Add request to get preferred run destination for a platform

Sources/SWBBuildService/Messages.swift

@@ -1528,6 +1528,31 @@ private struct ClearAllCaches: MessageHandler {
     }
 }
 
+private struct GetPlatformPreferredRunDestinationMsg: MessageHandler {
+    struct PlatformNotFoundError: Error, CustomStringConvertible {
+        let platform: String
+        var description: String { "Unknown platform '\(platform)'" }
+    }
+    struct UnknownPreferredArchError: Error, CustomStringConvertible {
+        let platform: String
+        var description: String { "Platform '\(platform)' does not specify a preferred architecture" }
+    }
+
+    func handle(request: Request, message: GetPlatformPreferredRunDestinationRequest) async throws -> GetPlatformPreferredRunDestinationResponse {
+        let session = try request.session(for: message)
+        guard let platform = session.core.platformRegistry.lookup(name: message.platform) else {
+            throw PlatformNotFoundError(platform: message.platform)
+        }
+        // All relevant platforms define a preferredArch, so the undefined_arch fallback case should never happen
+        // in practice, and indicates a serious issue occurred during plugin loading.
+        guard let targetArchitecture = platform.preferredArch else {
+            throw UnknownPreferredArchError(platform: message.platform)
+        }
+        let runDestination = RunDestinationInfo(platform: platform.name, sdk: platform.name, sdkVariant: nil, targetArchitecture: targetArchitecture, supportedArchitectures: [targetArchitecture], disableOnlyActiveArch: false, hostTargetedPlatform: nil)
+        return GetPlatformPreferredRunDestinationResponse(info: runDestination)
+    }
+}
+
 // MARK: ServiceExtension Support
 
 public struct ServiceSessionMessageHandlers: ServiceExtension {
@@ -1593,6 +1618,7 @@ package struct ServiceMessageHandlers: ServiceExtension {
         service.registerMessageHandler(GetStatisticsDumpMsg.self)
         service.registerMessageHandler(GetBuildSettingsDescriptionDumpMsg.self)
         service.registerMessageHandler(ExecuteCommandLineToolMsg.self)
+        service.registerMessageHandler(GetPlatformPreferredRunDestinationMsg.self)
 
         service.registerMessageHandler(CreateXCFrameworkHandler.self)
 

Sources/SWBProtocol/Message.swift

@@ -65,7 +65,7 @@ public struct PingRequest: RequestMessage, Equatable {
 
 public struct SetConfigItemRequest: RequestMessage, Equatable {
     public typealias ResponseMessage = VoidResponse
-    
+
     public static let name = "SET_CONFIG_ITEM"
 
     public let key: String
@@ -286,6 +286,33 @@ public struct GetBuildSettingsDescriptionRequest: SessionMessage, RequestMessage
     }
 }
 
+/// Get the preferred run destination for a given platform
+public struct GetPlatformPreferredRunDestinationRequest: SessionMessage, RequestMessage, Equatable, SerializableCodable {
+    public typealias ResponseMessage = GetPlatformPreferredRunDestinationResponse
+
+    public static let name = "GET_PLATFORM_PREFERRED_RUN_DESTINATION_REQUEST"
+
+    public let sessionHandle: String
+
+    /// The name of the platform to get the run destination for, eg. `macosx`, `iphoneos`, `iphonesimulator`
+    public let platform: String
+
+    public init(sessionHandle: String, platform: String) {
+        self.sessionHandle = sessionHandle
+        self.platform = platform
+    }
+}
+
+public struct GetPlatformPreferredRunDestinationResponse: Message, Equatable, SerializableCodable {
+    public static let name = "GET_PLATFORM_PREFERRED_RUN_DESTINATION_RESPONSE"
+
+    public let info: RunDestinationInfo
+
+    public init(info: RunDestinationInfo) {
+        self.info = info
+    }
+}
+
 public struct CreateXCFrameworkRequest: RequestMessage, Equatable, SerializableCodable {
     public typealias ResponseMessage = StringResponse
 
@@ -1171,6 +1198,9 @@ public struct IPCMessage: Serializable, Sendable {
         GetBuildSettingsDescriptionRequest.self,
         ExecuteCommandLineToolRequest.self,
 
+        GetPlatformPreferredRunDestinationRequest.self,
+        GetPlatformPreferredRunDestinationResponse.self,
+
         CreateSessionRequest.self,
         CreateSessionResponse.self,
         SetSessionSystemInfoRequest.self,

Sources/SwiftBuild/SWBBuildServiceSession.swift

@@ -611,6 +611,19 @@ public final class SWBBuildServiceSession: Sendable {
         try await service.send(request: DeveloperPathRequest(sessionHandle: uid)).value
     }
 
+    public func preferredRunDestination(forPlatform platformName: String) async throws -> SWBRunDestinationInfo {
+        let runDestination = try await service.send(request: GetPlatformPreferredRunDestinationRequest(sessionHandle: uid, platform: platformName)).info
+        return SWBRunDestinationInfo(
+            platform: runDestination.platform,
+            sdk: runDestination.sdk,
+            sdkVariant: runDestination.sdkVariant,
+            targetArchitecture: runDestination.targetArchitecture,
+            supportedArchitectures: runDestination.supportedArchitectures.elements,
+            disableOnlyActiveArch: runDestination.disableOnlyActiveArch,
+            hostTargetedPlatform: runDestination.hostTargetedPlatform
+        )
+    }
+
     /// Set the session system information.
     public func setSystemInfo(_ systemInfo: SWBSystemInfo) async throws {
         _ = try await service.send(request: SetSessionSystemInfoRequest(sessionHandle: uid, operatingSystemVersion: Version(systemInfo.operatingSystemVersion), productBuildVersion: systemInfo.productBuildVersion, nativeArchitecture: systemInfo.nativeArchitecture))

Tests/SwiftBuildTests/PreferredRunDestinationTests.swift

@@ -0,0 +1,53 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import Testing
+import SwiftBuild
+import SwiftBuildTestSupport
+import SWBTestSupport
+@_spi(Testing) import SWBUtil
+
+@Suite
+fileprivate struct PreferredRunDestinationTests {
+    @Test(.requireSDKs(.macOS))
+    func platformExists() async throws {
+        try await withTemporaryDirectory { (temporaryDirectory: NamedTemporaryDirectory) in
+            try await withAsyncDeferrable { deferrable in
+                let testSession = try await TestSWBSession(temporaryDirectory: temporaryDirectory)
+                await deferrable.addBlock {
+                    await #expect(throws: Never.self) {
+                        try await testSession.close()
+                    }
+                }
+                let runDestination = try await testSession.session.preferredRunDestination(forPlatform: "macosx")
+                #expect(runDestination.sdk == "macosx")
+            }
+        }
+    }
+
+    @Test
+    func unknownPlatform() async throws {
+        try await withTemporaryDirectory { (temporaryDirectory: NamedTemporaryDirectory) in
+            try await withAsyncDeferrable { deferrable in
+                let testSession = try await TestSWBSession(temporaryDirectory: temporaryDirectory)
+                await deferrable.addBlock {
+                    await #expect(throws: Never.self) {
+                        try await testSession.close()
+                    }
+                }
+                await #expect(throws: (any Error).self) {
+                    try await testSession.session.preferredRunDestination(forPlatform: "unknown_platform")
+                }
+            }
+        }
+    }
+}