// Copyright 2016 The Tulsi Authors. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

import Foundation


/// Provides functionality to generate an Xcode project from a TulsiGeneratorConfig.
final class XcodeProjectGenerator {
  enum ProjectGeneratorError: Error {
    /// General Xcode project creation failure with associated debug info.
    case serializationFailed(String)

    /// The aspect info for the labels could not be built.
    case labelAspectFailure(String)

    /// The given labels failed to resolve to valid targets.
    case labelResolutionFailed(Set<BuildLabel>)

    /// The given |path| to generate this Xcode project is invalid because it is within |reason|.
    case invalidXcodeProjectPath(path: String, reason: String)
  }

  /// Encapsulates the source paths of various resources (scripts, utilities, etc...) that will be
  /// copied into the generated Xcode project.
  struct ResourceSourcePathURLs {
    let buildScript: URL  // The script to run on "build" actions.
    let cleanScript: URL  // The script to run on "clean" actions.
    let extraBuildScripts: [URL] // Any additional scripts to install into the project bundle.
    let postProcessor: URL  // Binary post processor utility.
    let iOSUIRunnerEntitlements: URL  // Entitlements file template for iOS UI Test runner apps.
    let macOSUIRunnerEntitlements: URL  // Entitlements file template for macOS UI Test runner apps.
    let stubInfoPlist: URL  // Stub Info.plist (needed for Xcode 8).
    let stubIOSAppExInfoPlistTemplate: URL  // Stub Info.plist (needed for app extension targets).
    let stubWatchOS2InfoPlist: URL  // Stub Info.plist (needed for watchOS2 app targets).
    let stubWatchOS2AppExInfoPlist: URL  // Stub Info.plist (needed for watchOS2 appex targets).

    // In order to load tulsi_aspects, Tulsi constructs a Bazel repository inside of the generated
    // Xcode project. Its structure looks like this:
    // ├── Bazel
    // │   ├── WORKSPACE
    // │   └── tulsi
    // │       ├── file1
    // │       └── ...
    // These two items define the content of this repository, including the WORKSPACE file and the
    // "tulsi" package.
    let bazelWorkspaceFile: URL // Stub WORKSPACE file.
    let tulsiPackageFiles: [URL] // Files to copy into the "tulsi" package.
  }

  /// Path relative to PROJECT_FILE_PATH in which Tulsi generated files (scripts, artifacts, etc...)
  /// should be placed.
  private static let TulsiArtifactDirectory = ".tulsi"
  static let ScriptDirectorySubpath = "\(TulsiArtifactDirectory)/Scripts"
  static let BazelDirectorySubpath = "\(TulsiArtifactDirectory)/Bazel"
  static let TulsiPackageName = "tulsi"
  static let UtilDirectorySubpath = "\(TulsiArtifactDirectory)/Utils"
  static let ConfigDirectorySubpath = "\(TulsiArtifactDirectory)/Configs"
  static let ProjectResourcesDirectorySubpath = "\(TulsiArtifactDirectory)/Resources"
  private static let BuildScript = "bazel_build.py"
  private static let CleanScript = "bazel_clean.sh"
  private static let ShellCommandsUtil = "bazel_cache_reader"
  private static let ShellCommandsCleanScript = "clean_symbol_cache"
  private static let WorkspaceFile = "WORKSPACE"
  private static let PostProcessorUtil = "post_processor"
  private static let IOSUIRunnerEntitlements = "iOSXCTRunner.entitlements"
  private static let MacOSUIRunnerEntitlements = "macOSXCTRunner.entitlements"
  private static let StubInfoPlistFilename = "StubInfoPlist.plist"
  private static let StubWatchOS2InfoPlistFilename = "StubWatchOS2InfoPlist.plist"
  private static let StubWatchOS2AppExInfoPlistFilename = "StubWatchOS2AppExInfoPlist.plist"
  private static let CachedExecutionRootFilename = "execroot_path.py"
  private static let DefaultSwiftVersion = "4"
  private static let SupportScriptsPath = "Library/Application Support/Tulsi/Scripts/"

  /// Rules which should not be generated at the top level.
  private static let LibraryRulesForTopLevelWarning =
      Set(["objc_library", "swift_library", "cc_library"])

  private let workspaceRootURL: URL
  private let config: TulsiGeneratorConfig
  private let localizedMessageLogger: LocalizedMessageLogger
  private let fileManager: FileManager
  private let workspaceInfoExtractor: BazelWorkspaceInfoExtractorProtocol
  private let resourceURLs: ResourceSourcePathURLs
  private let tulsiVersion: String

  private let pbxTargetGeneratorType: PBXTargetGeneratorProtocol.Type

  /// Exposed for testing. Simply writes the given NSData to the given NSURL.
  /// TODO(dmishe): Use fileManager instance to perform writes, and remove this block.
  var writeDataHandler: (URL, Data) throws -> Void = { (outputFileURL: URL, data: Data) in
    try data.write(to: outputFileURL, options: NSData.WritingOptions.atomic)
  }

  /// Exposed for testing. Returns the current user name.
  var usernameFetcher: () -> String = NSUserName

  /// Exposed for testing. Suppresses writing any preprocessor defines integral to Bazel itself into
  /// the generated project.
  var suppressCompilerDefines = false

  /// Exposed for testing. Instead of writing the real workspace name into the generated project,
  /// write a stub value that will be the same regardless of the execution environment.
  var redactWorkspaceSymlink = false

  /// Exposed for testing. Suppresses creating folders for artifacts that are expected to be
  /// generated by Bazel.
  var suppressGeneratedArtifactFolderCreation = false

  var cachedDefaultSwiftVersion: String?

  /// Computed property to determine if DBGShellCommands is actively caching dSYM for this project.
  var disableDBGShellCommandsCaching: Bool {
    return config.options[.DisableDBGShellCommandsCaching].commonValueAsBool ?? true
  }

  /// Computed property to determine if all debugging info should be normalized, if possible.
  var disableNormalizedDebugPrefixMap: Bool {
    return config.options[.DisableNormalizedDebugPrefixMap].commonValueAsBool ?? true
  }

  init(workspaceRootURL: URL,
       config: TulsiGeneratorConfig,
       localizedMessageLogger: LocalizedMessageLogger,
       workspaceInfoExtractor: BazelWorkspaceInfoExtractorProtocol,
       resourceURLs: ResourceSourcePathURLs,
       tulsiVersion: String,
       fileManager: FileManager = FileManager.default,
       pbxTargetGeneratorType: PBXTargetGeneratorProtocol.Type = PBXTargetGenerator.self) {
    self.workspaceRootURL = workspaceRootURL
    self.config = config
    self.localizedMessageLogger = localizedMessageLogger
    self.workspaceInfoExtractor = workspaceInfoExtractor
    self.resourceURLs = resourceURLs
    self.tulsiVersion = tulsiVersion
    self.fileManager = fileManager
    self.pbxTargetGeneratorType = pbxTargetGeneratorType
  }

  /// Determines the "best" common SDKROOT for a sequence of RuleEntries.
  static func projectSDKROOT<T>(_ targetRules: T) -> String? where T: Sequence, T.Iterator.Element == RuleEntry {
    var discoveredSDKs = Set<String>()
    for entry in targetRules {
      if let sdkroot = entry.XcodeSDKRoot {
        discoveredSDKs.insert(sdkroot)
      }
    }

    if discoveredSDKs.count == 1 {
      return discoveredSDKs.first!
    }

    if discoveredSDKs.isEmpty {
      // In practice this should not happen since it'd indicate a project that won't be able to
      // build. It is possible that the user is in the process of creating a new project, so
      // rather than fail the generation a default is selected. Since iOS happens to be the best
      // supported type by Bazel at the time of this writing, it is chosen as the default.
      return "iphoneos"
    }

    if discoveredSDKs == ["iphoneos", "watchos"] {
      // Projects containing just an iPhone host and a watchOS app use iphoneos as the project SDK
      // to match Xcode's behavior.
      return "iphoneos"
    }

    // Projects that have a collection that is not mappable to a standard Xcode project simply
    // do not set the SDKROOT. Unfortunately this will cause "My Mac" to be listed as a target
    // device regardless of whether or not the selected build target supports it, but this is
    // a somewhat better user experience when compared to any other device SDK (in which Xcode
    // will display every simulator for that platform regardless of whether or not the build
    // target can be run on them).
    return nil
  }

