Provide explicit API for user to set user agent string prefix

2 files changed, 33 insertions, 8 deletions

diff --git a/src/cpp/client/create_channel.cc b/src/cpp/client/create_channel.cc
index fdaa28ffef..76a1b31e2f 100644
--- a/src/cpp/client/create_channel.cc
+++ b/src/cpp/client/create_channel.cc
@@ -32,7 +32,6 @@
  */
 
 #include <memory>
-#include <sstream>
 
 #include <grpc++/channel.h>
 #include <grpc++/create_channel.h>
@@ -56,13 +55,8 @@ std::shared_ptr<Channel> CreateCustomChannel(
     const ChannelArguments& args) {
   internal::GrpcLibrary
       init_lib;  // We need to call init in case of a bad creds.
-  ChannelArguments cp_args = args;
-  std::ostringstream user_agent_prefix;
-  user_agent_prefix << "grpc-c++/" << grpc_version_string();
-  cp_args.SetString(GRPC_ARG_PRIMARY_USER_AGENT_STRING,
-                    user_agent_prefix.str());
   return creds
-             ? creds->CreateChannel(target, cp_args)
+             ? creds->CreateChannel(target, args)
              : CreateChannelInternal("", grpc_lame_client_channel_create(
                                              NULL, GRPC_STATUS_INVALID_ARGUMENT,
                                              "Invalid credentials."));
diff --git a/src/cpp/common/channel_arguments.cc b/src/cpp/common/channel_arguments.cc
index 90cd5136af..e23c964797 100644
--- a/src/cpp/common/channel_arguments.cc
+++ b/src/cpp/common/channel_arguments.cc
@@ -30,14 +30,23 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  *
  */
-
 #include <grpc++/support/channel_arguments.h>
 
+#include <sstream>
+
+#include <grpc/impl/codegen/grpc_types.h>
 #include <grpc/support/log.h>
 #include "src/core/channel/channel_args.h"
 
 namespace grpc {
 
+ChannelArguments::ChannelArguments() {
+  std::ostringstream user_agent_prefix;
+  user_agent_prefix << "grpc-c++/" << grpc_version_string();
+  // This will be ignored if used on the server side.
+  SetString(GRPC_ARG_PRIMARY_USER_AGENT_STRING, user_agent_prefix.str());
+}
+
 ChannelArguments::ChannelArguments(const ChannelArguments& other)
     : strings_(other.strings_) {
   args_.reserve(other.args_.size());
@@ -81,6 +90,28 @@ void ChannelArguments::SetCompressionAlgorithm(
   SetInt(GRPC_COMPRESSION_ALGORITHM_ARG, algorithm);
 }
 
+// Note: a second call to this will add in front the result of the first call.
+void ChannelArguments::SetUserAgentPrefix(
+    const grpc::string& user_agent_prefix) {
+  if (user_agent_prefix.empty()) {
+    return;
+  }
+  bool replaced = false;
+  for (auto it = args_.begin(); it != args_.end(); ++it) {
+    const grpc_arg& arg = *it;
+    if (arg.type == GRPC_ARG_STRING &&
+        grpc::string(arg.key) == GRPC_ARG_PRIMARY_USER_AGENT_STRING) {
+      strings_.push_back(user_agent_prefix + " " + arg.value.string);
+      it->value.string = const_cast<char*>(strings_.back().c_str());
+      replaced = true;
+      break;
+    }
+  }
+  if (!replaced) {
+    SetString(GRPC_ARG_PRIMARY_USER_AGENT_STRING, user_agent_prefix);
+  }
+}
+
 void ChannelArguments::SetInt(const grpc::string& key, int value) {
   grpc_arg arg;
   arg.type = GRPC_ARG_INTEGER;