swift-build fails on Xcode 16.2 with "Error decoding response to CreateSessionRequest"

[chatwyn]

Describe the bug

When running swift-build as a service on macOS 15.3.1, the service fails to initialize with the following error:

This issue is specific to macOS 15.3.1 — the exact same workflow runs correctly on both macOS 14.5 and macOS 15.4.

Environment

• macOS version: 15.3.1 (failing), 14.5 & 15.4 (working)

Expected behavior

The service should create a session and proceed with builds as expected.

Actual behavior

Fails with deserialization error on the SESSION_CREATED IPC message.

Additional context

• This issue seems tied to the serialization logic or Swift versioning between macOS 15.3.1 and others.
• If helpful, I can provide a minimal repro or logs.

Please let me know if there's anything I can help provide to debug this further. Thanks!

[Coder-Star]

This issue occurs when the diagnostics in the CreateSessionResponse are not empty.

I have conducted a preliminary check and the reason may be that DiagnosticData's serialization did not handle optionName compatibility，I noticed that the deserializer has made compatibility processing for optionName

public struct DiagnosticData: Serializable, Equatable, Hashable, Sendable, Codable {
    public let description: String
    public let component: Component
    public let optionName: String?

    public init(_ description: String, component: Component = .default, optionName: String? = nil) {
        self.description = description
        self.component = component
        self.optionName = optionName
    }

    public func serialize<T: Serializer>(to serializer: T) {
        serializer.serializeAggregate(3) {
            serializer.serialize(description)
            serializer.serialize(component)
            serializer.serialize(optionName)
        }
    }

    public init(from deserializer: any Deserializer) throws {
        let count = try deserializer.beginAggregate(2...3)
        self.description = try deserializer.deserialize()
        self.component = try deserializer.deserialize()
        self.optionName = (count >= 3) ? try deserializer.deserialize() : nil
    }
}

[jakepetroules]

I can't think of any reason off hand that this would be specific to OS version. @chatwyn Can you confirm which Xcode versions were in use in each of your test cases?

[Coder-Star]

This has nothing to do with the OS version，This issue will occur on Xcode 16.0~16.2, but there is no problem on Xcode 16.3 @jakepetroules

[Coder-Star]

@jakepetroules I see that the minimum version of Xcode required for the current merged code is still 16.2, why not 16.3?

[jakepetroules]

The swift.org CI is still on 16.2, so we can't raise it until that is upgraded.

[jakepetroules]

Note that we've been moving many of the IPC messages to use SerializableCodable over time, which wraps a JSON payload in a MsgPack message, rather than encoding as a MsgPack message directly -- this makes older clients more resilient to changes, since decoding can simply ignore unknown JSON fields, unlike MsgPack where the field count must be exactly the same as the old client expected.

[Coder-Star]

The swift.org CI is still on 16.2, so we can't raise it until that is upgraded.

Based on our testing, we have only found this one compatibility issue. Can I suggest a PR compatibility issue for 16.0-16.2?