  /// Generates an Xcode project bundle in the given folder.
  /// NOTE: This may be a long running operation.
  func generateXcodeProjectInFolder(_ outputFolderURL: URL) throws -> URL {
    let generateProfilingToken = localizedMessageLogger.startProfiling("generating_project",
                                                                       context: config.projectName)
    defer { localizedMessageLogger.logProfilingEnd(generateProfilingToken) }
    try validateXcodeProjectPath(outputFolderURL)
    try resolveConfigReferences()

    let mainGroup = pbxTargetGeneratorType.mainGroupForOutputFolder(outputFolderURL,
                                                                    workspaceRootURL: workspaceRootURL)

    let projectResourcesDirectory = "${PROJECT_FILE_PATH}/\(XcodeProjectGenerator.ProjectResourcesDirectorySubpath)"
    let plistPaths = StubInfoPlistPaths(
      resourcesDirectory: projectResourcesDirectory,
      defaultStub: "\(projectResourcesDirectory)/\(XcodeProjectGenerator.StubInfoPlistFilename)",
      watchOSStub: "\(projectResourcesDirectory)/\(XcodeProjectGenerator.StubWatchOS2InfoPlistFilename)",
      watchOSAppExStub: "\(projectResourcesDirectory)/\(XcodeProjectGenerator.StubWatchOS2AppExInfoPlistFilename)")

    let projectInfo = try buildXcodeProjectWithMainGroup(mainGroup,
                                                         stubInfoPlistPaths: plistPaths)

    let serializingProgressNotifier = ProgressNotifier(name: SerializingXcodeProject,
                                                       maxValue: 1,
                                                       indeterminate: true)
    let serializer = OpenStepSerializer(rootObject: projectInfo.project,
                                        gidGenerator: ConcreteGIDGenerator())

    let serializingProfileToken = localizedMessageLogger.startProfiling("serializing_project",
                                                                        context: config.projectName)
    guard let serializedXcodeProject = serializer.serialize() else {
      throw ProjectGeneratorError.serializationFailed("OpenStep serialization failed")
    }
    localizedMessageLogger.logProfilingEnd(serializingProfileToken)

    let projectBundleName = config.xcodeProjectFilename
    let projectURL = outputFolderURL.appendingPathComponent(projectBundleName)
    if !createDirectory(projectURL) {
      throw ProjectGeneratorError.serializationFailed("Project directory creation failed")
    }

    let pbxproj = projectURL.appendingPathComponent("project.pbxproj")
    try writeDataHandler(pbxproj, serializedXcodeProject)
    serializingProgressNotifier.incrementValue()

    try installWorkspaceSettings(projectURL)
    try installXcodeSchemesForProjectInfo(projectInfo,
                                          projectURL: projectURL,
                                          projectBundleName: projectBundleName)
    installTulsiScripts(projectURL)
    installTulsiBazelPackage(projectURL)
    installUtilities(projectURL)
    installGeneratorConfig(projectURL)
    installGeneratedProjectResources(projectURL)
    installStubExtensionPlistFiles(projectURL,
                                   rules: projectInfo.buildRuleEntries.filter { $0.pbxTargetType?.isiOSAppExtension ?? false },
                                   plistPaths: plistPaths)
    linkTulsiWorkspace()
    return projectURL
  }

  // MARK: - Private methods

  /// Extracts the default swift version to use for targets without an explicit default by running
  /// 'xcrun swift --version' if not already fetched.
  private func fetchDefaultSwitchVersion() -> String {
    // Used the already computed default version if it is available.
    if let defaultSwiftVersion = cachedDefaultSwiftVersion {
      return defaultSwiftVersion
    }

    let semaphore = DispatchSemaphore(value: 0)
    var completionInfo: ProcessRunner.CompletionInfo?
    let process = TulsiProcessRunner.createProcess("/usr/bin/xcrun",
                                                   arguments: ["swift", "--version"],
                                                   messageLogger: self.localizedMessageLogger,
                                                   loggingIdentifier: "extract_default_swift_version") {
                                                    processCompletionInfo in
      defer { semaphore.signal() }

      completionInfo = processCompletionInfo
    }
    process.launch()
    _ = semaphore.wait(timeout: DispatchTime.distantFuture)

    guard let info = completionInfo else {
      self.localizedMessageLogger.warning("ExtractingDefaultSwiftVersionFailed",
                                          comment: "Default version in %1$@, additional error context in %2$@.",
                                          values: XcodeProjectGenerator.DefaultSwiftVersion,
                                          "Internal error, unable to find process information")
      cachedDefaultSwiftVersion = XcodeProjectGenerator.DefaultSwiftVersion
      return XcodeProjectGenerator.DefaultSwiftVersion
    }

    guard info.terminationStatus == 0,
      let stdout = NSString(data: info.stdout, encoding: String.Encoding.utf8.rawValue) else {
        let stderr = NSString(data: info.stderr, encoding: String.Encoding.utf8.rawValue) ?? "<no stderr>"
        self.localizedMessageLogger.warning("ExtractingDefaultSwiftVersionFailed",
                                            comment: "Default version in %1$@, additional error context in %2$@.",
                                            values: XcodeProjectGenerator.DefaultSwiftVersion,
                                            "xcrun swift --version returned exitcode \(info.terminationStatus) with stderr: \(stderr)")
        cachedDefaultSwiftVersion = XcodeProjectGenerator.DefaultSwiftVersion
        return XcodeProjectGenerator.DefaultSwiftVersion
    }
    // Example output format:
    // Apple Swift version 4.0.3 (swiftlang-900.0.74.1 clang-900.0.39.2)
    // Target: x86_64-apple-macosx10.9
    //
    // Note that we only care about the major and minor version number (e.g. 4.0, not 4.0.3).
    let pattern = "^Apple\\sSwift\\sversion\\s([0-9]+\\.?[0-9]?)"
    guard let regExpr = try? NSRegularExpression(pattern: pattern, options: .caseInsensitive) else {
      self.localizedMessageLogger.warning("ExtractingDefaultSwiftVersionFailed",
                                          comment: "Default version in %1$@, additional error context in %2$@.",
                                          values: XcodeProjectGenerator.DefaultSwiftVersion,
                                          "Internal error, unable to create regular expression")
      cachedDefaultSwiftVersion = XcodeProjectGenerator.DefaultSwiftVersion
      return XcodeProjectGenerator.DefaultSwiftVersion
    }
    guard let match = regExpr.firstMatch(in: stdout as String,
                                         range: NSMakeRange(0, stdout.length)) else {
      self.localizedMessageLogger.warning("ExtractingDefaultSwiftVersionFailed",
                                          comment: "Default version in %1$@, additional error context in %2$@.",
                                          values: XcodeProjectGenerator.DefaultSwiftVersion,
                                          "Unable to parse version from xcrun output. Output: \(stdout)")
      cachedDefaultSwiftVersion = XcodeProjectGenerator.DefaultSwiftVersion
      return XcodeProjectGenerator.DefaultSwiftVersion
    }
    cachedDefaultSwiftVersion = stdout.substring(with: match.range(at: 1))
    return stdout.substring(with: match.range(at: 1))
  }

  /// Encapsulates information about the results of a buildXcodeProjectWithMainGroup invocation.
  private struct GeneratedProjectInfo {
    /// The newly created PBXProject instance.
    let project: PBXProject

    /// RuleEntry's for which build targets were created. Note that this list may differ from the
    /// set of targets selected by the user as part of the generator config.
    let buildRuleEntries: Set<RuleEntry>

    /// RuleEntry's for test_suite's for which special test schemes should be created.
    let testSuiteRuleEntries: [BuildLabel: RuleEntry]

    /// A mapping of indexer targets by name.
    let indexerTargets: [String: PBXTarget]
  }

  /// Throws an exception if the Xcode project path is found to be in a forbidden location,
  /// assuming macOS default of a case-insensitive filesystem.
  private func validateXcodeProjectPath(_ outputPath: URL) throws {
    for (invalidPath, reason) in invalidXcodeProjectPathsWithReasons {
      if outputPath.absoluteString.lowercased().range(of: invalidPath.lowercased()) != nil {
        throw ProjectGeneratorError.invalidXcodeProjectPath(path: outputPath.path, reason: reason +
            " (\"\(invalidPath)\")")
      }
    }
  }

