Add the Killswitch client

NuGet.Server.Common.sln

@@ -62,6 +62,10 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "NuGet.Services.Sql", "src\N
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "NuGet.Services.Sql.Tests", "tests\NuGet.Services.Sql.Tests\NuGet.Services.Sql.Tests.csproj", "{FA2B3447-7242-495F-A8E1-D94181C0C9A5}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "NuGet.Services.KillSwitch", "src\NuGet.Services.KillSwitch\NuGet.Services.KillSwitch.csproj", "{BC8D8216-8A19-4402-9DA3-238D0162B0A6}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "NuGet.Services.Killswitch.Tests", "tests\NuGet.Services.Killswitch.Tests\NuGet.Services.Killswitch.Tests.csproj", "{4F73955E-3A98-4403-8BBE-43271FED1CA3}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -168,6 +172,14 @@ Global
 		{FA2B3447-7242-495F-A8E1-D94181C0C9A5}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{FA2B3447-7242-495F-A8E1-D94181C0C9A5}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{FA2B3447-7242-495F-A8E1-D94181C0C9A5}.Release|Any CPU.Build.0 = Release|Any CPU
+		{BC8D8216-8A19-4402-9DA3-238D0162B0A6}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{BC8D8216-8A19-4402-9DA3-238D0162B0A6}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{BC8D8216-8A19-4402-9DA3-238D0162B0A6}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{BC8D8216-8A19-4402-9DA3-238D0162B0A6}.Release|Any CPU.Build.0 = Release|Any CPU
+		{4F73955E-3A98-4403-8BBE-43271FED1CA3}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{4F73955E-3A98-4403-8BBE-43271FED1CA3}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{4F73955E-3A98-4403-8BBE-43271FED1CA3}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{4F73955E-3A98-4403-8BBE-43271FED1CA3}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
@@ -198,6 +210,8 @@ Global
 		{E29F54DF-DFB8-4E27-940D-21ECCB9B6FC1} = {7783A106-0F4C-4055-9AB4-413FB2C7B8F0}
 		{F5121B0A-669F-48BD-86DC-27C546D1A825} = {8415FED7-1BED-4227-8B4F-BB7C24E041CD}
 		{FA2B3447-7242-495F-A8E1-D94181C0C9A5} = {7783A106-0F4C-4055-9AB4-413FB2C7B8F0}
+		{BC8D8216-8A19-4402-9DA3-238D0162B0A6} = {8415FED7-1BED-4227-8B4F-BB7C24E041CD}
+		{4F73955E-3A98-4403-8BBE-43271FED1CA3} = {7783A106-0F4C-4055-9AB4-413FB2C7B8F0}
 	EndGlobalSection
 	GlobalSection(ExtensibilityGlobals) = postSolution
 		SolutionGuid = {AA413DB0-5475-4B5D-A3AF-6323DA8D538B}

build.ps1

@@ -72,8 +72,9 @@ Invoke-BuildStep 'Set version metadata in AssemblyInfo.cs' { `
             "$PSScriptRoot\src\NuGet.Services.ServiceBus\Properties\AssemblyInfo.g.cs", `
             "$PSScriptRoot\src\NuGet.Services.Validation\Properties\AssemblyInfo.g.cs", `
             "$PSScriptRoot\src\NuGet.Services.Validation.Issues\Properties\AssemblyInfo.g.cs", `
-            "$PSScriptRoot\src\NuGet.Services.Sql\Properties\AssemblyInfo.g.cs"
-
+            "$PSScriptRoot\src\NuGet.Services.Sql\Properties\AssemblyInfo.g.cs", `
+            "$PSScriptRoot\src\NuGet.Services.Killswitch\Properties\AssemblyInfo.g.cs"
+
         $versionMetadata | ForEach-Object {
             Set-VersionInfo -Path $_ -Version $SimpleVersion -Branch $Branch -Commit $CommitSHA
         }
@@ -105,8 +106,9 @@ Invoke-BuildStep 'Creating artifacts' { `
             "src\NuGet.Services.ServiceBus\NuGet.Services.ServiceBus.csproj", `
             "src\NuGet.Services.Validation\NuGet.Services.Validation.csproj", `
             "src\NuGet.Services.Validation.Issues\NuGet.Services.Validation.Issues.csproj", `
-            "src\NuGet.Services.Sql\NuGet.Services.Sql.csproj"
-
+            "src\NuGet.Services.Sql\NuGet.Services.Sql.csproj", `
+            "src\NuGet.Services.Killswitch\NuGet.Services.Killswitch.csproj"
+
         $projects | ForEach-Object {
             New-Package (Join-Path $PSScriptRoot $_) -Configuration $Configuration -Symbols -IncludeReferencedProjects -MSBuildVersion "15"
         }

