/// <summary>
        /// Builds and runs the header tool and touches the header directories.
        /// Performs any early outs if headers need no changes, given the UObject modules, tool path, game name, and configuration
        /// </summary>
        public static bool ExecuteHeaderToolIfNecessary(UEToolChain ToolChain, UEBuildTarget Target, CPPEnvironment GlobalCompileEnvironment, List <UHTModuleInfo> UObjectModules, FileReference ModuleInfoFileName, ref ECompilationResult UHTResult)
        {
            if (ProgressWriter.bWriteMarkup)
            {
                Log.WriteLine(LogEventType.Console, "@progress push 5%");
            }
            using (ProgressWriter Progress = new ProgressWriter("Generating code...", false))
            {
                // We never want to try to execute the header tool when we're already trying to build it!
                var bIsBuildingUHT = Target.GetTargetName().Equals("UnrealHeaderTool", StringComparison.InvariantCultureIgnoreCase);

                var RootLocalPath = Path.GetFullPath(ProjectFileGenerator.RootRelativePath);

                // check if UHT is out of date
                DateTime HeaderToolTimestamp = DateTime.MaxValue;
                bool     bHaveHeaderTool     = !bIsBuildingUHT && GetHeaderToolTimestamp(out HeaderToolTimestamp);

                // ensure the headers are up to date
                bool bUHTNeedsToRun = (UEBuildConfiguration.bForceHeaderGeneration == true || !bHaveHeaderTool || AreGeneratedCodeFilesOutOfDate(UObjectModules, HeaderToolTimestamp));
                if (bUHTNeedsToRun || UnrealBuildTool.IsGatheringBuild)
                {
                    // Since code files are definitely out of date, we'll now finish computing information about the UObject modules for UHT.  We
                    // want to save this work until we know that UHT actually needs to be run to speed up best-case iteration times.
                    if (UnrealBuildTool.IsGatheringBuild)                               // In assembler-only mode, PCH info is loaded from our UBTMakefile!
                    {
                        foreach (var UHTModuleInfo in UObjectModules)
                        {
                            // Only cache the PCH name if we don't already have one.  When running in 'gather only' mode, this will have already been cached
                            if (string.IsNullOrEmpty(UHTModuleInfo.PCH))
                            {
                                UHTModuleInfo.PCH = "";

                                // We need to figure out which PCH header this module is including, so that UHT can inject an include statement for it into any .cpp files it is synthesizing
                                var DependencyModuleCPP      = (UEBuildModuleCPP)Target.GetModuleByName(UHTModuleInfo.ModuleName);
                                var ModuleCompileEnvironment = DependencyModuleCPP.CreateModuleCompileEnvironment(GlobalCompileEnvironment);
                                DependencyModuleCPP.CachePCHUsageForModuleSourceFiles(ModuleCompileEnvironment);
                                if (DependencyModuleCPP.ProcessedDependencies.UniquePCHHeaderFile != null)
                                {
                                    UHTModuleInfo.PCH = DependencyModuleCPP.ProcessedDependencies.UniquePCHHeaderFile.AbsolutePath;
                                }
                            }
                        }
                    }
                }

                // @todo ubtmake: Optimization: Ideally we could avoid having to generate this data in the case where UHT doesn't even need to run!  Can't we use the existing copy?  (see below use of Manifest)
                UHTManifest Manifest = new UHTManifest(Target, RootLocalPath, ToolChain.ConvertPath(RootLocalPath + '\\'), UObjectModules);

                if (!bIsBuildingUHT && bUHTNeedsToRun)
                {
                    // Always build UnrealHeaderTool if header regeneration is required, unless we're running within a Rocket ecosystem or hot-reloading
                    if (UnrealBuildTool.RunningRocket() == false &&
                        UEBuildConfiguration.bDoNotBuildUHT == false &&
                        UEBuildConfiguration.bHotReloadFromIDE == false &&
                        !(bHaveHeaderTool && !UnrealBuildTool.IsGatheringBuild && UnrealBuildTool.IsAssemblingBuild))                           // If running in "assembler only" mode, we assume UHT is already up to date for much faster iteration!
                    {
                        // If it is out of date or not there it will be built.
                        // If it is there and up to date, it will add 0.8 seconds to the build time.
                        Log.TraceInformation("Building UnrealHeaderTool...");

                        var UBTArguments = new StringBuilder();

                        UBTArguments.Append("UnrealHeaderTool");

                        // Which desktop platform do we need to compile UHT for?
                        UBTArguments.Append(" " + BuildHostPlatform.Current.Platform.ToString());
                        // NOTE: We force Development configuration for UHT so that it runs quickly, even when compiling debug
                        UBTArguments.Append(" " + UnrealTargetConfiguration.Development.ToString());

                        // NOTE: We disable mutex when launching UBT from within UBT to compile UHT
                        UBTArguments.Append(" -NoMutex");

                        if (UnrealBuildTool.CommandLineContains("-noxge"))
                        {
                            UBTArguments.Append(" -noxge");
                        }

                        // Propagate command-line option
                        if (UnrealBuildTool.CommandLineContains("-2015"))
                        {
                            UBTArguments.Append(" -2015");
                        }
                        if (UnrealBuildTool.CommandLineContains("-2013"))
                        {
                            UBTArguments.Append(" -2013");
                        }

