JavaScriptServices/src/Microsoft.AspNetCore.SpaServices.Extensions/Prerendering/SpaPrerenderingExtensions.cs

// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Http.Features;
using Microsoft.AspNetCore.NodeServices;
using Microsoft.AspNetCore.SpaServices;
using Microsoft.AspNetCore.SpaServices.Extensions.Util;
using Microsoft.AspNetCore.SpaServices.Prerendering;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Net.Http.Headers;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Microsoft.AspNetCore.Builder
{
    /// <summary>
    /// Extension methods for configuring prerendering of a Single Page Application.
    /// </summary>
    public static class SpaPrerenderingExtensions
    {
        private static TimeSpan BuildTimeout = TimeSpan.FromSeconds(50); // Note that the HTTP request itself by default times out after 60s, so you only get useful error information if this is shorter

        /// <summary>
        /// Enables server-side prerendering middleware for a Single Page Application.
        /// </summary>
        /// <param name="spaBuilder">The <see cref="ISpaBuilder"/>.</param>
        /// <param name="configuration">Supplies configuration for the prerendering middleware.</param>
        public static void UseSpaPrerendering(
            this ISpaBuilder spaBuilder,
            Action<SpaPrerenderingOptions> configuration)
        {
            if (spaBuilder == null)
            {
                throw new ArgumentNullException(nameof(spaBuilder));
            }

            if (configuration == null)
            {
                throw new ArgumentNullException(nameof(configuration));
            }

            var options = new SpaPrerenderingOptions();
            configuration.Invoke(options);

            var capturedBootModulePath = options.BootModulePath;
            if (string.IsNullOrEmpty(capturedBootModulePath))
            {
                throw new InvalidOperationException($"To use {nameof(UseSpaPrerendering)}, you " +
                    $"must set a nonempty value on the ${nameof(SpaPrerenderingOptions.BootModulePath)} " +
                    $"property on the ${nameof(SpaPrerenderingOptions)}.");
            }

            // If we're building on demand, start that process in the background now
            var buildOnDemandTask = options.BootModuleBuilder?.Build(spaBuilder);

            // Get all the necessary context info that will be used for each prerendering call
            var applicationBuilder = spaBuilder.ApplicationBuilder;
            var serviceProvider = applicationBuilder.ApplicationServices;
            var nodeServices = GetNodeServices(serviceProvider);
            var applicationStoppingToken = serviceProvider.GetRequiredService<IApplicationLifetime>()
                .ApplicationStopping;
            var applicationBasePath = serviceProvider.GetRequiredService<IHostingEnvironment>()
                .ContentRootPath;
            var moduleExport = new JavaScriptModuleExport(capturedBootModulePath);
            var excludePathStrings = (options.ExcludeUrls ?? Array.Empty<string>())
                .Select(url => new PathString(url))
                .ToArray();

            applicationBuilder.Use(async (context, next) =>
            {
                // If this URL is excluded, skip prerendering.
                // This is typically used to ensure that static client-side resources
                // (e.g., /dist/*.css) are served normally or through SPA development
                // middleware, and don't return the prerendered index.html page.
                foreach (var excludePathString in excludePathStrings)
                {
                    if (context.Request.Path.StartsWithSegments(excludePathString))
                    {
                        await next();
                        return;
                    }
                }

                // If we're building on demand, wait for that to finish, or raise any build errors
                if (buildOnDemandTask != null && !buildOnDemandTask.IsCompleted)
                {
                    // For better debuggability, create a per-request timeout that makes it clear if the
                    // prerendering builder took too long for this request, but without aborting the
                    // underlying build task so that subsequent requests could still work.
                    await buildOnDemandTask.WithTimeout(BuildTimeout,
                        $"The prerendering build process did not complete within the " +
                        $"timeout period of {BuildTimeout.Seconds} seconds. " +
                        $"Check the log output for error information.");
                }

                // It's no good if we try to return a 304. We need to capture the actual
                // HTML content so it can be passed as a template to the prerenderer.
                RemoveConditionalRequestHeaders(context.Request);

                // Make sure we're not capturing compressed content, because then we'd have
                // to decompress it. Since this sub-request isn't leaving the machine, there's
                // little to no benefit in having compression on it.
                var originalAcceptEncodingValue = GetAndRemoveAcceptEncodingHeader(context.Request);

                // Capture the non-prerendered responses, which in production will typically only
                // be returning the default SPA index.html page (because other resources will be
                // served statically from disk). We will use this as a template in which to inject
                // the prerendered output.
                using (var outputBuffer = new MemoryStream())
                {
                    var originalResponseStream = context.Response.Body;
                    context.Response.Body = outputBuffer;

                    try
                    {
                        await next();
                        outputBuffer.Seek(0, SeekOrigin.Begin);
                    }
                    finally
                    {
                        context.Response.Body = originalResponseStream;

                        if (!string.IsNullOrEmpty(originalAcceptEncodingValue))
                        {
                            context.Request.Headers[HeaderNames.AcceptEncoding] = originalAcceptEncodingValue;
                        }
                    }

