Prevent NullReferenceException when using HttpClient to initialize Kubernetes client. Fixes #335 (#336)

* Preventing null ref exception when initializing with HttpClient and adding support for generating a default client handler from a config object

* Updating example to make use of config-based client handler generation

examples/httpClientFactory/Program.cs

@@ -31,7 +31,8 @@ namespace httpClientFactory
                             return new Kubernetes(
                                 serviceProvider.GetRequiredService<KubernetesClientConfiguration>(),
                                 httpClient);
-                        });
+                        })
+                        .ConfigurePrimaryHttpMessageHandler(config.CreateDefaultHttpClientHandler);
 
                     // Add the class that uses the client
                     services.AddHostedService<PodListHostedService>();

src/KubernetesClient/Kubernetes.ConfigInit.cs

@@ -43,7 +43,7 @@ namespace k8s
             ValidateConfig(config);
             CaCerts = config.SslCaCerts;
             SkipTlsVerify = config.SkipTlsVerify;
-            InitializeFromConfig(config);
+            SetCredentials(config); 
         }
 
         /// <summary>
@@ -151,8 +151,9 @@ namespace k8s
                 }
             }
 
-            // set credentails for the kubernernet client
-            SetCredentials(config, HttpClientHandler);
+            // set credentails for the kubernetes client
+            SetCredentials(config);
+            config.AddCertificates(HttpClientHandler);
         }
 
         private X509Certificate2Collection CaCerts { get; }
@@ -195,9 +196,7 @@ namespace k8s
         ///     Set credentials for the Client
         /// </summary>
         /// <param name="config">k8s client configuration</param>
-        /// <param name="handler">http client handler for the rest client</param>
-        /// <returns>Task</returns>
-        private void SetCredentials(KubernetesClientConfiguration config, HttpClientHandler handler)
+        private void SetCredentials(KubernetesClientConfiguration config)
         {
             // set the Credentails for token based auth
             if (!string.IsNullOrWhiteSpace(config.AccessToken))
@@ -212,25 +211,6 @@ namespace k8s
                     Password = config.Password
                 };
             }
-
-#if XAMARINIOS1_0 || MONOANDROID8_1
-            // handle.ClientCertificates is not implemented in Xamarin.
-            return;
-#endif
-
-            if ((!string.IsNullOrWhiteSpace(config.ClientCertificateData) ||
-                    !string.IsNullOrWhiteSpace(config.ClientCertificateFilePath)) &&
-                    (!string.IsNullOrWhiteSpace(config.ClientCertificateKeyData) ||
-                    !string.IsNullOrWhiteSpace(config.ClientKeyFilePath)))
-            {
-                var cert = CertUtils.GeneratePfx(config);
-
-#if NET452
-                ((WebRequestHandler) handler).ClientCertificates.Add(cert);
-#else
-                handler.ClientCertificates.Add(cert);
-#endif
-            }
         }
 
         /// <summary>

src/KubernetesClient/KubernetesClientConfiguration.HttpClientHandler.cs

@@ -0,0 +1,56 @@
+using System;
+using System.Net.Http;
+
+namespace k8s
+{
+    public partial class KubernetesClientConfiguration {
+        public HttpClientHandler CreateDefaultHttpClientHandler() {
+            var httpClientHandler = new HttpClientHandler();
+
+#if !NET452
+            var uriScheme = new Uri(this.Host).Scheme;
+
+            if(uriScheme == "https")
+            {
+                if(this.SkipTlsVerify)
+                {
+                    httpClientHandler.ServerCertificateCustomValidationCallback = 
+                        (sender, certificate, chain, sslPolicyErrors) => true;
+                }
+                else
+                {
+                    httpClientHandler.ServerCertificateCustomValidationCallback = (sender, certificate, chain, sslPolicyErrors) =>
+                    {
+                        return Kubernetes.CertificateValidationCallBack(sender, this.SslCaCerts, certificate, chain, sslPolicyErrors);
+                    };
+                }
+            }
+#endif
+
+            AddCertificates(httpClientHandler);
+
+            return httpClientHandler;
+        }
+
+        public void AddCertificates(HttpClientHandler handler) {
+#if XAMARINIOS1_0 || MONOANDROID8_1
+            // handle.ClientCertificates is not implemented in Xamarin.
+            return;
+#endif
+
+            if ((!string.IsNullOrWhiteSpace(this.ClientCertificateData) ||
+                    !string.IsNullOrWhiteSpace(this.ClientCertificateFilePath)) &&
+                    (!string.IsNullOrWhiteSpace(this.ClientCertificateKeyData) ||
+                    !string.IsNullOrWhiteSpace(this.ClientKeyFilePath)))
+            {
+                var cert = CertUtils.GeneratePfx(this);
+
+#if NET452
+                ((WebRequestHandler) handler).ClientCertificates.Add(cert);
+#else
+                handler.ClientCertificates.Add(cert);
+#endif
+            }
+        }
+    }
+}