  /// Invokes Bazel to load any missing information in the config file.
  private func resolveConfigReferences() throws {
    let ruleEntryMap = try loadRuleEntryMap()
    let unresolvedLabels = config.buildTargetLabels.filter {
      !ruleEntryMap.hasAnyRuleEntry(withBuildLabel: $0)
    }
    if !unresolvedLabels.isEmpty {
      throw ProjectGeneratorError.labelResolutionFailed(Set<BuildLabel>(unresolvedLabels))
    }
    for label in config.buildTargetLabels {
      if let entry = ruleEntryMap.anyRuleEntry(withBuildLabel: label),
         XcodeProjectGenerator.LibraryRulesForTopLevelWarning.contains(entry.type) {
        localizedMessageLogger.warning("TopLevelLibraryTarget",
                                       comment: "Warning when a library target is used as a top level buildTarget. Target in %1$@, target type in %2$@.",
                                       values: entry.label.description, entry.type)
      }
    }
  }

  // Generates a PBXProject and a returns it along with a set of build, test and indexer targets.
  private func buildXcodeProjectWithMainGroup(_ mainGroup: PBXGroup,
                                              stubInfoPlistPaths: StubInfoPlistPaths) throws -> GeneratedProjectInfo {
    let xcodeProject = PBXProject(name: config.projectName, mainGroup: mainGroup)

    if let enabled = config.options[.SuppressSwiftUpdateCheck].commonValueAsBool, enabled {
      xcodeProject.lastSwiftUpdateCheck = "0710"
    }

    let buildScriptPath = "${PROJECT_FILE_PATH}/\(XcodeProjectGenerator.ScriptDirectorySubpath)/\(XcodeProjectGenerator.BuildScript)"
    let cleanScriptPath = "${PROJECT_FILE_PATH}/\(XcodeProjectGenerator.ScriptDirectorySubpath)/\(XcodeProjectGenerator.CleanScript)"


    let generator = pbxTargetGeneratorType.init(bazelURL: config.bazelURL,
                                                bazelBinPath: workspaceInfoExtractor.bazelBinPath,
                                                project: xcodeProject,
                                                buildScriptPath: buildScriptPath,
                                                stubInfoPlistPaths: stubInfoPlistPaths,
                                                tulsiVersion: tulsiVersion,
                                                options: config.options,
                                                localizedMessageLogger: localizedMessageLogger,
                                                workspaceRootURL: workspaceRootURL,
                                                suppressCompilerDefines: suppressCompilerDefines,
                                                redactWorkspaceSymlink: redactWorkspaceSymlink)

    if let additionalFilePaths = config.additionalFilePaths {
      generator.generateFileReferencesForFilePaths(additionalFilePaths)
    }

    let ruleEntryMap = try loadRuleEntryMap()
    var expandedTargetLabels = Set<BuildLabel>()
    var testSuiteRules = [BuildLabel: RuleEntry]()
    // Ideally this should use a generic SequenceType, but Swift 2.2 sometimes crashes in this case.
    // TODO(abaire): Go back to using a generic here when support for Swift 2.2 is removed.
    func expandTargetLabels(_ labels: Set<BuildLabel>) {
      for label in labels {
        // Effectively we will only be using the last RuleEntry in the case of duplicates.
        // We could log about duplicates here, but this would only lead to duplicate logging.
        let ruleEntries = ruleEntryMap.ruleEntries(buildLabel: label)
        for ruleEntry in ruleEntries {
          if ruleEntry.type != "test_suite" {
            // Add the RuleEntry itself and any registered extensions.
            expandedTargetLabels.insert(label)
            expandedTargetLabels.formUnion(ruleEntry.extensions)

            // Recursively expand extensions. Currently used by App -> Watch App -> Watch Extension.
            expandTargetLabels(ruleEntry.extensions)
          } else {
            // Expand the test_suite to its set of tests.
            testSuiteRules[ruleEntry.label] = ruleEntry
            expandTargetLabels(ruleEntry.testSuiteDependencies)
          }
        }
      }
    }
    let buildTargetLabels = Set(config.buildTargetLabels)
    expandTargetLabels(buildTargetLabels)

    var targetRules = Set<RuleEntry>()
    var hostTargetLabels = [BuildLabel: BuildLabel]()

    func profileAction(_ name: String, action: () throws -> Void) rethrows {
      let profilingToken = localizedMessageLogger.startProfiling(name, context: config.projectName)
      try action()
      localizedMessageLogger.logProfilingEnd(profilingToken)
    }

    profileAction("gathering_sources_for_indexers") {
      // Map from RuleEntry to cumulative preprocessor framework search paths.
      // This is used to propagate framework search paths up the graph while also making sure that
      // each RuleEntry is only registered once.
      var processedEntries = [RuleEntry: (NSOrderedSet)]()
      let progressNotifier = ProgressNotifier(name: GatheringIndexerSources,
                                              maxValue: expandedTargetLabels.count)
      for label in expandedTargetLabels {
        progressNotifier.incrementValue()
        let ruleEntries = ruleEntryMap.ruleEntries(buildLabel: label)
        guard !ruleEntries.isEmpty else {
          localizedMessageLogger.error("UnknownTargetRule",
                                       comment: "Failure to look up a Bazel target that was expected to be present. The target label is %1$@",
                                       context: config.projectName,
                                       values: label.value)
          continue
        }
        for ruleEntry in ruleEntries {
          targetRules.insert(ruleEntry)
          for hostTargetLabel in ruleEntry.linkedTargetLabels {
            hostTargetLabels[hostTargetLabel] = ruleEntry.label
          }
          autoreleasepool {
            generator.registerRuleEntryForIndexer(ruleEntry,
                                                  ruleEntryMap: ruleEntryMap,
                                                  pathFilters: config.pathFilters,
                                                  processedEntries: &processedEntries)
          }
        }
      }
    }
    var indexerTargets = [String: PBXTarget]()
    profileAction("generating_indexers") {
      let progressNotifier = ProgressNotifier(name: GeneratingIndexerTargets,
                                              maxValue: 1,
                                              indeterminate: true)
      indexerTargets = generator.generateIndexerTargets()
      progressNotifier.incrementValue()
    }

    if let includeSkylarkSources = config.options[.IncludeBuildSources].commonValueAsBool,
       includeSkylarkSources {
      profileAction("adding_buildfiles") {
        let buildfiles = workspaceInfoExtractor.extractBuildfiles(expandedTargetLabels)
        let paths = buildfiles.map() { $0.asFileName! }
        generator.generateFileReferencesForFilePaths(paths, pathFilters: config.pathFilters)
      }
    }

    // Add RuleEntrys for any test hosts to ensure that selected tests can be executed in Xcode.
    for (hostLabel, _) in hostTargetLabels {
      if config.buildTargetLabels.contains(hostLabel) { continue }
      guard let recoveredHostRuleEntry = ruleEntryMap.anyRuleEntry(withBuildLabel: hostLabel) else {
        // Already reported MissingTestHost warning in PBXTargetGenerator within
        // generateBuildTargetsForRuleEntries(...).
        continue
      }
      // Add the recovered test host target.
      targetRules.insert(recoveredHostRuleEntry)
    }

    let workingDirectory = pbxTargetGeneratorType.workingDirectoryForPBXGroup(mainGroup)
    profileAction("generating_clean_target") {
      generator.generateBazelCleanTarget(cleanScriptPath, workingDirectory: workingDirectory)
    }
    profileAction("generating_top_level_build_configs") {
      var buildSettings = [String: String]()
      if let sdkroot = XcodeProjectGenerator.projectSDKROOT(targetRules) {
        buildSettings = ["SDKROOT": sdkroot]
      }
      // Pull in transitive settings from the top level targets.
      for entry in targetRules {
        if let swiftVersion = entry.attributes[.swift_language_version] as? String {
          buildSettings["SWIFT_VERSION"] = swiftVersion
        } else if entry.attributes[.has_swift_dependency] as? Bool ?? false {
          buildSettings["SWIFT_VERSION"] = fetchDefaultSwitchVersion()
        }
        if let swiftToolchain = entry.attributes[.swift_toolchain] as? String {
          buildSettings["TOOLCHAINS"] = swiftToolchain
        }
      }