                    // If it isn't an HTML page that we can use as the template for prerendering,
                    //  - ... because it's not text/html
                    //  - ... or because it's an error
                    // then prerendering doesn't apply to this request, so just pass through the
                    // response as-is. Note that the non-text/html case is not an error: this is
                    // typically how the SPA dev server responses for static content are returned
                    // in development mode.
                    var canPrerender = IsSuccessStatusCode(context.Response.StatusCode)
                        && IsHtmlContentType(context.Response.ContentType);
                    if (!canPrerender)
                    {
                        await outputBuffer.CopyToAsync(context.Response.Body);
                        return;
                    }

                    // Most prerendering logic will want to know about the original, unprerendered
                    // HTML that the client would be getting otherwise. Typically this is used as
                    // a template from which the fully prerendered page can be generated.
                    var customData = new Dictionary<string, object>
                    {
                        { "originalHtml", Encoding.UTF8.GetString(outputBuffer.GetBuffer()) }
                    };

                    // If the developer wants to use custom logic to pass arbitrary data to the
                    // prerendering JS code (e.g., to pass through cookie data), now's their chance
                    options.SupplyData?.Invoke(context, customData);

                    var (unencodedAbsoluteUrl, unencodedPathAndQuery)
                        = GetUnencodedUrlAndPathQuery(context);
                    var renderResult = await Prerenderer.RenderToString(
                        applicationBasePath,
                        nodeServices,
                        applicationStoppingToken,
                        moduleExport,
                        unencodedAbsoluteUrl,
                        unencodedPathAndQuery,
                        customDataParameter: customData,
                        timeoutMilliseconds: 0,
                        requestPathBase: context.Request.PathBase.ToString());

                    await ServePrerenderResult(context, renderResult);
                }
            });
        }

        private static bool IsHtmlContentType(string contentType)
        {
            if (string.Equals(contentType, "text/html", StringComparison.Ordinal))
            {
                return true;
            }

            return contentType != null
                && contentType.StartsWith("text/html;", StringComparison.Ordinal);
        }

        private static bool IsSuccessStatusCode(int statusCode)
            => statusCode >= 200 && statusCode < 300;

        private static void RemoveConditionalRequestHeaders(HttpRequest request)
        {
            request.Headers.Remove(HeaderNames.IfMatch);
            request.Headers.Remove(HeaderNames.IfModifiedSince);
            request.Headers.Remove(HeaderNames.IfNoneMatch);
            request.Headers.Remove(HeaderNames.IfUnmodifiedSince);
            request.Headers.Remove(HeaderNames.IfRange);
        }

        private static string GetAndRemoveAcceptEncodingHeader(HttpRequest request)
        {
            var headers = request.Headers;
            var value = (string)null;

            if (headers.ContainsKey(HeaderNames.AcceptEncoding))
            {
                value = headers[HeaderNames.AcceptEncoding];
                headers.Remove(HeaderNames.AcceptEncoding);
            }

            return value;
        }

        private static (string, string) GetUnencodedUrlAndPathQuery(HttpContext httpContext)
        {
            // This is a duplicate of code from Prerenderer.cs in the SpaServices package.
            // Once the SpaServices.Extension package implementation gets merged back into
            // SpaServices, this duplicate can be removed. To remove this, change the code
            // above that calls Prerenderer.RenderToString to use the internal overload
            // that takes an HttpContext instead of a url/path+query pair.
            var requestFeature = httpContext.Features.Get<IHttpRequestFeature>();
            var unencodedPathAndQuery = requestFeature.RawTarget;
            var request = httpContext.Request;
            var unencodedAbsoluteUrl = $"{request.Scheme}://{request.Host}{unencodedPathAndQuery}";
            return (unencodedAbsoluteUrl, unencodedPathAndQuery);
        }

        private static async Task ServePrerenderResult(HttpContext context, RenderToStringResult renderResult)
        {
            context.Response.Clear();

            if (!string.IsNullOrEmpty(renderResult.RedirectUrl))
            {
                var permanentRedirect = renderResult.StatusCode.GetValueOrDefault() == 301;
                context.Response.Redirect(renderResult.RedirectUrl, permanentRedirect);
            }
            else
            {
                // The Globals property exists for back-compatibility but is meaningless
                // for prerendering that returns complete HTML pages
                if (renderResult.Globals != null)
                {
                    throw new InvalidOperationException($"{nameof(renderResult.Globals)} is not " +
                        $"supported when prerendering via {nameof(UseSpaPrerendering)}(). Instead, " +
                        $"your prerendering logic should return a complete HTML page, in which you " +
                        $"embed any information you wish to return to the client.");
                }

                if (renderResult.StatusCode.HasValue)
                {
                    context.Response.StatusCode = renderResult.StatusCode.Value;
                }

                context.Response.ContentType = "text/html";
                await context.Response.WriteAsync(renderResult.Html);
            }
        }

        private static INodeServices GetNodeServices(IServiceProvider serviceProvider)
        {
            // Use the registered instance, or create a new private instance if none is registered
            var instance = (INodeServices)serviceProvider.GetService(typeof(INodeServices));
            return instance ?? NodeServicesFactory.CreateNodeServices(
                new NodeServicesOptions(serviceProvider));
        }
    }
}