Add ConfigAwarePlaceholders class (#997)

* Add ConfigAwarePlaceholders class

This allows placeholder values to be sourced from IConfiguration in
addition to set values.

* Rework how IPlaceholders is decorated in container

src/Ocelot/DependencyInjection/IOcelotBuilder.cs

@@ -24,5 +24,7 @@ namespace Ocelot.DependencyInjection
 
         IOcelotBuilder AddTransientDefinedAggregator<T>()
             where T : class, IDefinedAggregator;
+
+        IOcelotBuilder AddConfigPlaceholders();
     }
 }

src/Ocelot/DependencyInjection/OcelotBuilder.cs

@@ -37,6 +37,7 @@ namespace Ocelot.DependencyInjection
     using Ocelot.Security.IPSecurity;
     using Ocelot.ServiceDiscovery;
     using System;
+    using System.Linq;
     using System.Net.Http;
     using System.Reflection;
 
@@ -209,5 +210,39 @@ namespace Ocelot.DependencyInjection
 
             return this;
         }
+
+        public IOcelotBuilder AddConfigPlaceholders()
+        {
+            // see: https://greatrexpectations.com/2018/10/25/decorators-in-net-core-with-dependency-injection
+            var wrappedDescriptor = Services.First(x => x.ServiceType == typeof(IPlaceholders));
+            
+            var objectFactory = ActivatorUtilities.CreateFactory(
+                typeof(ConfigAwarePlaceholders),
+                new[] { typeof(IPlaceholders) });
+
+            Services.Replace(ServiceDescriptor.Describe(
+                typeof(IPlaceholders),
+                s => (IPlaceholders) objectFactory(s,
+                    new[] {CreateInstance(s, wrappedDescriptor)}),
+                wrappedDescriptor.Lifetime
+            ));
+
+            return this;
+        }
+        
+        private static object CreateInstance(IServiceProvider services, ServiceDescriptor descriptor)
+        {
+            if (descriptor.ImplementationInstance != null)
+            {
+                return descriptor.ImplementationInstance;
+            }
+
+            if (descriptor.ImplementationFactory != null)
+            {
+                return descriptor.ImplementationFactory(services);
+            }
+
+            return ActivatorUtilities.GetServiceOrCreateInstance(services, descriptor.ImplementationType);
+        }
     }
 }

src/Ocelot/Infrastructure/ConfigAwarePlaceholders.cs

@@ -0,0 +1,61 @@
+namespace Ocelot.Infrastructure
+{
+    using System;
+    using System.Text.RegularExpressions;
+    using Microsoft.Extensions.Configuration;
+    using Request.Middleware;
+    using Responses;
+
+    public class ConfigAwarePlaceholders : IPlaceholders
+    {
+        private readonly IConfiguration _configuration;
+        private readonly IPlaceholders _placeholders;
+
+        public ConfigAwarePlaceholders(IConfiguration configuration, IPlaceholders placeholders)
+        {
+            _configuration = configuration;
+            _placeholders = placeholders;
+        }
+        
+        public Response<string> Get(string key)
+        {
+            var placeholderResponse = _placeholders.Get(key);
+
+            if (!placeholderResponse.IsError)
+            {
+                return placeholderResponse;
+            }
+
+            return GetFromConfig(CleanKey(key));
+        }
+
+        public Response<string> Get(string key, DownstreamRequest request)
+        {
+            var placeholderResponse = _placeholders.Get(key, request);
+            
+            if (!placeholderResponse.IsError)
+            {
+                return placeholderResponse;
+            }
+
+            return GetFromConfig(CleanKey(key));
+        }
+
+        public Response Add(string key, Func<Response<string>> func)
+            => _placeholders.Add(key, func);
+
+        public Response Remove(string key)
+            => _placeholders.Remove(key);
+
+        private string CleanKey(string key) 
+            => Regex.Replace(key, @"[{}]", string.Empty, RegexOptions.None);
+
+        private Response<string> GetFromConfig(string key)
+        {
+            var valueFromConfig = _configuration[key];
+            return valueFromConfig == null
+                ? (Response<string>) new ErrorResponse<string>(new CouldNotFindPlaceholderError(key))
+                : new OkResponse<string>(valueFromConfig);
+        }
+    }
+}