      // Update this project's build settings with the latest feature flags.
      for featureFlag in bazelBuildSettingsFeatures {
        buildSettings[featureFlag] = "YES"
      }

      if (self.disableDBGShellCommandsCaching) {
        buildSettings["TULSI_UPDATE_DSYM_CACHE"] = "NO"
      } else {
        buildSettings["TULSI_UPDATE_DSYM_CACHE"] = "YES"
      }

      if (self.disableNormalizedDebugPrefixMap) {
        buildSettings["TULSI_NORMALIZED_DEBUG_INFO"] = "NO"
      } else {
        buildSettings["TULSI_NORMALIZED_DEBUG_INFO"] = "YES"
      }

      buildSettings["TULSI_PROJECT"] = config.projectName
      generator.generateTopLevelBuildConfigurations(buildSettings)
    }

    try profileAction("generating_build_targets") {
      try generator.generateBuildTargetsForRuleEntries(targetRules,
                                                       ruleEntryMap: ruleEntryMap)
    }

    let referencePatcher = BazelXcodeProjectPatcher(fileManager: fileManager)
    profileAction("patching_bazel_relative_references") {
      referencePatcher.patchBazelRelativeReferences(xcodeProject, workspaceRootURL)
    }
    profileAction("patching_external_repository_references") {
      referencePatcher.patchExternalRepositoryReferences(xcodeProject)
    }
    profileAction("updating_dbgshellcommands") {
      do {
        try updateShellCommands()
      } catch {
        self.localizedMessageLogger.warning("UpdatingDBGShellCommandsFailed",
                                            comment: LocalizedMessageLogger.bugWorthyComment("Failed to update the script to find cached dSYM bundles via DBGShellCommands."),
                                            context: self.config.projectName,
                                            values: "\(error)")
      }
    }
    if (!self.disableDBGShellCommandsCaching) {
      profileAction("cleaning_cached_dsym_paths") {
        cleanCachedDsymPaths()
      }
    }
    return GeneratedProjectInfo(project: xcodeProject,
                                buildRuleEntries: targetRules,
                                testSuiteRuleEntries: testSuiteRules,
                                indexerTargets: indexerTargets)
  }

  private func installWorkspaceSettings(_ projectURL: URL) throws {
    func writeWorkspaceSettings(_ workspaceSettings: [String: Any],
                                toDirectoryAtURL directoryURL: URL,
                                replaceIfExists: Bool = false) throws {

      let workspaceSettingsURL = directoryURL.appendingPathComponent("WorkspaceSettings.xcsettings")
      if (!replaceIfExists && fileManager.fileExists(atPath: workspaceSettingsURL.path)) ||
          !createDirectory(directoryURL) {
        return
      }

      let data = try PropertyListSerialization.data(fromPropertyList: workspaceSettings,
                                                                      format: .xml,
                                                                      options: 0)
      try writeDataHandler(workspaceSettingsURL, data)
    }


    let workspaceSharedDataURL = projectURL.appendingPathComponent("project.xcworkspace/xcshareddata")
    try writeWorkspaceSettings(["IDEWorkspaceSharedSettings_AutocreateContextsIfNeeded": false as AnyObject],
                               toDirectoryAtURL: workspaceSharedDataURL,
                               replaceIfExists: true)

    let workspaceUserDataURL = projectURL.appendingPathComponent("project.xcworkspace/xcuserdata/\(usernameFetcher()).xcuserdatad")
    let perUserWorkspaceSettings: [String: Any] = [
        "LiveSourceIssuesEnabled": true,
        "IssueFilterStyle": "ShowAll",
    ]
    try writeWorkspaceSettings(perUserWorkspaceSettings, toDirectoryAtURL: workspaceUserDataURL)
  }

  private func loadRuleEntryMap() throws -> RuleEntryMap {
    do {
      return try workspaceInfoExtractor.ruleEntriesForLabels(config.buildTargetLabels,
                                                             startupOptions: config.options[.BazelBuildStartupOptionsDebug],
                                                             buildOptions: config.options[.BazelBuildOptionsDebug],
                                                             useAspectForTestSuitesOption: config.options[.UseAspectForTestSuites])
    } catch BazelWorkspaceInfoExtractorError.aspectExtractorFailed(let info) {
      throw ProjectGeneratorError.labelAspectFailure(info)
    }
  }

  // Links tulsi-workspace to the current Bazel execution root. This may be overwritten during
  // builds, but is useful to include in project generation for users who have local_repository
  // references.
  private func linkTulsiWorkspace() {
    // Don't create the tulsi-workspace symlink for tests.
    guard !self.redactWorkspaceSymlink else { return }

    let path = workspaceRootURL.appendingPathComponent(PBXTargetGenerator.TulsiWorkspacePath,
                                                       isDirectory: false).path
    let bazelExecRoot = self.workspaceInfoExtractor.bazelExecutionRoot;

    // See if tulsi-includes is already present.
    if let attributes = try? fileManager.attributesOfItem(atPath: path) {
      // If tulsi-includes is already a symlink, we only need to change it if it points to the wrong
      // Bazel exec root.
      if attributes[FileAttributeKey.type] as? FileAttributeType == FileAttributeType.typeSymbolicLink {
        do {
          let oldBazelExecRoot = try self.fileManager.destinationOfSymbolicLink(atPath: path)
          guard oldBazelExecRoot != bazelExecRoot else { return }
        } catch {
          self.localizedMessageLogger.warning("UpdatingTulsiWorkspaceSymlinkFailed",
                                              comment: "Warning shown when failing to update the tulsi-workspace symlink in %1$@ to the Bazel execution root, additional context %2$@.",
                                              context: config.projectName,
                                              values: path, "Unable to read old symlink. Was it modified?")
          return
        }
      }

      // The symlink exists but points to the wrong path or is a different file type. Remove it.
      do {
        try fileManager.removeItem(atPath: path)
      } catch {
        self.localizedMessageLogger.warning("UpdatingTulsiWorkspaceSymlinkFailed",
                                            comment: "Warning shown when failing to update the tulsi-workspace symlink in %1$@ to the Bazel execution root, additional context %2$@.",
                                            context: config.projectName,
                                            values: path, "Unable to remove the old tulsi-workspace symlink. Trying removing it and try again.")
        return
      }
    }

    // Symlink tulsi-workspace ->  Bazel exec root.
    do {
      try self.fileManager.createSymbolicLink(atPath: path, withDestinationPath: bazelExecRoot)
    } catch {
      self.localizedMessageLogger.warning("UpdatingTulsiWorkspaceSymlinkFailed",
                                          comment: "Warning shown when failing to update the tulsi-workspace symlink in %1$@ to the Bazel execution root, additional context %2$@.",
                                          context: config.projectName,
                                          values: path, "Creating symlink failed. Is it already present?")
    }
  }

  // Writes Xcode schemes for non-indexer targets if they don't already exist.
  private func installXcodeSchemesForProjectInfo(_ info: GeneratedProjectInfo,
                                                 projectURL: URL,
                                                 projectBundleName: String) throws {
    let xcschemesURL = projectURL.appendingPathComponent("xcshareddata/xcschemes")
    guard createDirectory(xcschemesURL) else { return }

    func targetForLabel(_ label: BuildLabel) -> PBXTarget? {
      if let pbxTarget = info.project.targetByName[label.targetName!] {
        return pbxTarget
      } else if let pbxTarget = info.project.targetByName[label.asFullPBXTargetName!] {
        return pbxTarget
      }
      return nil
    }

    func commandlineArguments(for ruleEntry: RuleEntry) -> [String] {
      return config.options[.CommandlineArguments, ruleEntry.label.value]?.components(separatedBy: " ") ?? []
    }

    func environmentVariables(for ruleEntry: RuleEntry) -> [String: String] {
      var environmentVariables: [String: String] = [:]
      config.options[.EnvironmentVariables, ruleEntry.label.value]?.components(separatedBy: .newlines).forEach() { keyValueString in
        let components = keyValueString.components(separatedBy: "=")
        let key = components.first ?? ""
        if !key.isEmpty {
          let value = components[1..<components.count].joined(separator: "=")
          environmentVariables[key] = value
        }
      }
      return environmentVariables
    }

