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Add trailer support to URLSession backend #122

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Merged
guoye-zhang merged 7 commits into from
Mar 18, 2026
Merged

Add trailer support to URLSession backend #122

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d531316
Add trailer support to URLSession backend
guoye-zhang Feb 27, 2026
a758667
Set on the input stream instead
guoye-zhang Mar 7, 2026
a15a232
Merge main into urlsession-trailer
guoye-zhang Mar 7, 2026
0c4811c
Merge main into urlsession-trailer
guoye-zhang Mar 12, 2026
1caccc2
Merge branch 'main' into urlsession-trailer
guoye-zhang Mar 12, 2026
5f4a72f
Enable testTrailerRead
guoye-zhang Mar 12, 2026
5d5867b
Merge branch 'main' into urlsession-trailer
guoye-zhang Mar 18, 2026
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13 changes: 12 additions & 1 deletion Sources/URLSessionHTTPClient/URLSessionRequestStreamBridge.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -105,8 +105,19 @@ final class URLSessionRequestStreamBridge: NSObject, StreamDelegate, Sendable {
}
}

func close() {
func close(trailerFields: HTTPFields?) {
self.lockedState.withLock { state in
if let trailerFields {
var trailerNames: Set<HTTPField.Name> = []
for field in trailerFields {
trailerNames.insert(field.name)
}
var trailerDictionary: [String: String] = [:]
for name in trailerNames {
trailerDictionary[name.rawName] = trailerFields[name]
}
state.inputStream.setProperty(trailerDictionary, forKey: .init("_kCFStreamPropertyHTTPTrailer"))
}
state.outputStream.close()
}
}
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2 changes: 1 addition & 1 deletion Sources/URLSessionHTTPClient/URLSessionTaskDelegateBridge+ConcludingAsyncReader.swift
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Expand Up @@ -21,7 +21,7 @@ extension URLSessionTaskDelegateBridge: ConcludingAsyncReader {
func consumeAndConclude<Return, Failure: Error>(
body: (consuming sending URLSessionTaskDelegateBridge) async throws(Failure) -> Return
) async throws(Failure) -> (Return, HTTPFields?) {
try await (body(self), nil)
try await (body(self), self.responseTrailerFields)
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@guoye-zhang guoye-zhang Feb 28, 2026

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One of the reasons I think early returning before completing the read should throw is that we don't know whether nil means a trailer isn't present or it hasn't been received.

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@xbhatnag xbhatnag Mar 12, 2026

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I'd like to understand why the distinction matters.

What is the difference in corrective action that a user will take if:

  • the trailer is missing cause the server didn't provide it
  • the client under-read and didn't process trailers as a result

My point is, either way, the outcome is that they didn't receive trailers and there isn't much corrective action because the request is over either way.

I could be wrong though, so an example would help me understand why.

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@guoye-zhang guoye-zhang Mar 12, 2026

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Suppose that a checksum is sometimes sent in the trailer for data integrity. With the current API behavior, collect silently truncates and gives nil trailer, so all checks are skipped. It’s really easy to introduce data truncation issues with the current set of APIs.

}
}

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24 changes: 20 additions & 4 deletions Sources/URLSessionHTTPClient/URLSessionTaskDelegateBridge.swift
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Expand Up @@ -74,10 +74,15 @@ final class URLSessionTaskDelegateBridge: NSObject, Sendable, URLSessionDataDele
}
var state: State = .awaitingResponse
var completionContinuation: CheckedContinuation<Void, Never>? = nil
var responseTrailerFields: HTTPFields?
}

private let state: Mutex<TaskState> = .init(.init())

var responseTrailerFields: HTTPFields? {
self.state.withLock { $0.responseTrailerFields }
}

func urlSession(
_ session: URLSession,
dataTask: URLSessionDataTask,
Expand Down Expand Up @@ -153,6 +158,15 @@ final class URLSessionTaskDelegateBridge: NSObject, Sendable, URLSessionDataDele
state.state = .awaitingConsumption(existingData, complete: true, error: error, suspendedTask: nil)
}
state.completionContinuation = nil
if let trailerDictionary = task.value(forKey: "_trailers") as? [String: String] {
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var trailerFields = HTTPFields()
for (name, value) in trailerDictionary {
if let name = HTTPField.Name(name) {
trailerFields.append(.init(name: name, value: value))
}
}
state.responseTrailerFields = trailerFields
}
}
return state
}
Expand Down Expand Up @@ -247,15 +261,16 @@ final class URLSessionTaskDelegateBridge: NSObject, Sendable, URLSessionDataDele
let bridge = URLSessionRequestStreamBridge(task: task)
completionHandler(bridge.inputStream)
do {
_ = try await requestBody.produce(into: bridge)
let trailerFields = try await requestBody.produce(into: bridge)
bridge.close(trailerFields: trailerFields)
} catch {
if bridge.writeFailed {
// Ignore error
} else {
self.requestBodyStreamFailed(with: error)
}
bridge.close(trailerFields: nil)
}
bridge.close()
}
}
}
Expand All @@ -277,15 +292,16 @@ final class URLSessionTaskDelegateBridge: NSObject, Sendable, URLSessionDataDele
let bridge = URLSessionRequestStreamBridge(task: task)
completionHandler(bridge.inputStream)
do {
_ = try await requestBody.produce(offset: offset, into: bridge)
let trailerFields = try await requestBody.produce(offset: offset, into: bridge)
bridge.close(trailerFields: trailerFields)
} catch {
if bridge.writeFailed {
// Ignore error
} else {
self.requestBodyStreamFailed(with: error)
}
bridge.close(trailerFields: nil)
}
bridge.close()
}
}
}
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3 changes: 1 addition & 2 deletions Tests/HTTPClientTests/DarwinHTTPClientTests.swift
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Expand Up @@ -42,8 +42,7 @@ struct DarwinHTTPClientTests {
// TODO: Writing just an empty span causes an indefinite stall. The terminating chunk (size 0) is not written out on the wire.
.testEmptyChunkedBody,

// TODO: Trailers are not supported by URLSession
.testTrailerRead,
// TODO: Trailer writes are not supported by URLSession
.testTrailerWrite,
]) {
return DefaultHTTPClient.shared
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