Fix setting SPECIALIZATION_SDK_OPTIONS, .ARCHS (swiftlang#9379)

The code to build up the SPECIALIZATION_SDK_OPTIONS list was appending
to the existing list, but the list is not guarenteed to be initialized.
If it isn't initialized, the append never happens and the setting is
dropped.

Author: plemarquand

Fixtures/PIFBuilder/PackageWithSDKSpecialization/Package.swift

@@ -0,0 +1,23 @@
+// swift-tools-version: 6.2
+
+import PackageDescription
+
+let package = Package(
+    name: "PackageWithSDKSpecialization",
+    platforms: [ .macOS("10.15.foo") ],
+    products: [
+        .library(
+            name: "PackageWithSDKSpecialization",
+            targets: ["PackageWithSDKSpecialization"]),
+    ],
+    targets: [
+        .target(
+            name: "PackageWithSDKSpecialization",
+            dependencies: []
+        ),
+        .target(
+            name: "Executable",
+            dependencies: ["PackageWithSDKSpecialization"]
+        ),
+    ]
+)

Fixtures/PIFBuilder/PackageWithSDKSpecialization/Sources/Executable/main.swift

@@ -0,0 +1 @@
+print("Hello, world!")

Fixtures/PIFBuilder/PackageWithSDKSpecialization/Sources/PackageWithSDKSpecialization/PackageWithSDKSpecialization.swift

@@ -0,0 +1,3 @@
+struct PackageWithSDKSpecialization {
+    var text = "Hello, World!"
+}

Sources/SwiftBuildSupport/PackagePIFBuilder.swift

@@ -581,7 +581,7 @@ public final class PackagePIFBuilder {
                 log(.warning, "Ignoring options '\(platformOptions.joined(separator: " "))' specified for unknown platform \(platform.name)")
                 continue
             }
-            settings[.SPECIALIZATION_SDK_OPTIONS, pifPlatform]?.append(contentsOf: platformOptions)
+            settings[.SPECIALIZATION_SDK_OPTIONS, pifPlatform] = (settings[.SPECIALIZATION_SDK_OPTIONS, pifPlatform] ?? []) + platformOptions
         }
 
         let deviceFamilyIDs: Set<Int> = self.delegate.deviceFamilyIDs()
@@ -607,7 +607,7 @@ public final class PackagePIFBuilder {
                 } catch {
                     preconditionFailure("Unhandled arm64e platform: \(error)")
                 }
-                settings[.ARCHS, pifPlatform]?.append(contentsOf: ["arm64e"])
+                settings[.ARCHS, pifPlatform] = (settings[.ARCHS, pifPlatform] ?? []) + ["arm64e"]
             }
         }
 

Tests/SwiftBuildSupportTests/PIFBuilderTests.swift

@@ -118,6 +118,19 @@ extension SwiftBuildSupport.PIF.Project {
             throw StringError("Multiple targets named \(name) in PIF project")
         }
     }
+
+    fileprivate func buildConfig(named name: String) throws -> SwiftBuild.ProjectModel.BuildConfig {
+        let matchingConfigs = underlying.buildConfigs.filter {
+            $0.name == name
+        }
+        if matchingConfigs.isEmpty {
+            throw StringError("No config named \(name) in PIF project")
+        } else if matchingConfigs.count > 1 {
+            throw StringError("Multiple configs named \(name) in PIF project")
+        } else {
+            return matchingConfigs[0]
+        }
+    }
 }
 
 extension SwiftBuild.ProjectModel.BaseTarget {
@@ -177,6 +190,19 @@ struct PIFBuilderTests {
         }
     }
 
+    @Test func packageWithInternal() async throws {
+        try await withGeneratedPIF(fromFixture: "PIFBuilder/PackageWithSDKSpecialization") { pif, observabilitySystem in
+            let errors = observabilitySystem.diagnostics.filter { $0.severity == .error }
+            #expect(errors.isEmpty, "Expected no errors during PIF generation, but got: \(errors)")
+
+            let releaseConfig = try pif.workspace
+                .project(named: "PackageWithSDKSpecialization")
+                .buildConfig(named: "Release")
+
+            #expect(releaseConfig.settings[.SPECIALIZATION_SDK_OPTIONS, .macOS] == ["foo"])
+        }
+    }
+
     @Test func pluginWithBinaryTargetDependency() async throws {
         try await withGeneratedPIF(fromFixture: "Miscellaneous/Plugins/BinaryTargetExePlugin") { pif, observabilitySystem in
             // Verify that PIF generation succeeds for a package with a plugin that depends on a binary target