Make host.json optional, and use a default host id from machine key

Author: davidebbo

src/WebJobs.Script.WebHost/App_Start/AutofacBootstrap.cs

@@ -1,8 +1,10 @@
 using System;
+using System.Configuration;
 using System.IO;
 using System.Threading;
 using System.Threading.Tasks;
 using System.Web;
+using System.Web.Configuration;
 using System.Web.Hosting;
 using Autofac;
 using Microsoft.Azure.WebJobs.Script;
@@ -40,6 +42,15 @@ internal static void Initialize(ContainerBuilder builder)
                 RootLogPath = logFilePath,
                 FileLoggingEnabled = true
             };
+
+            // If there is an explicit machine key, it makes a good default host id. It can still be
+            // overridden in host.json
+            var section = (MachineKeySection)ConfigurationManager.GetSection("system.web/machineKey");
+            if (section.Decryption != "Auto" && section.ValidationKey.Length >= 32)
+            {
+                scriptHostConfig.HostConfig.HostId = section.ValidationKey.Substring(0, 32);
+            }
+
             WebScriptHostManager scriptHostManager = new WebScriptHostManager(scriptHostConfig);
             builder.RegisterInstance<WebScriptHostManager>(scriptHostManager);
 

src/WebJobs.Script/Host/ScriptHost.cs

@@ -21,7 +21,7 @@ namespace Microsoft.Azure.WebJobs.Script
     public class ScriptHost : JobHost
     {
         private const string HostAssemblyName = "ScriptHost";
-        private const string HostFileName = "host.json";
+        private const string HostConfigFileName = "host.json";
         internal const string FunctionConfigFileName = "function.json";
         private readonly TraceWriter _traceWriter;
         private readonly AutoResetEvent _restartEvent = new AutoResetEvent(false);
@@ -125,7 +125,14 @@ protected virtual void Initialize()
             }
 
             // read host.json and apply to JobHostConfiguration
-            string hostConfigFilePath = Path.Combine(ScriptConfig.RootScriptPath, HostFileName);
+            string hostConfigFilePath = Path.Combine(ScriptConfig.RootScriptPath, HostConfigFileName);
+
+            // If it doesn't exist, create an empty JSON file
+            if (!File.Exists(hostConfigFilePath))
+            {
+                File.WriteAllText(hostConfigFilePath, "{}");
+            }
+
             _traceWriter.Verbose(string.Format("Reading host configuration file '{0}'", hostConfigFilePath));
             string json = File.ReadAllText(hostConfigFilePath);
             JObject hostConfig = JObject.Parse(json);
@@ -370,12 +377,16 @@ internal static void ApplyConfiguration(JObject config, ScriptHostConfiguration
         {
             JobHostConfiguration hostConfig = scriptConfig.HostConfig;
 
+            // We may already have a host id, but the one from the JSON takes precedence
             JToken hostId = (JToken)config["id"];
-            if (hostId == null)
+            if (hostId != null)
+            {
+                hostConfig.HostId = (string)hostId;
+            }
+            else if (hostConfig.HostId == null)
             {
                 throw new InvalidOperationException("An 'id' must be specified in the host configuration.");
             }
-            hostConfig.HostId = (string)hostId;
 
             JToken watchFiles = (JToken)config["watchFiles"];
             if (watchFiles != null && watchFiles.Type == JTokenType.Boolean)
@@ -465,7 +476,7 @@ private void OnFileChanged(object sender, FileSystemEventArgs e)
         {
             string fileName = Path.GetFileName(e.Name);
 
-            if (((string.Compare(fileName, HostFileName, ignoreCase: true) == 0) || string.Compare(fileName, FunctionConfigFileName, ignoreCase: true) == 0) ||
+            if (((string.Compare(fileName, HostConfigFileName, ignoreCase: true) == 0) || string.Compare(fileName, FunctionConfigFileName, ignoreCase: true) == 0) ||
                 ((Directory.EnumerateDirectories(ScriptConfig.RootScriptPath).Count() != _directoryCountSnapshot)))
             {
                 // a host level configuration change has been made which requires a

src/WebJobs.Script/Host/ScriptHostManager.cs

@@ -54,7 +54,6 @@ public ScriptHost Instance
             {
                 IsRunning = false;
 
-                _config.HostConfig = new JobHostConfiguration();
                 ScriptHost newInstance = ScriptHost.Create(_config);
 
                 // TODO: consider using StartAsync here to speed up