[core] Allow overriding the default toolchain using env TOOLCHAINS on Darwin

To match the behaviour of `xcrun`.

Author: benlangmuir

Sources/SKCore/ToolchainRegistry.swift

@@ -36,6 +36,14 @@ public final class ToolchainRegistry {
   /// Mutex for registering and accessing toolchains.
   var queue: DispatchQueue = DispatchQueue(label: "toolchain-registry-queue")
 
+  /// The currently selected toolchain identifier on Darwin.
+  public internal(set) lazy var darwinToolchainOverride: String? = {
+    if let id = getenv("TOOLCHAINS"), !id.isEmpty, id != "default" {
+      return id
+    }
+    return nil
+  }()
+
   /// Creates an empty toolchain registry.
   public init() {}
 }
@@ -75,16 +83,16 @@ extension ToolchainRegistry {
 
   /// The default toolchain.
   ///
-  /// On Darwin, this is typically the toolchain with the identifier
-  /// `darwinDefaultToolchainIdentifier`, i.e. the default toolchain of the active Xcode. Otherwise
-  /// it is the first toolchain that was registered, if any.
+  /// On Darwin, this is typically the toolchain with the identifier `darwinToolchainIdentifier`,
+  /// i.e. the default toolchain of the active Xcode. Otherwise it is the first toolchain that was
+  /// registered, if any.
   ///
   /// The default toolchain must be only of the registered toolchains.
   public var `default`: Toolchain? {
     get {
       return queue.sync {
         if _default == nil {
-          if let tc = toolchainIdentifiers[ToolchainRegistry.darwinDefaultToolchainIdentifier] {
+          if let tc = toolchainIdentifiers[darwinToolchainIdentifier] {
             _default = tc
           } else {
             _default = _toolchains.first
@@ -108,10 +116,16 @@ extension ToolchainRegistry {
   }
 
   /// The standard default toolchain identifier on Darwin.
+  public static let darwinDefaultToolchainIdentifier: String = "com.apple.dt.toolchain.XcodeDefault"
+
+  /// The current toolchain identifier on Darwin, which is either specified byt the `TOOLCHAINS`
+  /// environment variable, or defaults to `darwinDefaultToolchainIdentifier`.
   ///
   /// The value of `default.identifier` may be different if the default toolchain has been
-  /// explicitly overridden, or if there is no toolchain with this identifier.
-  public static let darwinDefaultToolchainIdentifier: String = "com.apple.dt.toolchain.XcodeDefault"
+  /// explicitly overridden in code, or if there is no toolchain with this identifier.
+  public var darwinToolchainIdentifier: String {
+    return darwinToolchainOverride ?? ToolchainRegistry.darwinDefaultToolchainIdentifier
+  }
 
   /// All toolchains, in the order they were added.
   public var toolchains: [Toolchain] {

Tests/SKCoreTests/ToolchainRegistryTests.swift

@@ -37,6 +37,7 @@ final class ToolchainRegistryTests: XCTestCase {
     Platform.currentPlatform = .darwin
 
     let tr = ToolchainRegistry()
+    tr.darwinToolchainOverride = nil
     XCTAssertNil(tr.default)
     let a = Toolchain(identifier: "a", displayName: "a", path: nil)
     try! tr.registerToolchain(a)
@@ -69,6 +70,7 @@ final class ToolchainRegistryTests: XCTestCase {
 #if os(macOS)
     let fs = InMemoryFileSystem()
     let tr1 = ToolchainRegistry(fs)
+    tr1.darwinToolchainOverride = nil
 
     let xcodeDeveloper = ToolchainRegistry.currentXcodeDeveloperPath!
     let toolchains = xcodeDeveloper.appending(components: "Toolchains")
@@ -90,6 +92,7 @@ final class ToolchainRegistryTests: XCTestCase {
     XCTAssertEqual(tr1.toolchains.count, 1)
 
     let tr = ToolchainRegistry(fs)
+    tr.darwinToolchainOverride = nil
 
     XCTAssertEqual(tr.default?.identifier, ToolchainRegistry.darwinDefaultToolchainIdentifier)
     XCTAssertEqual(tr.default?.path, toolchains.appending(component: "XcodeDefault.xctoolchain"))
@@ -161,6 +164,11 @@ final class ToolchainRegistryTests: XCTestCase {
     let tc = Toolchain(path, fs)
     XCTAssertNotNil(tc)
     XCTAssertEqual(tc?.identifier, "org.fake.explicit")
+
+    let overrideReg = ToolchainRegistry(fs)
+    overrideReg.darwinToolchainOverride = "org.fake.global.B"
+    XCTAssertEqual(overrideReg.darwinToolchainIdentifier, "org.fake.global.B")
+    XCTAssertEqual(overrideReg.default?.identifier, "org.fake.global.B")
 #endif
   }