    func preActionScripts(for ruleEntry: RuleEntry) -> [XcodeActionType: String] {
        var preActionScripts: [XcodeActionType: String] = [:]
        preActionScripts[.BuildAction] = config.options[.BuildActionPreActionScript, ruleEntry.label.value] ?? nil
        preActionScripts[.LaunchAction] = config.options[.LaunchActionPreActionScript, ruleEntry.label.value] ?? nil
        preActionScripts[.TestAction] = config.options[.TestActionPreActionScript, ruleEntry.label.value] ?? nil
        return preActionScripts
    }

    func postActionScripts(for ruleEntry: RuleEntry) -> [XcodeActionType: String] {
        var postActionScripts: [XcodeActionType: String] = [:]
        postActionScripts[.BuildAction] = config.options[.BuildActionPostActionScript, ruleEntry.label.value] ?? nil
        postActionScripts[.LaunchAction] = config.options[.LaunchActionPostActionScript, ruleEntry.label.value] ?? nil
        postActionScripts[.TestAction] = config.options[.TestActionPostActionScript, ruleEntry.label.value] ?? nil
        return postActionScripts
    }
    // Build a map of extension targets to hosts so the hosts may be referenced as additional build
    // requirements. This is necessary for watchOS2 targets (Xcode will spawn an error when
    // attempting to run the app without the scheme linkage, even though Bazel will create the
    // embedded host correctly) and does not harm other extensions.
    var extensionHosts = [BuildLabel: RuleEntry]()
    for entry in info.buildRuleEntries {
      for extensionLabel in entry.extensions {
        extensionHosts[extensionLabel] = entry
      }
    }

    let runTestTargetBuildConfigPrefix = pbxTargetGeneratorType.getRunTestTargetBuildConfigPrefix()
    for entry in info.buildRuleEntries {
      // Generate an XcodeScheme with a test action set up to allow tests to be run without Xcode
      // attempting to compile code.
      let target: PBXNativeTarget
      if let pbxTarget = targetForLabel(entry.label) as? PBXNativeTarget {
        target = pbxTarget
      } else {
        localizedMessageLogger.warning("XCSchemeGenerationFailed",
                                       comment: "Warning shown when generation of an Xcode scheme failed for build target %1$@",
                                       context: config.projectName,
                                       values: entry.label.value)
        continue
      }

      let filename = target.name + ".xcscheme"

      let url = xcschemesURL.appendingPathComponent(filename)
      let targetType = entry.pbxTargetType ?? .Application

      var appExtension: Bool = false
      var extensionType: String? = nil
      var launchActionBuildConfig: String = "Debug"
      var launchStyle: XcodeScheme.LaunchStyle? = .Normal
      var runnableDebuggingMode: XcodeScheme.RunnableDebuggingMode = .Default

      if targetType.isiOSAppExtension {
        appExtension = true
        launchStyle = .AppExtension
        extensionType = entry.extensionType
      } else if targetType.isWatchApp {
        runnableDebuggingMode = .Remote
      } else if targetType.isLibrary {
        launchStyle = nil
      } else if targetType.isTest {
        // Test targets should be Buildable but not Runnable. Since Xcode will use the build config
        // from the LaunchAction, we need to use the test build config for the LaunchAction.
        launchStyle = nil
        launchActionBuildConfig = runTestTargetBuildConfigPrefix + "Debug"
      }

      var additionalBuildTargets = target.buildActionDependencies.map() {
        ($0, projectBundleName, XcodeScheme.makeBuildActionEntryAttributes())
      }
      if let host = extensionHosts[entry.label] {
        guard let hostTarget = targetForLabel(host.label) else {
          localizedMessageLogger.warning("XCSchemeGenerationFailed",
                                         comment: "Warning shown when generation of an Xcode scheme failed for build target %1$@",
                                         details: "Extension host could not be resolved.",
                                         context: config.projectName,
                                         values: entry.label.value)
          continue
        }
        let hostTargetTuple =
            (hostTarget, projectBundleName, XcodeScheme.makeBuildActionEntryAttributes())
        additionalBuildTargets.append(hostTargetTuple)
      }

      let scheme = XcodeScheme(target: target,
                               project: info.project,
                               projectBundleName: projectBundleName,
                               testActionBuildConfig: runTestTargetBuildConfigPrefix + "Debug",
                               launchActionBuildConfig: launchActionBuildConfig,
                               profileActionBuildConfig: runTestTargetBuildConfigPrefix + "Release",
                               appExtension: appExtension,
                               extensionType: extensionType,
                               launchStyle: launchStyle,
                               runnableDebuggingMode: runnableDebuggingMode,
                               additionalBuildTargets: additionalBuildTargets,
                               commandlineArguments: commandlineArguments(for: entry),
                               environmentVariables: environmentVariables(for: entry),
                               preActionScripts:preActionScripts(for: entry),
                               postActionScripts:postActionScripts(for: entry),
                               localizedMessageLogger: localizedMessageLogger)
      let xmlDocument = scheme.toXML()


      let data = xmlDocument.xmlData(options: XMLNode.Options.nodePrettyPrint)
      try writeDataHandler(url, data)
    }

    func extractTestTargets(_ testSuite: RuleEntry) -> (Set<PBXTarget>, PBXTarget?) {
      var suiteHostTarget: PBXTarget? = nil
      var validTests = Set<PBXTarget>()
      for testEntryLabel in testSuite.testSuiteDependencies {
        if let recursiveTestSuite = info.testSuiteRuleEntries[testEntryLabel] {
          let (recursiveTests, recursiveSuiteHostTarget) = extractTestTargets(recursiveTestSuite)
          validTests.formUnion(recursiveTests)
          if suiteHostTarget == nil {
            suiteHostTarget = recursiveSuiteHostTarget
          }
          continue
        }

        guard let testTarget = targetForLabel(testEntryLabel) as? PBXNativeTarget else {
          localizedMessageLogger.warning("TestSuiteUsesUnresolvedTarget",
                                         comment: "Warning shown when a test_suite %1$@ refers to a test label %2$@ that was not resolved and will be ignored",
                                         context: config.projectName,
                                         values: testSuite.label.value, testEntryLabel.value)
          continue
        }

        // Non XCTests are treated as standalone applications and cannot be included in an Xcode
        // test scheme.
        if testTarget.productType == .Application {
          localizedMessageLogger.warning("TestSuiteIncludesNonXCTest",
                                         comment: "Warning shown when a non XCTest %1$@ is included in a test suite %2$@ and will be ignored.",
                                         context: config.projectName,
                                         values: testEntryLabel.value, testSuite.label.value)
          continue
        }

        // Only UnitTests do not need a test host; they are considered 'logic tests'.
        let testHostTarget = info.project.linkedHostForTestTarget(testTarget) as? PBXNativeTarget
        if testHostTarget == nil && testTarget.productType != .UnitTest {
          localizedMessageLogger.warning("TestSuiteTestHostResolutionFailed",
                                         comment: "Warning shown when the test host for a test %1$@ inside test suite %2$@ could not be found. The test will be ignored, but this state is unexpected and should be reported.",
                                         context: config.projectName,
                                         values: testEntryLabel.value, testSuite.label.value)
          continue
        }

        if suiteHostTarget == nil {
          suiteHostTarget = testHostTarget
        }

        validTests.insert(testTarget)
      }

      return (validTests, suiteHostTarget)
    }

    func installSchemesForIndexerTargets() throws {
      let indexerTargets = info.indexerTargets.values
      guard !indexerTargets.isEmpty else { return }

      let filename = "_idx_Scheme.xcscheme"
      let url = xcschemesURL.appendingPathComponent(filename)

      let additionalBuildTargets = indexerTargets.map() {
        ($0, projectBundleName, XcodeScheme.makeBuildActionEntryAttributes())
      }

      let scheme = XcodeScheme(target: nil,
                               project: info.project,
                               projectBundleName: projectBundleName,
                               launchStyle: nil,
                               additionalBuildTargets: additionalBuildTargets,
                               preActionScripts: [:],
                               postActionScripts: [:],
                               localizedMessageLogger: localizedMessageLogger)
      let xmlDocument = scheme.toXML()

