Add an optional `target_name_override` to support multiple-endpoints with SSL. (#89)

Resolves #87.

Signed-off-by: Tao He <sighingnow@gmail.com>

README.md

@@ -238,6 +238,39 @@ transport security using openssl.
 
 We also provide a tool [`setup-ca.sh`](./security-config/setup-ca.sh) as a helper for development and testing.
 
+#### transport security & Multiple endpoints
+
+If you want to use multiple `https://` endpoints, and you are working with self-signed certificates, you
+may encountered errors like
+
+```
+error: 14: connections to all backends failing
+```
+
+That means your DNS have some problems with your DNS resolver and SSL authority, you could put a domain
+name (a host name) to the `SANS` field when self-signing your certificate, e.g,
+
+```
+"sans": [
+  "etcd",
+  "127.0.0.1",
+  "127.0.0.2",
+  "127.0.0.3"
+],
+```
+
+And pass a `target_name_override` arguments to `WithSSL`,
+
+```cpp
+  etcd::Client *etcd = etcd::Client::WithSSL(
+                    "https://127.0.0.1:2379,https://127.0.0.2:2479",
+                    "example.rootca.cert", "example.cert", "example.key", "etcd");
+
+```
+
+For more discussion about this feature, see also [#87](https://github.com/etcd-cpp-apiv3/etcd-cpp-apiv3/issues/87),
+[grpc#20186](https://github.com/grpc/grpc/issues/20186) and [grpc#22119](https://github.com/grpc/grpc/issues/22119).
+
 ### Reading a value
 
 You can read a value with the `get()` method of the client instance. The only parameter is the

etcd/Client.hpp

@@ -98,6 +98,7 @@ namespace etcd
            std::string const & ca,
            std::string const & cert,
            std::string const & key,
+           std::string const & target_name_override,
            std::string const & load_balancer);
 
     /**
@@ -108,12 +109,16 @@ namespace etcd
      * @param ca   root CA file for SSL/TLS connection.
      * @param cert cert chain file for SSL/TLS authentication, could be empty string.
      * @param key  private key file for SSL/TLS authentication, could be empty string.
+     * @param target_name_override Override the target host name if you want to pass multiple address
+     * for load balancing with SSL, and there's no DNS. The @target_name_override@ must exist in the
+     * SANS of your SSL certificate.
      * @param load_balancer is the load balance strategy, can be one of round_robin/pick_first/grpclb/xds.
      */
     static etcd::Client *WithSSL(std::string const & etcd_url,
            std::string const & ca,
            std::string const & cert = "",
            std::string const & key = "",
+           std::string const & target_name_override = "",
            std::string const & load_balancer = "round_robin");
 
     /**

src/Client.cpp

@@ -239,6 +239,7 @@ etcd::Client::Client(std::string const & address,
                      std::string const & ca,
                      std::string const & cert,
                      std::string const & key,
+                     std::string const & target_name_override,
                      std::string const & load_balancer)
 {
   // create channels
@@ -249,6 +250,9 @@ etcd::Client::Client(std::string const & address,
   std::shared_ptr<grpc::ChannelCredentials> creds = grpc::SslCredentials(
       etcd::detail::make_ssl_credentials(ca, cert, key));
   grpc_args.SetLoadBalancingPolicyName(load_balancer);
+  if (!target_name_override.empty()) {
+    grpc_args.SetString(GRPC_SSL_TARGET_NAME_OVERRIDE_ARG, target_name_override);
+  }
   this->channel = grpc::CreateCustomChannel(addresses, creds, grpc_args);
 
   // setup stubs
@@ -264,8 +268,9 @@ etcd::Client *etcd::Client::WithSSL(std::string const & etcd_url,
                                     std::string const & ca,
                                     std::string const & cert,
                                     std::string const & key,
+                                    std::string const & target_name_override,
                                     std::string const & load_balancer) {
-  return new etcd::Client(etcd_url, ca, cert, key, load_balancer);
+  return new etcd::Client(etcd_url, ca, cert, key, target_name_override, load_balancer);
 }
 
 pplx::task<etcd::Response> etcd::Client::head()