                        // Add UHT plugins to UBT command line as external plugins
                        if (Target.UnrealHeaderToolPlugins != null && Target.UnrealHeaderToolPlugins.Count > 0)
                        {
                            foreach (PluginInfo Plugin in Target.UnrealHeaderToolPlugins)
                            {
                                UBTArguments.Append(" -PLUGIN \"" + Plugin.File + "\"");
                            }
                        }

                        if (RunExternalExecutable(UnrealBuildTool.GetUBTPath(), UBTArguments.ToString()) != 0)
                        {
                            return(false);
                        }
                    }

                    Progress.Write(1, 3);

                    var ActualTargetName = String.IsNullOrEmpty(Target.GetTargetName()) ? "UE4" : Target.GetTargetName();
                    Log.TraceInformation("Parsing headers for {0}", ActualTargetName);

                    string HeaderToolPath = GetHeaderToolPath();
                    if (!File.Exists(HeaderToolPath))
                    {
                        throw new BuildException("Unable to generate headers because UnrealHeaderTool binary was not found ({0}).", Path.GetFullPath(HeaderToolPath));
                    }

                    // Disable extensions when serializing to remove the $type fields
                    Directory.CreateDirectory(ModuleInfoFileName.Directory.FullName);
                    System.IO.File.WriteAllText(ModuleInfoFileName.FullName, fastJSON.JSON.Instance.ToJSON(Manifest, new fastJSON.JSONParameters {
                        UseExtensions = false
                    }));

                    string CmdLine = (Target.ProjectFile != null) ? "\"" + Target.ProjectFile.FullName + "\"" : Target.GetTargetName();
                    CmdLine += " \"" + ModuleInfoFileName + "\" -LogCmds=\"loginit warning, logexit warning, logdatabase error\" -Unattended -WarningsAsErrors";
                    if (UnrealBuildTool.IsEngineInstalled())
                    {
                        CmdLine += " -installed";
                    }

                    if (UEBuildConfiguration.bFailIfGeneratedCodeChanges)
                    {
                        CmdLine += " -FailIfGeneratedCodeChanges";
                    }

                    if (!bInvalidateUHTMakefile && BuildConfiguration.bUseUHTMakefiles)
                    {
                        CmdLine += " -UseMakefiles";
                    }

                    if (!UEBuildConfiguration.bCompileAgainstEngine)
                    {
                        CmdLine += " -NoEnginePlugins";
                    }

                    Log.TraceInformation("  Running UnrealHeaderTool {0}", CmdLine);

                    Stopwatch s = new Stopwatch();
                    s.Start();
                    UHTResult = (ECompilationResult)RunExternalExecutable(ExternalExecution.GetHeaderToolPath(), CmdLine);
                    s.Stop();

                    if (UHTResult != ECompilationResult.Succeeded)
                    {
                        // On Linux and Mac, the shell will return 128+signal number exit codes if UHT gets a signal (e.g. crashes or is interrupted)
                        if ((BuildHostPlatform.Current.Platform == UnrealTargetPlatform.Linux ||
                             BuildHostPlatform.Current.Platform == UnrealTargetPlatform.Mac) &&
                            (int)(UHTResult) >= 128
                            )
                        {
                            // SIGINT is 2, so 128 + SIGINT is 130
                            UHTResult = ((int)(UHTResult) == 130) ? ECompilationResult.Canceled : ECompilationResult.CrashOrAssert;
                        }

                        Log.TraceInformation("Error: Failed to generate code for {0} - error code: {2} ({1})", ActualTargetName, (int)UHTResult, UHTResult.ToString());
                        return(false);
                    }

                    Log.TraceInformation("Reflection code generated for {0} in {1} seconds", ActualTargetName, s.Elapsed.TotalSeconds);
                    if (BuildConfiguration.bPrintPerformanceInfo)
                    {
                        Log.TraceInformation("UnrealHeaderTool took {1}", ActualTargetName, (double)s.ElapsedMilliseconds / 1000.0);
                    }

                    // Now that UHT has successfully finished generating code, we need to update all cached FileItems in case their last write time has changed.
                    // Otherwise UBT might not detect changes UHT made.
                    DateTime StartTime = DateTime.UtcNow;
                    FileItem.ResetInfos();
                    double ResetDuration = (DateTime.UtcNow - StartTime).TotalSeconds;
                    Log.TraceVerbose("FileItem.ResetInfos() duration: {0}s", ResetDuration);
                }
                else
                {
                    Log.TraceVerbose("Generated code is up to date.");
                }

                Progress.Write(2, 3);

                // There will never be generated code if we're building UHT, so this should never be called.
                if (!bIsBuildingUHT)
                {
                    // Allow generated code to be sync'd to remote machines if needed. This needs to be done even if UHT did not run because
                    // generated headers include other generated headers using absolute paths which in case of building remotely are already
                    // the remote machine absolute paths. Because of that parsing headers will not result in finding all includes properly.
                    // @todo ubtmake: Need to figure out what this does in the assembler case, and whether we need to run it
                    ToolChain.PostCodeGeneration(Manifest);
                }

                // touch the directories
                UpdateDirectoryTimestamps(UObjectModules);

                Progress.Write(3, 3);
            }
            if (ProgressWriter.bWriteMarkup)
            {
                Log.WriteLine(LogEventType.Console, "@progress pop");
            }
            return(true);
        }