src/NuGet.Services.KillSwitch/HttpKillswitchClient.cs

@@ -0,0 +1,128 @@
+// 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 System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Net.Http;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.Logging;
+using Newtonsoft.Json;
+
+namespace NuGet.Services.KillSwitch
+{
+    /// <summary>
+    /// A killswitch service that queries an HTTP service periodically for the list of active killswitches.
+    /// </summary>
+    public class HttpKillswitchClient : IKillswitchClient
+    {
+        private readonly HttpClient _httpClient;
+        private readonly HttpKillswitchConfig _config;
+        private readonly ILogger<HttpKillswitchClient> _logger;
+
+        private int _started;
+        private KillswitchCache _cache;
+
+        public HttpKillswitchClient(
+            HttpClient httpClient,
+            HttpKillswitchConfig config,
+            ILogger<HttpKillswitchClient> logger)
+        {
+            _httpClient = httpClient ?? throw new ArgumentNullException(nameof(httpClient));
+            _config = config ?? throw new ArgumentNullException(nameof(config));
+            _logger = logger ?? throw new ArgumentNullException(nameof(logger));
+
+            _started = 0;
+            _cache = null;
+        }
+
+        public async Task StartAsync(CancellationToken cancellationToken)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+
+            // Ensure that this client has not already been started.
+            if (Interlocked.CompareExchange(ref _started, 1, 0) != 0)
+            {
+                _logger.LogError("The killswitch client has already been started!");
+
+                throw new InvalidOperationException("The killswitch client has already been started!");
+            }
+
+            // Prime the killswitch cache.
+            await RefreshAsync(cancellationToken);
+
+            // Refresh the killswitch cache on a background thread periodically.
+#pragma warning disable CS4014 // Because this call is not awaited, execution of the current method continues before the call is completed
+            Task.Run(async () =>
+            {
+                while (!cancellationToken.IsCancellationRequested)
+                {
+                    try
+                    {
+                        await Task.Delay(_config.RefreshInterval);
+                        await RefreshAsync(cancellationToken);
+                    }
+                    catch (Exception e)
+                    {
+                        _logger.LogError(0, e, "Failed to refresh the killswitch cache due to exception");
+                    }
+                }
+            });
+#pragma warning restore CS4014 // Because this call is not awaited, execution of the current method continues before the call is completed
+        }
+
+        public async Task RefreshAsync(CancellationToken cancellationToken)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+
+            _logger.LogInformation("Refreshing killswitches...");
+
+            var duration = Stopwatch.StartNew();
+            var response = await _httpClient.GetAsync(_config.KillswitchEndpoint, cancellationToken);
+            var content = await response.Content.ReadAsStringAsync();
+
+            var items = JsonConvert.DeserializeObject<List<string>>(content);
+
+            _cache = new KillswitchCache(items);
+
+            _logger.LogInformation("Refreshed killswitches after {ElapsedTime}", duration.Elapsed);
+        }
+
+        public bool IsActive(string name)
+        {
+            var cache = _cache;
+
+            if (cache == null)
+            {
+                _logger.LogError("Cannot check the killswitch {Name} as the client isn't started", name);
+
+                throw new InvalidOperationException("Cannot check the killswitch as the client isn't started");
+            }
+
+            if (cache.Staleness.Elapsed >= _config.MaximumStaleness)
+            {
+                _logger.LogError(
+                    "Failed to determine whether the killswitch {Name} is active as the client's cache is {Staleness} stale",
+                    name,
+                    cache.Staleness.Elapsed);
+
+                throw new InvalidOperationException("Reached maximum killswitch staleness threshold");
+            }
+
+            return cache.Items.Contains(name);
+        }
+
+        private class KillswitchCache
+        {
+            public KillswitchCache(List<string> items)
+            {
+                Staleness = Stopwatch.StartNew();
+                Items = new HashSet<string>(items, StringComparer.OrdinalIgnoreCase);
+            }
+
+            public Stopwatch Staleness { get; }
+            public HashSet<string> Items { get; }
+        }
+    }
+}

src/NuGet.Services.KillSwitch/HttpKillswitchConfig.cs

@@ -0,0 +1,29 @@
+// 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 System;
+
+namespace NuGet.Services.KillSwitch
+{
+    /// <summary>
+    /// The configuration for <see cref="HttpKillswitchClient"/>.
+    /// </summary>
+    public class HttpKillswitchConfig
+    {
+        /// <summary>
+        /// The REST endpoint that contains the list of activated killswitch.
+        /// </summary>
+        public Uri KillswitchEndpoint { get; set; }
+
+        /// <summary>
+        /// How frequently the endpoint should be queried.
+        /// </summary>
+        public TimeSpan RefreshInterval { get; set; } = TimeSpan.FromMinutes(5);
+
+        /// <summary>
+        /// The maximum allowed staleness by <see cref="HttpKillswitchClient.IsActive(string)"/>.
+        /// The method will throw if this threshold is reached.
+        /// </summary>
+        public TimeSpan MaximumStaleness { get; set; } = TimeSpan.FromMinutes(20);
+    }
+}

