New: Return static response to requests while app is starting

(cherry picked from commit 303fc5d786ccf2ad14c8523fc239696c5d37ea53)

src/NzbDrone.Common/EnvironmentInfo/IRuntimeInfo.cs

@@ -9,6 +9,7 @@ public interface IRuntimeInfo
         bool IsAdmin { get; }
         bool IsWindowsService { get; }
         bool IsWindowsTray { get; }
+        bool IsStarting { get; set; }
         bool IsExiting { get; set; }
         bool IsTray { get; }
         RuntimeMode Mode { get; }

src/NzbDrone.Common/EnvironmentInfo/RuntimeInfo.cs

@@ -19,6 +19,7 @@ public RuntimeInfo(Logger logger, IHostLifetime hostLifetime = null)
             _logger = logger;
 
             IsWindowsService = hostLifetime is WindowsServiceLifetime;
+            IsStarting = true;
 
             // net6.0 will return Radarr.dll for entry assembly, we need the actual
             // executable name (Radarr on linux).  On mono this will return the location of
@@ -82,6 +83,7 @@ public bool IsAdmin
 
         public bool IsWindowsService { get; private set; }
 
+        public bool IsStarting { get; set; }
         public bool IsExiting { get; set; }
         public bool IsTray
         {

src/NzbDrone.Host/AppLifetime.cs

@@ -56,6 +56,7 @@ public Task StopAsync(CancellationToken cancellationToken)
 
         private void OnAppStarted()
         {
+            _runtimeInfo.IsStarting = false;
             _runtimeInfo.IsExiting = false;
 
             if (!_startupContext.Flags.Contains(StartupContext.NO_BROWSER)

src/NzbDrone.Host/Startup.cs

@@ -264,6 +264,7 @@ public void Configure(IApplicationBuilder app,
 
             app.UseMiddleware<VersionMiddleware>();
             app.UseMiddleware<UrlBaseMiddleware>(configFileProvider.UrlBase);
+            app.UseMiddleware<StartingUpMiddleware>();
             app.UseMiddleware<CacheHeaderMiddleware>();
             app.UseMiddleware<IfModifiedMiddleware>();
             app.UseMiddleware<BufferingMiddleware>(new List<string> { "/api/v3/command" });

src/Sonarr.Http/Middleware/StartingUpMiddleware.cs

@@ -0,0 +1,43 @@
+using System.Buffers;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Http;
+using NzbDrone.Common.EnvironmentInfo;
+using NzbDrone.Common.Serializer;
+using NzbDrone.Core.Lifecycle;
+using NzbDrone.Core.Messaging.Events;
+using Sonarr.Http.Extensions;
+
+namespace Sonarr.Http.Middleware
+{
+    public class StartingUpMiddleware
+    {
+        private readonly RequestDelegate _next;
+        private readonly IRuntimeInfo _runtimeInfo;
+        private static readonly string MESSAGE = "Sonarr is starting up, please try again later";
+
+        public StartingUpMiddleware(RequestDelegate next, IRuntimeInfo runtimeInfo)
+        {
+            _next = next;
+            _runtimeInfo = runtimeInfo;
+        }
+
+        public async Task InvokeAsync(HttpContext context)
+        {
+            if (_runtimeInfo.IsStarting)
+            {
+                var isJson = context.Request.IsApiRequest();
+                var message = isJson ? STJson.ToJson(new { ErrorMessage = MESSAGE }) : MESSAGE;
+                var bytes = Encoding.UTF8.GetBytes(message);
+
+                context.Response.StatusCode = 503;
+                context.Response.ContentType = isJson ? "application/json" : "text/plain";
+                await context.Response.Body.WriteAsync(bytes, 0, bytes.Length);
+
+                return;
+            }
+
+            await _next(context);
+        }
+    }
+}