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Fix race condition in LanguageModelSession when streaming responses #126

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Merged
mattt merged 4 commits into from
Feb 17, 2026
Merged

Fix race condition in LanguageModelSession when streaming responses #126

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cb249fc
Simplify LanguageModelSession state management
atdrendel Feb 14, 2026
68328a7
Replace OSAllocatedUnfairLock with Locked
atdrendel Feb 14, 2026
0967c49
Re-enable observation for LanguageModelSession.isResponding and trans…
atdrendel Feb 14, 2026
3592070
Replace os_unfair_lock and pthread_mutex_t with NSLock
atdrendel Feb 17, 2026
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94 changes: 55 additions & 39 deletions Sources/AnyLanguageModel/LanguageModelSession.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -3,8 +3,17 @@ import Observation

@Observable
public final class LanguageModelSession: @unchecked Sendable {
public private(set) var isResponding: Bool = false
public private(set) var transcript: Transcript
public var isResponding: Bool {
access(keyPath: \.isResponding)

@atdrendel atdrendel Feb 15, 2026

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According to Philippe Hausler on the Swift Forums, the access() and withMutation() calls need to happen outside of locks.

return state.access { $0.isResponding }
}

public var transcript: Transcript {
access(keyPath: \.transcript)
return state.access { $0.transcript }
}

@ObservationIgnored private let state: Locked<State>

private let model: any LanguageModel
public let tools: [any Tool]
Expand All @@ -20,8 +29,6 @@ public final class LanguageModelSession: @unchecked Sendable {
/// with the Foundation Models framework.
@ObservationIgnored public var toolExecutionDelegate: (any ToolExecutionDelegate)?

@ObservationIgnored private let respondingState = RespondingState()

public convenience init(
model: any LanguageModel,
tools: [any Tool] = [],
Expand Down Expand Up @@ -84,37 +91,33 @@ public final class LanguageModelSession: @unchecked Sendable {
}
}

self.transcript = finalTranscript
self.state = .init(.init(finalTranscript))
}

public func prewarm(promptPrefix: Prompt? = nil) {
model.prewarm(for: self, promptPrefix: promptPrefix)
}

nonisolated private func beginResponding() async {
let count = await respondingState.increment()
let active = count > 0
await MainActor.run {
self.isResponding = active
nonisolated private func beginResponding() {
withMutation(keyPath: \.isResponding) {
state.access { $0.beginResponding() }
Comment on lines +102 to +103

@atdrendel atdrendel Feb 15, 2026

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According to Philippe Hausler on the Swift Forums, the access() and withMutation() calls need to happen outside of locks.

}
}

nonisolated private func endResponding() async {
let count = await respondingState.decrement()
let active = count > 0
await MainActor.run {
self.isResponding = active
nonisolated private func endResponding() {
withMutation(keyPath: \.isResponding) {
state.access { $0.endResponding() }
}
}

nonisolated private func wrapRespond<T>(_ operation: () async throws -> T) async throws -> T {
await beginResponding()
beginResponding()
do {
let result = try await operation()
await endResponding()
endResponding()
return result
} catch {
await endResponding()
endResponding()
throw error
}
}
Expand All @@ -127,7 +130,7 @@ public final class LanguageModelSession: @unchecked Sendable {
let relay = AsyncThrowingStream<ResponseStream<Content>.Snapshot, any Error> { continuation in
let stream = upstream
Task {
await session.beginResponding()
session.beginResponding()
var lastSnapshot: ResponseStream<Content>.Snapshot?
do {
for try await snapshot in stream {
Expand All @@ -152,14 +155,14 @@ public final class LanguageModelSession: @unchecked Sendable {
segments: [.text(.init(content: textContent))]
)
)
await MainActor.run {
session.transcript.append(responseEntry)
session.withMutation(keyPath: \.transcript) {
session.state.access { $0.transcript.append(responseEntry) }
}
}
} catch {
continuation.finish(throwing: error)
}
await session.endResponding()
session.endResponding()
}
}
return ResponseStream(stream: relay)
Expand Down Expand Up @@ -202,8 +205,8 @@ public final class LanguageModelSession: @unchecked Sendable {
responseFormat: nil
)
)
await MainActor.run {
self.transcript.append(promptEntry)
withMutation(keyPath: \.transcript) {
state.access { $0.transcript.append(promptEntry) }
}

let response = try await model.respond(
Expand All @@ -230,9 +233,11 @@ public final class LanguageModelSession: @unchecked Sendable {
)

// Add tool entries and response to transcript
await MainActor.run {
self.transcript.append(contentsOf: response.transcriptEntries)
self.transcript.append(responseEntry)
withMutation(keyPath: \.transcript) {
state.access { state in
state.transcript.append(contentsOf: response.transcriptEntries)
state.transcript.append(responseEntry)
}
}

return response
Expand All @@ -253,7 +258,9 @@ public final class LanguageModelSession: @unchecked Sendable {
responseFormat: nil
)
)
transcript.append(promptEntry)
withMutation(keyPath: \.transcript) {
state.access { $0.transcript.append(promptEntry) }
}

return wrapStream(
model.streamResponse(
Expand Down Expand Up @@ -547,8 +554,8 @@ extension LanguageModelSession {
responseFormat: nil
)
)
await MainActor.run {
self.transcript.append(promptEntry)
withMutation(keyPath: \.transcript) {
state.access { $0.transcript.append(promptEntry) }
}

// Extract text content for the Prompt parameter
Expand Down Expand Up @@ -578,9 +585,11 @@ extension LanguageModelSession {
)