src/NuGet.Services.KillSwitch/IKillswitchClient.cs

@@ -0,0 +1,39 @@
+// 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 System.Threading;
+using System.Threading.Tasks;
+
+namespace NuGet.Services.KillSwitch
+{
+    /// <summary>
+    /// Use killswitches to dynamically disable features in your service.
+    /// </summary>
+    public interface IKillswitchClient
+    {
+        /// <summary>
+        /// Start tracking the killswitches. This method can be called only once per instance.
+        /// </summary>
+        /// <param name="cancellationToken">Token to stop tracking the killswitches.</param>
+        /// <returns>A task that completes after the killswitches have been loaded.</returns>
+        Task StartAsync(CancellationToken cancellationToken);
+
+        /// <summary>
+        /// Force a refresh of the list of activated killswitches. This can be called as frequently
+        /// as desired, regardless of whether <see cref="StartAsync(CancellationToken)"/> has been called.
+        /// </summary>
+        /// <param name="cancellationToken">Cancel the refresh operation.</param>
+        /// <returns>A task that completes when the known killswitches have been refreshed.</returns>
+        Task RefreshAsync(CancellationToken cancellationToken);
+
+        /// <summary>
+        /// Check whether the killswitch is known to be active. The list of known activated killswitches MUST be
+        /// loaded (through either <see cref="StartAsync(CancellationToken)"/> or <see cref="RefreshAsync"/>) prior
+        /// to calling this method. If you'd like to guarantee the result isn't stale, call <see cref="RefreshAsync"/>
+        /// prior to calling this method.
+        /// </summary>
+        /// <param name="name">The name of the killswitch.</param>
+        /// <returns>True if the killswitch is known to be active.</returns>
+        bool IsActive(string name);
+    }
+}

src/NuGet.Services.KillSwitch/NuGet.Services.KillSwitch.csproj

@@ -0,0 +1,53 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="15.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
+  <PropertyGroup>
+    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
+    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
+    <ProjectGuid>{BC8D8216-8A19-4402-9DA3-238D0162B0A6}</ProjectGuid>
+    <OutputType>Library</OutputType>
+    <AppDesignerFolder>Properties</AppDesignerFolder>
+    <RootNamespace>NuGet.Services.KillSwitch</RootNamespace>
+    <AssemblyName>NuGet.Services.KillSwitch</AssemblyName>
+    <TargetFrameworkVersion>v4.5</TargetFrameworkVersion>
+    <FileAlignment>512</FileAlignment>
+    <TargetFrameworkProfile />
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
+    <DebugSymbols>true</DebugSymbols>
+    <DebugType>full</DebugType>
+    <Optimize>false</Optimize>
+    <OutputPath>bin\Debug\</OutputPath>
+    <DefineConstants>DEBUG;TRACE</DefineConstants>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
+    <DebugType>pdbonly</DebugType>
+    <Optimize>true</Optimize>
+    <OutputPath>bin\Release\</OutputPath>
+    <DefineConstants>TRACE</DefineConstants>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+  </PropertyGroup>
+  <ItemGroup>
+    <Reference Include="System" />
+    <Reference Include="System.Core" />
+    <Reference Include="System.Xml.Linq" />
+    <Reference Include="System.Data.DataSetExtensions" />
+    <Reference Include="Microsoft.CSharp" />
+    <Reference Include="System.Data" />
+    <Reference Include="System.Xml" />
+  </ItemGroup>
+  <ItemGroup>
+    <Compile Include="IKillswitchClient.cs" />
+    <Compile Include="HttpKillswitchConfig.cs" />
+    <Compile Include="HttpKillswitchClient.cs" />
+    <Compile Include="Properties\AssemblyInfo.cs" />
+  </ItemGroup>
+  <ItemGroup>
+    <None Include="project.json" />
+  </ItemGroup>
+  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
+  <Import Project="..\..\build\sign.targets" />
+</Project>

src/NuGet.Services.KillSwitch/Properties/AssemblyInfo.cs

@@ -0,0 +1,15 @@
+// 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 System.Reflection;
+using System.Runtime.InteropServices;
+
+[assembly: AssemblyTitle("NuGet.Services.Killswitch")]
+[assembly: AssemblyDescription("Disable NuGet service features at runtime")]
+[assembly: AssemblyProduct("NuGet.Services.Killswitch")]
+[assembly: AssemblyCopyright("Copyright © .NET Foundation 2018")]
+[assembly: ComVisible(false)]
+[assembly: Guid("bc8d8216-8a19-4402-9da3-238d0162b0a6")]
+[assembly: AssemblyCompany(".NET Foundation")]
+
+// Assembly version info is set at build time

src/NuGet.Services.KillSwitch/project.json

@@ -0,0 +1,13 @@
+{
+  "dependencies": {
+    "System.Net.Http": "4.3.3",
+    "Microsoft.Extensions.Logging": "1.1.2",
+    "Newtonsoft.Json": "9.0.1"
+  },
+  "frameworks": {
+    "net45": {}
+  },
+  "runtimes": {
+    "win": {}
+  }
+}