From 72b9890f1a15af9eeb73476ae334969bd226c1a8 Mon Sep 17 00:00:00 2001
From: yang-g <yangg@google.com>
Date: Tue, 24 Sep 2019 09:25:51 -0700
Subject: [PATCH] Global channel args mutator when creating a client channel

---
 src/core/lib/channel/channel_args.cc | 16 ++++++++++++++++
 src/core/lib/channel/channel_args.h  | 14 ++++++++++++++
 src/core/lib/surface/channel.cc      |  7 +++++++
 3 files changed, 37 insertions(+)

diff --git a/src/core/lib/channel/channel_args.cc b/src/core/lib/channel/channel_args.cc
index 60248f9d8fc..211b79d57f5 100644
--- a/src/core/lib/channel/channel_args.cc
+++ b/src/core/lib/channel/channel_args.cc
@@ -16,6 +16,8 @@
  *
  */
 
+#include <grpc/impl/codegen/grpc_types.h>
+#include <grpc/impl/codegen/log.h>
 #include <grpc/support/port_platform.h>
 
 #include <limits.h>
@@ -361,3 +363,17 @@ char* grpc_channel_args_string(const grpc_channel_args* args) {
   gpr_strvec_destroy(&v);
   return result;
 }
+
+namespace {
+grpc_channel_args_client_channel_creation_mutator g_mutator = nullptr;
+}  // namespace
+
+void grpc_channel_args_set_client_channel_creation_mutator(
+    grpc_channel_args_client_channel_creation_mutator cb) {
+  GPR_DEBUG_ASSERT(g_mutator == nullptr);
+  g_mutator = cb;
+}
+grpc_channel_args_client_channel_creation_mutator
+grpc_channel_args_get_client_channel_creation_mutator() {
+  return g_mutator;
+}
diff --git a/src/core/lib/channel/channel_args.h b/src/core/lib/channel/channel_args.h
index 5928802f288..f6b4ace2c87 100644
--- a/src/core/lib/channel/channel_args.h
+++ b/src/core/lib/channel/channel_args.h
@@ -23,6 +23,8 @@
 
 #include <grpc/grpc.h>
 
+#include "src/core/lib/surface/channel_stack_type.h"
+
 // Channel args are intentionally immutable, to avoid the need for locking.
 
 /** Copy the arguments in \a src into a new instance */
@@ -108,4 +110,16 @@ grpc_arg grpc_channel_arg_pointer_create(char* name, void* value,
 // Callers takes ownership of result.
 char* grpc_channel_args_string(const grpc_channel_args* args);
 
+// Takes ownership of the old_args
+typedef grpc_channel_args* (*grpc_channel_args_client_channel_creation_mutator)(
+    const char* target, grpc_channel_args* old_args,
+    grpc_channel_stack_type type);
+
+// Should be called only once globaly before grpc is init'ed.
+void grpc_channel_args_set_client_channel_creation_mutator(
+    grpc_channel_args_client_channel_creation_mutator cb);
+// This will be called at the creation of each channel.
+grpc_channel_args_client_channel_creation_mutator
+grpc_channel_args_get_client_channel_creation_mutator();
+
 #endif /* GRPC_CORE_LIB_CHANNEL_CHANNEL_ARGS_H */
diff --git a/src/core/lib/surface/channel.cc b/src/core/lib/surface/channel.cc
index 24813407e28..7f48f258318 100644
--- a/src/core/lib/surface/channel.cc
+++ b/src/core/lib/surface/channel.cc
@@ -257,6 +257,13 @@ grpc_channel* grpc_channel_create(const char* target,
       get_default_authority(input_args);
   grpc_channel_args* args =
       build_channel_args(input_args, default_authority.get());
+  if (grpc_channel_stack_type_is_client(channel_stack_type)) {
+    auto channel_args_mutator =
+        grpc_channel_args_get_client_channel_creation_mutator();
+    if (channel_args_mutator != nullptr) {
+      args = channel_args_mutator(target, args, channel_stack_type);
+    }
+  }
   grpc_channel_stack_builder_set_channel_arguments(builder, args);
   grpc_channel_args_destroy(args);
   grpc_channel_stack_builder_set_target(builder, target);