// Add tool entries and response to transcript
await MainActor.run {
self.transcript.append(contentsOf: response.transcriptEntries)
self.transcript.append(responseEntry)
withMutation(keyPath: \.transcript) {
state.access { state in
state.transcript.append(contentsOf: response.transcriptEntries)
state.transcript.append(responseEntry)
}
}

return response
Expand Down Expand Up @@ -651,7 +660,9 @@ extension LanguageModelSession {
responseFormat: nil
)
)
transcript.append(promptEntry)
withMutation(keyPath: \.transcript) {
state.access { $0.transcript.append(promptEntry) }
}

// Extract text content for the Prompt parameter
let textPrompt = Prompt(prompt)
Expand Down Expand Up @@ -902,16 +913,21 @@ private enum ResponseStreamError: Error {

// MARK: -

private actor RespondingState {
private struct State: Equatable, Sendable {
var transcript: Transcript

var isResponding: Bool { count > 0 }
private var count = 0

func increment() -> Int {
init(_ transcript: Transcript) {
self.transcript = transcript
}

mutating func beginResponding() {
count += 1
return count
}

func decrement() -> Int {
mutating func endResponding() {
count = max(0, count - 1)
return count
}
}
16 changes: 16 additions & 0 deletions Sources/AnyLanguageModel/Locked.swift
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Original file line number Diff line number Diff line change
@@ -0,0 +1,16 @@
import Foundation

final class Locked<State> {
private let lock = NSLock()
private var state: State

init(_ state: State) {
self.state = state
}

func access<T>(_ block: (inout State) throws -> T) rethrows -> T {
try lock.withLock { try block(&self.state) }
}
}

extension Locked: @unchecked Sendable where State: Sendable {}
152 changes: 152 additions & 0 deletions Tests/AnyLanguageModelTests/LockedTests.swift
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Original file line number Diff line number Diff line change
@@ -0,0 +1,152 @@
import Foundation
import Testing

@testable import AnyLanguageModel

@Suite("Locked")
struct LockedTests {
@Test("Read access returns the initial value")
func readAccess() {
let locked = Locked(42)
let value = locked.access { $0 }
#expect(value == 42)
}

@Test("Write access mutates the state")
func writeAccess() {
let locked = Locked(0)
locked.access { $0 = 99 }
let value = locked.access { $0 }
#expect(value == 99)
}

@Test("Access returns the value from the closure")
func returnValue() {
let locked = Locked("hello")
let result = locked.access { state -> Int in
state += " world"
return state.count
}
#expect(result == 11)
#expect(locked.access { $0 } == "hello world")
}

@Test("Access propagates thrown errors")
func throwsPropagation() {
struct TestError: Error, Equatable {}

let locked = Locked(0)
#expect(throws: TestError.self) {
try locked.access { _ in throw TestError() }
}
}

@Test("Works with complex state types")
func complexState() {
struct State {
var name: String
var count: Int
var tags: [String]
}

let locked = Locked(State(name: "initial", count: 0, tags: []))
locked.access { state in
state.name = "updated"
state.count = 5
state.tags.append("a")
state.tags.append("b")
}

let snapshot = locked.access { $0 }
#expect(snapshot.name == "updated")
#expect(snapshot.count == 5)
#expect(snapshot.tags == ["a", "b"])
}

@Test("Concurrent increments produce the correct final count")
func concurrentCounter() async {
let locked = Locked(0)
let iterations = 100_000

await withTaskGroup(of: Void.self) { group in
for _ in 0 ..< iterations {
group.addTask { locked.access { $0 += 1 } }
}
}

let finalValue = locked.access { $0 }
#expect(finalValue == iterations)
}

@Test("Concurrent increments with mixed task priorities")
func concurrentCounterWithMixedPriorities() async {
let locked = Locked(0)
let iterations = 100_000

await withTaskGroup(of: Void.self) { group in
for i in 0 ..< iterations {
let priority: TaskPriority = i.isMultiple(of: 2) ? .high : .background
group.addTask(priority: priority) {
locked.access { $0 += 1 }
}
}
}

let finalValue = locked.access { $0 }
#expect(finalValue == iterations)
}

@Test("Concurrent appends to an array")
func concurrentArrayAppend() async {
let locked = Locked<[Int]>([])
let iterations = 10_000

await withTaskGroup(of: Void.self) { group in
for i in 0 ..< iterations {
group.addTask { locked.access { $0.append(i) } }
}
}

let finalArray = locked.access { $0 }
#expect(finalArray.count == iterations)
}

@Test("Multiple independent locked values mutated concurrently")
func multipleLockedValues() async {
let lockedA = Locked(0)
let lockedB = Locked(0)
let iterations = 50_000

await withTaskGroup(of: Void.self) { group in
for _ in 0 ..< iterations {
group.addTask { lockedA.access { $0 += 1 } }
group.addTask { lockedB.access { $0 += 1 } }
}
}

#expect(lockedA.access { $0 } == iterations)
#expect(lockedB.access { $0 } == iterations)
}

@Test("Can wrap a non-Sendable type")
func nonSendableElement() {
class Box {
var value: Int
init(_ value: Int) { self.value = value }
}

let locked = Locked(Box(10))
locked.access { $0.value += 5 }
let result = locked.access { $0.value }
#expect(result == 15)
}

@Test("Copies share the same underlying storage")
func copySharesStorage() {
let original = Locked(0)
let copy = original
original.access { $0 = 42 }
let value = copy.access { $0 }
#expect(value == 42)
}
}
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