      let data = xmlDocument.xmlData(options: XMLNode.Options.nodePrettyPrint)
      try writeDataHandler(url, data)
    }
    try installSchemesForIndexerTargets()

    func installSchemeForTestSuite(_ suite: RuleEntry, named suiteName: String) throws {
      let (validTests, extractedHostTarget) = extractTestTargets(suite)
      guard !validTests.isEmpty else {
        localizedMessageLogger.warning("TestSuiteHasNoValidTests",
                                       comment: "Warning shown when none of the tests of a test suite %1$@ were able to be resolved.",
                                       context: config.projectName,
                                       values: suite.label.value)
        return
      }

      let filename = suiteName + "_Suite.xcscheme"

      let url = xcschemesURL.appendingPathComponent(filename)
      let scheme = XcodeScheme(target: extractedHostTarget,
                               project: info.project,
                               projectBundleName: projectBundleName,
                               testActionBuildConfig: runTestTargetBuildConfigPrefix + "Debug",
                               profileActionBuildConfig: runTestTargetBuildConfigPrefix + "Release",
                               launchStyle: .Normal,
                               explicitTests: Array(validTests),
                               commandlineArguments: commandlineArguments(for: suite),
                               environmentVariables: environmentVariables(for: suite),
                               preActionScripts: preActionScripts(for: suite),
                               postActionScripts:postActionScripts(for: suite),
                               localizedMessageLogger: localizedMessageLogger)
      let xmlDocument = scheme.toXML()


      let data = xmlDocument.xmlData(options: XMLNode.Options.nodePrettyPrint)
      try writeDataHandler(url, data)
    }

    var testSuiteSchemes = [String: [RuleEntry]]()
    for (label, entry) in info.testSuiteRuleEntries {
      let shortName = label.targetName!
      if let _ = testSuiteSchemes[shortName] {
        testSuiteSchemes[shortName]!.append(entry)
      } else {
        testSuiteSchemes[shortName] = [entry]
      }
    }
    for testSuites in testSuiteSchemes.values {
      for suite in testSuites {
        let suiteName: String
        if testSuites.count > 1 {
          suiteName = suite.label.asFullPBXTargetName!
        } else {
          suiteName = suite.label.targetName!
        }
        try installSchemeForTestSuite(suite, named: suiteName)
      }
    }
  }

  /// Create a file that contains the execution root for the workspace of the generated project.
  private func installCachedExecutionRoot(_ scriptDirectoryURL: URL) {
    let executionRootFileURL = scriptDirectoryURL.appendingPathComponent(XcodeProjectGenerator.CachedExecutionRootFilename)

    let execroot = workspaceInfoExtractor.bazelExecutionRoot.replacingOccurrences(of: "'",
                                                                                  with: "")

    // Entire script is one variable, directly referenced within bazel_build.py. If this is an empty
    // string, the path will return False in an os.path.exists(...) call.
    let script = "BAZEL_EXECUTION_ROOT = '\(execroot)'\n"

    var errorInfo: String? = nil
    do {
      try writeDataHandler(executionRootFileURL, script.data(using: .utf8)!)
    } catch let e as NSError {
      errorInfo = e.localizedDescription
    } catch {
      errorInfo = "Unexpected exception"
    }
    if let errorInfo = errorInfo {
      // Return an error, as failing to create the file will leave us without a buildable project.
      localizedMessageLogger.error("BazelExecutionRootCacheFailed",
                                   comment: XcodeProjectGenerator.CachedExecutionRootFilename +
                                            "could not be created. \(errorInfo)",
                                   context: config.projectName)
      return
    }
  }

  /// Copy the bazel_cache_reader to a subfolder in the user's Library, return the absolute path.
  private func installShellCommands(atURL supportScriptsAbsoluteURL: URL) throws -> String {

    // Create all intermediate directories if they aren't present.
    var isDir = ObjCBool(false)
    if !fileManager.fileExists(atPath: supportScriptsAbsoluteURL.path, isDirectory: &isDir) {
      try fileManager.createDirectory(atPath: supportScriptsAbsoluteURL.path,
                                      withIntermediateDirectories: true,
                                      attributes: nil)
    }

    // Find bazel_cache_reader in Tulsi.app's Utilities folder.
    let bundle = Bundle(for: type(of: self))
    let symbolCacheSourceURL = bundle.url(forResource: XcodeProjectGenerator.ShellCommandsUtil,
                                          withExtension: "",
                                          subdirectory: "Utilities")!

    // Copy bazel_cache_reader to ~/Library/Application Support/Tulsi/Scripts
    installFiles([(symbolCacheSourceURL, XcodeProjectGenerator.ShellCommandsUtil)],
                 toDirectory: supportScriptsAbsoluteURL)

    // Return the absolute path to ~/Library/Application Support/Tulsi/Scripts/bazel_cache_reader.
    let shellCommandsURL =
        supportScriptsAbsoluteURL.appendingPathComponent(XcodeProjectGenerator.ShellCommandsUtil)

    return shellCommandsURL.path
  }

  /// Update the global user defaults to reference bazel_cache_reader
  private func updateGlobalUserDefaultsWithShellCommands(shellCommandsPath: String) {

    // Check that bazel_cache_reader exists at the given path. If not, do nothing.
    var isDir = ObjCBool(false)
    guard fileManager.fileExists(atPath: shellCommandsPath, isDirectory: &isDir) else {
      return
    }

    // Find if there is an existing entry for com.apple.DebugSymbols.
    let dbgDefaults = UserDefaults.standard.persistentDomain(forName: "com.apple.DebugSymbols")

    guard var currentDBGDefaults = dbgDefaults else {
      // If no com.apple.DebugSymbols ever existed, create a new dictionary with our script for
      // DBGShellCommands, and set DBGSpotlightPaths to an empty array to continue using Spotlight
      // as a fallback for dSYM searches via LLDB and Instruments.
      UserDefaults.standard.setPersistentDomain(["DBGShellCommands": [shellCommandsPath],
                                                 "DBGSpotlightPaths": []],
                                                forName: "com.apple.DebugSymbols")
      return
    }

    // If there is one...
    var newShellCommands : [String] = []

    if let currentShellCommands = currentDBGDefaults["DBGShellCommands"] as? [String] {
      // Check if shellCommandsPath is already in DBGShellCommands's array of Strings.
      guard !currentShellCommands.contains(shellCommandsPath) else {
        // Do nothing if it is.
        return
      }
      // Copy all the current shell commands to the new DBGShellCommands array.
      newShellCommands = currentShellCommands

    } else if let currentShellCommand = currentDBGDefaults["DBGShellCommands"] as? String {
      // Check that the single path at DBGShellCommands is not the same as shellCommandsPath.
      if currentShellCommand != shellCommandsPath {
        // Add it to our new DBGShellCommands array in progress if it's not.
        newShellCommands.append(currentShellCommand)
      }
    }
    // Add shellCommandsPath to the new DBGShellCommands array.
    newShellCommands.append(shellCommandsPath)

    // Replace DBGShellCommands in the existing com.apple.DebugSymbols defaults.
    currentDBGDefaults["DBGShellCommands"] = newShellCommands
    UserDefaults.standard.setPersistentDomain(currentDBGDefaults, forName: "com.apple.DebugSymbols")
  }

  /// Remove the bazel_cache_reader from a subfolder in ~/Library and in global user defaults.
  private func removeShellCommands(atURL shellCommandsURL: URL) throws -> String? {

    // If a file exists at ~/Library/Application Support/Tulsi/Scripts/bazel_cache_reader...
    var isDir = ObjCBool(false)
    guard fileManager.fileExists(atPath: shellCommandsURL.path, isDirectory: &isDir) else {
      // Exit early if it doesn't.
      return nil
    }

    // ...otherwise, remove it.
    try fileManager.removeItem(at: shellCommandsURL)

    // Return the path to the bazel_cache_reader after removal.
    return shellCommandsURL.path
  }

  /// Update the global user defaults to reference bazel_cache_reader
  private func removeUnusedShellCommandsFromGlobalUserDefaults(shellCommandsPath : String) {

    // Check that bazel_cache_reader does not exist at the given path. If it does, do nothing.
    var isDir = ObjCBool(false)
    guard !fileManager.fileExists(atPath: shellCommandsPath, isDirectory: &isDir) else {
      return
    }

