Suppressing external startup types when offline

Author: fabiocav

src/WebJobs.Script.WebHost/DefaultScriptHostBuilder.cs

@@ -27,6 +27,11 @@ public IHost BuildHost(bool skipHostStartup, bool skipHostConfigurationParsing)
         {
             var builder = new HostBuilder();
 
+            if (skipHostStartup)
+            {
+                builder.Properties[ScriptConstants.SkipHostInitializationKey] = bool.TrueString;
+            }
+
             if (skipHostConfigurationParsing)
             {
                 builder.ConfigureAppConfiguration((context, _) =>
@@ -42,10 +47,10 @@ public IHost BuildHost(bool skipHostStartup, bool skipHostConfigurationParsing)
             {
                 builder.ConfigureServices(services =>
                 {
-                    // When offline, we need most general services registered so admin
+                    // When skipping host startup (e.g. offline), we need most general services registered so admin
                     // APIs can function. However, we want to prevent the ScriptHost from
                     // actually starting up. To accomplish this, we remove the host service
-                    // responsible for starting the job hst.
+                    // responsible for starting the job host.
                     var jobHostService = services.FirstOrDefault(p => p.ImplementationType == typeof(JobHostService));
                     services.Remove(jobHostService);
                 });

src/WebJobs.Script.WebHost/WebJobsScriptHostService.cs

@@ -263,6 +263,7 @@ private void OnHostInitializing(object sender, EventArgs e)
 
         private IHost BuildHost(bool skipHostStartup = false, bool skipHostJsonConfiguration = false)
         {
+            _logger.LogInformation("Building host: startup suppressed:{skipHostStartup}, configuration suppressed: {skipHostJsonConfiguration}", skipHostStartup, skipHostJsonConfiguration);
             return _scriptHostBuilder.BuildHost(skipHostStartup, skipHostJsonConfiguration);
         }
 

src/WebJobs.Script/ScriptConstants.cs

@@ -51,6 +51,7 @@ public static class ScriptConstants
         public const string LogCategoryMigration = "Host.Migration";
 
         public const string SkipHostJsonConfigurationKey = "MS_SkipHostJsonConfiguration";
+        public const string SkipHostInitializationKey = "MS_SkipHostInitialization";
 
         // Define all system parameters we inject with a prefix to avoid collisions
         // with user parameters

src/WebJobs.Script/ScriptHostBuilderExtensions.cs

@@ -80,6 +80,8 @@ public static IHostBuilder AddScriptHost(this IHostBuilder builder, ScriptApplic
 
         public static IHostBuilder AddScriptHostCore(this IHostBuilder builder, ScriptApplicationHostOptions applicationHostOptions, Action<IWebJobsBuilder> configureWebJobs = null)
         {
+            var skipHostInitialization = builder.Properties.ContainsKey(ScriptConstants.SkipHostInitializationKey);
+
             builder.ConfigureWebJobs(webJobsBuilder =>
             {
                 // Built in binding registrations
@@ -94,7 +96,12 @@ public static IHostBuilder AddScriptHostCore(this IHostBuilder builder, ScriptAp
                 .AddTimers()
                 .AddManualTrigger();
 
-                webJobsBuilder.UseScriptExternalStartup(applicationHostOptions.ScriptPath);
+                if (!skipHostInitialization)
+                {
+                    // Only set our external startup if we're not suppressing host initialization
+                    // as we don't want to load user assemblies otherwise.
+                    webJobsBuilder.UseScriptExternalStartup(applicationHostOptions.ScriptPath);
+                }
 
                 configureWebJobs?.Invoke(webJobsBuilder);
             }, o => o.AllowPartialHostStartup = true);