, case-insensitive lookup via .first(name:))
• payload — either Data (DataTask) or a streamable AsyncBytes (DownloadTask)

A consumer can write their own RequestTaskModifier that reads the expected checksum from the response headers, computes the actual hash over payload using swift-crypto/CryptoKit, and throws on mismatch:

struct ChecksumValidator: RequestTaskModifier {
    func body(_ task: Content) async throws -> TaskResult<Data> {
        let result = try await task.result()
        guard let expected = result.head.headers.first(name: "Content-MD5") else {
            return result
        }
        let actual = Insecure.MD5.hash(data: result.payload)
            .map { String(format: "%02x", $0) }.joined()
        guard actual == expected else { throw ChecksumMismatch() }
        return result
    }
}

applied via .modifier(ChecksumValidator()).

This holds even for large/streamed downloads: DownloadTask's payload is an AsyncBytes async sequence, which a consumer can already feed into an incremental hasher chunk by chunk, without RequestDL needing to buffer or expose anything new.

Decision

Closing as out of scope for the network layer — this is response post-processing (the same category as .decode(...)), not something that needs PropertyNode/Internals access, and the hashing primitives themselves belong in swift-crypto, not this package. If a named convenience (e.g. a bundled Modifiers.Checksum) is ever wanted purely for discoverability, that can be revisited as pure sugar — it wouldn't add new capability, just a shortcut for the pattern above.

[o-nnerb]

Closing this — see the updated body. Payload checksum validation is already fully achievable today via a custom RequestTaskModifier reading the expected checksum from TaskResult.head.headers and hashing TaskResult.payload (works for both buffered Data and streamed AsyncBytes). No core/network-layer change is needed. Happy to reopen if there's demand for a bundled convenience modifier purely for discoverability.