    // Update DBGShellCommands such that the path to bazel_cache_reader isn't there anymore.
    let dbgDefaults = UserDefaults.standard.persistentDomain(forName: "com.apple.DebugSymbols")

    guard var currentDBGDefaults = dbgDefaults else {
      return
    }
    if var shellCommands = currentDBGDefaults["DBGShellCommands"] as? [String] {

      // Check if shellCommandsPath is already in DBGShellCommands's array of Strings.
      guard let pathFound = shellCommands.index(of: shellCommandsPath) else {
        return
      }

      // Remove it if it was.
      shellCommands.remove(at: pathFound)

      // Update with the modified com.apple.DebugSymbols global defaults.
      currentDBGDefaults["DBGShellCommands"] = shellCommands

    } else if let shellCommand = currentDBGDefaults["DBGShellCommands"] as? String {

      // Cover the single value case, which (3/13) would only apply if the user manually set it.
      if shellCommand == shellCommandsPath {
        currentDBGDefaults.removeValue(forKey: "DBGShellCommands")
      } else {
        return
      }

    } else {
      return
    }

    // Update com.apple.DebugSymbols if we need to update DBGShellCommands.
    UserDefaults.standard.setPersistentDomain(currentDBGDefaults,
                                              forName: "com.apple.DebugSymbols")
  }

  /// Install the latest bazel_cache_reader or remove it, as requested by config options.
  private func updateShellCommands() throws {

    // Construct a URL to ~/Library/Application Support/Tulsi/Scripts.
    let supportScriptsAbsoluteURL = fileManager.homeDirectoryForCurrentUser.appendingPathComponent(
      XcodeProjectGenerator.SupportScriptsPath, isDirectory: true)

    // If the config disabled DBGShellCommands caching...
    if (self.disableDBGShellCommandsCaching) {

      // Create a URL to ~/Library/Application Support/Tulsi/Scripts/bazel_cache_reader.
      let shellCommandsURL =
        supportScriptsAbsoluteURL.appendingPathComponent(XcodeProjectGenerator.ShellCommandsUtil)

      // Attempt to remove the app at that path.
      if let shellCommandsAppPath = try removeShellCommands(atURL: shellCommandsURL) {

        // If the delete was successful, remove its reference from global user defaults.
        removeUnusedShellCommandsFromGlobalUserDefaults(shellCommandsPath: shellCommandsAppPath)
      }
    } else {

      // If DBGShellCommands caching is enabled, install the latest version of the app to
      // ~/Library/Application Support/Tulsi/Scripts/
      let shellCommandsAppPath = try installShellCommands(atURL: supportScriptsAbsoluteURL)

      // Add a reference to it in global user defaults.
      updateGlobalUserDefaultsWithShellCommands(shellCommandsPath: shellCommandsAppPath)
    }
  }

  private func cleanCachedDsymPaths() {
    // Execute the script to clean up missing dSYM bundles asynchronously.
    let bundle = Bundle(for: type(of: self))
    let cleanSymbolsSourceURL = bundle.url(forResource: XcodeProjectGenerator.ShellCommandsCleanScript, withExtension: "py")!

    let process = ProcessRunner.createProcess(cleanSymbolsSourceURL.path,
                                              arguments: [String]()) {
      completionInfo in
      if let stderr = NSString(data: completionInfo.stderr,
                               encoding: String.Encoding.utf8.rawValue) {
        guard !stderr.trimmingCharacters(in: .whitespaces).isEmpty else {
          return
        }
        Thread.doOnMainQueue {
          self.localizedMessageLogger.warning("CleanCachedDsymsFailed",
                                              comment: LocalizedMessageLogger.bugWorthyComment("Failed to clean cached references to existing dSYM bundles."),
                                              context: self.config.projectName,
                                              values: stderr)
        }
      }
    }
    process.launch()
  }

  private func installTulsiScripts(_ projectURL: URL) {

    let scriptDirectoryURL = projectURL.appendingPathComponent(XcodeProjectGenerator.ScriptDirectorySubpath,
                                                                    isDirectory: true)
    if createDirectory(scriptDirectoryURL) {
      let profilingToken = localizedMessageLogger.startProfiling("installing_scripts",
                                                                 context: config.projectName)
      let progressNotifier = ProgressNotifier(name: InstallingScripts, maxValue: 1)
      defer { progressNotifier.incrementValue() }
      localizedMessageLogger.infoMessage("Installing scripts")
      installFiles([(resourceURLs.buildScript, XcodeProjectGenerator.BuildScript),
                    (resourceURLs.cleanScript, XcodeProjectGenerator.CleanScript),
                   ],
                   toDirectory: scriptDirectoryURL)
      installFiles(resourceURLs.extraBuildScripts.map { ($0, $0.lastPathComponent) },
                   toDirectory: scriptDirectoryURL)
      installCachedExecutionRoot(scriptDirectoryURL)

      localizedMessageLogger.logProfilingEnd(profilingToken)
    }
  }

  private func installTulsiBazelPackage(_ projectURL: URL) {

    let bazelWorkspaceURL = projectURL.appendingPathComponent(XcodeProjectGenerator.BazelDirectorySubpath,
                                                              isDirectory: true)
    let bazelPackageURL = bazelWorkspaceURL.appendingPathComponent(XcodeProjectGenerator.TulsiPackageName,
                                                                   isDirectory: true)

    if createDirectory(bazelPackageURL) {
      let profilingToken = localizedMessageLogger.startProfiling("installing_package",
                                                                 context: config.projectName)
      let progressNotifier = ProgressNotifier(name: InstallingScripts, maxValue: 1)
      defer { progressNotifier.incrementValue() }
      localizedMessageLogger.infoMessage("Installing Bazel integration package")

      installFiles([(resourceURLs.bazelWorkspaceFile, XcodeProjectGenerator.WorkspaceFile)],
                   toDirectory: bazelWorkspaceURL)
      installFiles(resourceURLs.tulsiPackageFiles.map { ($0, $0.lastPathComponent) },
                   toDirectory: bazelPackageURL)

      localizedMessageLogger.logProfilingEnd(profilingToken)
    }
  }

  private func installUtilities(_ projectURL: URL) {
    let utilDirectoryURL = projectURL.appendingPathComponent(XcodeProjectGenerator.UtilDirectorySubpath,
                                                                  isDirectory: true)
    if createDirectory(utilDirectoryURL) {
      let profilingToken = localizedMessageLogger.startProfiling("installing_utilities",
                                                                 context: config.projectName)
      let progressNotifier = ProgressNotifier(name: InstallingUtilities, maxValue: 1)
      defer { progressNotifier.incrementValue() }
      localizedMessageLogger.infoMessage("Installing utilities")
      installFiles([(resourceURLs.postProcessor, XcodeProjectGenerator.PostProcessorUtil)],
                   toDirectory: utilDirectoryURL)
      localizedMessageLogger.logProfilingEnd(profilingToken)
    }
  }

  private func installGeneratorConfig(_ projectURL: URL) {
    let configDirectoryURL = projectURL.appendingPathComponent(XcodeProjectGenerator.ConfigDirectorySubpath,
                                                                    isDirectory: true)
    guard createDirectory(configDirectoryURL, failSilently: true) else { return }
    let profilingToken = localizedMessageLogger.startProfiling("installing_generator_config",
                                                               context: config.projectName)
    let progressNotifier = ProgressNotifier(name: InstallingGeneratorConfig, maxValue: 1)
    defer { progressNotifier.incrementValue() }
    localizedMessageLogger.infoMessage("Installing generator config")

    let configURL = configDirectoryURL.appendingPathComponent(config.defaultFilename)
    var errorInfo: String? = nil
    do {
      let data = try config.save()
      try writeDataHandler(configURL, data as Data)
    } catch let e as NSError {
      errorInfo = e.localizedDescription
    } catch {
      errorInfo = "Unexpected exception"
    }
    if let errorInfo = errorInfo {
      localizedMessageLogger.syslogMessage("Generator config serialization failed. \(errorInfo)",
                                           context: config.projectName)
      return
    }

