Re-enable SwiftInterfaceTests.testSystemModuleInterface

Sources/SKTestSupport/TestServer.swift

@@ -15,7 +15,7 @@ import SKCore
 import LanguageServerProtocol
 import LanguageServerProtocolJSONRPC
 import SourceKitLSP
-import class Foundation.Pipe
+import Foundation
 import LSPTestSupport
 
 public final class TestSourceKitServer {
@@ -36,22 +36,40 @@ public final class TestSourceKitServer {
 
   public static let serverOptions: SourceKitServer.Options = SourceKitServer.Options()
 
+  /// If the server is not using the global module cache, the path of the local
+  /// module cache.
+  ///
+  /// This module cache will be deleted when the test server is destroyed.
+  private let moduleCache: URL?
+
   public let client: TestClient
   let connImpl: ConnectionImpl
 
   public var hasShutdown: Bool = false
 
   /// The server, if it is in the same process.
   public let server: SourceKitServer?
-
-  public init(connectionKind: ConnectionKind = .local) {
+
+  /// - Parameters:
+  ///   - useGlobalModuleCache: If `false`, the server will use its own module
+  ///     cache in an empty temporary directory instead of the global module cache.
+  public init(connectionKind: ConnectionKind = .local, useGlobalModuleCache: Bool = true) {
+    if !useGlobalModuleCache {
+      moduleCache = URL(fileURLWithPath: NSTemporaryDirectory()).appendingPathComponent(UUID().uuidString)
+    } else {
+      moduleCache = nil
+    }
+    var serverOptions = Self.serverOptions
+    if let moduleCache {
+      serverOptions.buildSetup.flags.swiftCompilerFlags += ["-module-cache-path", moduleCache.path]
+    }
 
     switch connectionKind {
       case .local:
         let clientConnection = LocalConnection()
         let serverConnection = LocalConnection()
         client = TestClient(server: serverConnection)
-        server = SourceKitServer(client: clientConnection, options: Self.serverOptions, onExit: {
+        server = SourceKitServer(client: clientConnection, options: serverOptions, onExit: {
           clientConnection.close()
         })
 
@@ -76,7 +94,7 @@ public final class TestSourceKitServer {
         )
 
         client = TestClient(server: clientConnection)
-        server = SourceKitServer(client: serverConnection, options: Self.serverOptions, onExit: {
+        server = SourceKitServer(client: serverConnection, options: serverOptions, onExit: {
           serverConnection.close()
         })
 
@@ -101,6 +119,10 @@ public final class TestSourceKitServer {
 
   deinit {
     close()
+
+    if let moduleCache {
+      try? FileManager.default.removeItem(at: moduleCache)
+    }
   }
 
   func close() {

Tests/SourceKitLSPTests/SwiftInterfaceTests.swift

@@ -33,7 +33,12 @@ final class SwiftInterfaceTests: XCTestCase {
   }
 
   override func setUp() {
-    connection = TestSourceKitServer()
+    // This is the only test that references modules from the SDK (Foundation).
+    // `testSystemModuleInterface` has been flaky for a long while and a
+    // hypothesis is that it was failing because of a malformed global module
+    // cache that might still be present from previous CI runs. If we use a
+    // local module cache, we define away that source of bugs.
+    connection = TestSourceKitServer(useGlobalModuleCache: false)
     sk = connection.client
     _ = try! sk.sendSync(InitializeRequest(
       processId: nil,
@@ -56,20 +61,8 @@ final class SwiftInterfaceTests: XCTestCase {
     sk = nil
     connection = nil
   }
-
-  func testSystemModuleInterface() throws {
-    try XCTSkipIf(true, "Test is flaky - rdar://108256204")
-    // This test is failing non-deterministically in CI becaue the file contents
-    // of the generated interface just contain a newline.
-    // I cannot reproduce the failure locally. Add some logging to determine
-    // whether the issue is sourcekitd not returning an empty generated
-    // interface or something around how the file is handled.
-    // Remove this lowering of the log level once we have determined what the
-    // issue is (rdar://104871745).
-    let previousLogLevel = Logger.shared.currentLevel
-    defer { Logger.shared.setLogLevel(previousLogLevel.description) }
-    Logger.shared.setLogLevel("debug")
 
+  func testSystemModuleInterface() throws {
     let url = URL(fileURLWithPath: "/\(UUID())/a.swift")
     let uri = DocumentURI(url)