    let perUserConfigURL = configDirectoryURL.appendingPathComponent(TulsiGeneratorConfig.perUserFilename)
    errorInfo = nil
    do {
      if let data = try config.savePerUserSettings() {
        try writeDataHandler(perUserConfigURL, data as Data)
      }
    } catch let e as NSError {
      errorInfo = e.localizedDescription
    } catch {
      errorInfo = "Unexpected exception"
    }
    if let errorInfo = errorInfo {
      localizedMessageLogger.syslogMessage("Generator per-user config serialization failed. \(errorInfo)",
                                           context: config.projectName)
      return
    }
    localizedMessageLogger.logProfilingEnd(profilingToken)
  }

  private func installGeneratedProjectResources(_ projectURL: URL) {

    let targetDirectoryURL = projectURL.appendingPathComponent(XcodeProjectGenerator.ProjectResourcesDirectorySubpath,
                                                                    isDirectory: true)
    guard createDirectory(targetDirectoryURL) else { return }
    let profilingToken = localizedMessageLogger.startProfiling("installing_project_resources",
                                                               context: config.projectName)
    localizedMessageLogger.infoMessage("Installing project resources")

    installFiles([(resourceURLs.iOSUIRunnerEntitlements, XcodeProjectGenerator.IOSUIRunnerEntitlements),
                  (resourceURLs.macOSUIRunnerEntitlements, XcodeProjectGenerator.MacOSUIRunnerEntitlements),
                  (resourceURLs.stubInfoPlist, XcodeProjectGenerator.StubInfoPlistFilename),
                  (resourceURLs.stubWatchOS2InfoPlist, XcodeProjectGenerator.StubWatchOS2InfoPlistFilename),
                  (resourceURLs.stubWatchOS2AppExInfoPlist, XcodeProjectGenerator.StubWatchOS2AppExInfoPlistFilename),
                 ],
                 toDirectory: targetDirectoryURL)


    localizedMessageLogger.logProfilingEnd(profilingToken)
  }

  private func installStubExtensionPlistFiles(_ projectURL: URL, rules: [RuleEntry], plistPaths: StubInfoPlistPaths) {
    let targetDirectoryURL = projectURL.appendingPathComponent(XcodeProjectGenerator.ProjectResourcesDirectorySubpath,
                                                               isDirectory: true)
    guard createDirectory(targetDirectoryURL) else { return }
    let profilingToken = localizedMessageLogger.startProfiling("installing_plist_files",
                                                               context: config.projectName)
    localizedMessageLogger.infoMessage("Installing plist files")

    let templatePath = resourceURLs.stubIOSAppExInfoPlistTemplate.path
    guard let plistTemplateData = fileManager.contents(atPath: templatePath) else {
      localizedMessageLogger.error("PlistTemplateNotFound",
                                   comment: LocalizedMessageLogger.bugWorthyComment("Failed to load a plist template"),
                                   context: config.projectName,
                                   values: templatePath)
      return
    }

    let plistTemplate: NSDictionary
    do {
      plistTemplate = try PropertyListSerialization.propertyList(from: plistTemplateData,
                                                                 options: PropertyListSerialization.ReadOptions.mutableContainers,
                                                                 format: nil) as! NSDictionary
    } catch let e {
      localizedMessageLogger.error("PlistDeserializationFailed",
                                   comment: LocalizedMessageLogger.bugWorthyComment("Failed to deserialize a plist template"),
                                   context: config.projectName,
                                   values: resourceURLs.stubIOSAppExInfoPlistTemplate.path, e.localizedDescription)
      return
    }

    for entry in rules {
      plistTemplate.setValue(entry.extensionType, forKeyPath: "NSExtension.NSExtensionPointIdentifier")

      let plistName = plistPaths.plistFilename(forRuleEntry: entry)
      let targetURL = URL(string: plistName, relativeTo: targetDirectoryURL)!

      let data: Data
      do {
        data = try PropertyListSerialization.data(fromPropertyList: plistTemplate, format: .xml, options: 0)
      } catch let e {
        localizedMessageLogger.error("SerializingPlistFailed",
                                     comment: LocalizedMessageLogger.bugWorthyComment("Failed to serialize a plist template"),
                                     context: config.projectName,
                                     values: e.localizedDescription)
        return
      }

      guard fileManager.createFile(atPath: targetURL.path, contents: data, attributes: nil) else {
        localizedMessageLogger.error("WritingPlistFailed",
                                     comment: LocalizedMessageLogger.bugWorthyComment("Failed to write a plist template"),
                                     context: config.projectName,
                                     values: targetURL.path)
        return
      }
    }


    localizedMessageLogger.logProfilingEnd(profilingToken)
  }

  private func createDirectory(_ resourceDirectoryURL: URL, failSilently: Bool = false) -> Bool {
    do {
      try fileManager.createDirectory(at: resourceDirectoryURL,
                                           withIntermediateDirectories: true,
                                           attributes: nil)
    } catch let e as NSError {
      if !failSilently {
        localizedMessageLogger.error("DirectoryCreationFailed",
                                     comment: "Failed to create an important directory. The resulting project will most likely be broken. A bug should be reported.",
                                     context: config.projectName,
                                     values: resourceDirectoryURL as NSURL, e.localizedDescription)
      }
      return false
    }
    return true
  }

  private func installFiles(_ files: [(sourceURL: URL, filename: String)],
                            toDirectory directory: URL, failSilently: Bool = false) {
    for (sourceURL, filename) in files {
      guard let targetURL = URL(string: filename, relativeTo: directory) else {
        if !failSilently {
          localizedMessageLogger.error("CopyingResourceFailed",
                                       comment: "Failed to copy an important file resource, the resulting project will most likely be broken. A bug should be reported.",
                                       context: config.projectName,
                                       values: sourceURL as NSURL, filename, "Target URL is invalid")
        }
        continue
      }

      let errorInfo: String?
      do {
        if fileManager.fileExists(atPath: targetURL.path) {
          try fileManager.removeItem(at: targetURL)
        }
        try fileManager.copyItem(at: sourceURL, to: targetURL)
        errorInfo = nil
      } catch let e as NSError {
        errorInfo = e.localizedDescription
      } catch {
        errorInfo = "Unexpected exception"
      }
      if !failSilently, let errorInfo = errorInfo {
        let targetURLString = targetURL.absoluteString
        localizedMessageLogger.error("CopyingResourceFailed",
                                     comment: "Failed to copy an important file resource, the resulting project will most likely be broken. A bug should be reported.",
                                     context: config.projectName,
                                     values: sourceURL as NSURL, targetURLString, errorInfo)
      }
    }
  }


  func logPendingMessages() {
    if workspaceInfoExtractor.hasQueuedInfoMessages() {
      localizedMessageLogger.debugMessage("Printing Bazel logs that could contain the error.")
      workspaceInfoExtractor.logQueuedInfoMessages()
    }
  }


  /// Models a node in a path trie.
  private class PathTrie {
    private var root = PathNode(pathElement: "")

    func insert(_ path: URL) {
      let components = path.pathComponents
      guard !components.isEmpty else {
        return
      }
      root.addPath(components)
    }

    func leafPaths() -> [URL] {
      var ret = [URL]()
      for n in root.children.values {
        for path in n.leafPaths() {
          // TODO(dmishe): Swicth to an appropriate URL method.
          guard let url = NSURL.fileURL(withPathComponents: path) else {
            continue
          }
          ret.append(url as URL)
        }
      }
      return ret
    }

    private class PathNode {
      let value: String
      var children = [String: PathNode]()

      init(pathElement: String) {
        self.value = pathElement
      }

      func addPath<T: Collection>(_ pathComponents: T)
                  where T.SubSequence : Collection,
                  T.Iterator.Element == String {
        guard let firstComponent = pathComponents.first else {
          return
        }

        let node: PathNode
        if let existingNode = children[firstComponent] {
          node = existingNode
        } else {
          node = PathNode(pathElement: firstComponent)
          children[firstComponent] = node
        }
        let remaining = pathComponents.dropFirst()
        if !remaining.isEmpty {
          node.addPath(remaining)
        }
      }

      func leafPaths() -> [[String]] {
        if children.isEmpty {
          return [[value]]
        }
        var ret = [[String]]()
        for n in children.values {
          for childPath in n.leafPaths() {
            var subpath = [value]
            subpath.append(contentsOf: childPath)
            ret.append(subpath)
          }
        }
        return ret
      }
    }
  }
}