Transport channel arg (#28802)

Instead of passing transport optionally as an argument adjacent to channel args, pass it as a channel arg directly.
Doing so does not affect semantics, but does allow a cleaner API for channel creation which will become increasingly important as we move towards promises.

Co-authored-by: ctiller <ctiller@users.noreply.github.com>

src/core/ext/filters/client_channel/dynamic_filters.cc

@@ -140,8 +140,7 @@ std::pair<grpc_channel_stack*, grpc_error_handle> CreateChannelStack(
   // Initialize stack.
   grpc_error_handle error = grpc_channel_stack_init(
       /*initial_refs=*/1, DestroyChannelStack, channel_stack, filters.data(),
-      filters.size(), args, /*optional_transport=*/nullptr, "DynamicFilters",
-      channel_stack);
+      filters.size(), args, "DynamicFilters", channel_stack);
   if (error != GRPC_ERROR_NONE) {
     gpr_log(GPR_ERROR, "error initializing client internal stack: %s",
             grpc_error_std_string(error).c_str());

src/core/ext/filters/http/client/http_client_filter.cc

@@ -32,6 +32,7 @@
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
 
+#include "src/core/lib/channel/channel_args.h"
 #include "src/core/lib/gpr/string.h"
 #include "src/core/lib/gprpp/manual_constructor.h"
 #include "src/core/lib/profiling/timers.h"
@@ -489,12 +490,14 @@ static grpc_error_handle http_client_init_channel_elem(
   channel_data* chand = static_cast<channel_data*>(elem->channel_data);
   new (chand) channel_data();
   GPR_ASSERT(!args->is_last);
-  GPR_ASSERT(args->optional_transport != nullptr);
+  auto* transport = grpc_channel_args_find_pointer<grpc_transport>(
+      args->channel_args, GRPC_ARG_TRANSPORT);
+  GPR_ASSERT(transport != nullptr);
   chand->static_scheme = scheme_from_args(args->channel_args);
   chand->max_payload_size_for_get =
       max_payload_size_from_args(args->channel_args);
-  chand->user_agent = grpc_core::Slice(user_agent_from_args(
-      args->channel_args, args->optional_transport->vtable->name));
+  chand->user_agent = grpc_core::Slice(
+      user_agent_from_args(args->channel_args, transport->vtable->name));
   return GRPC_ERROR_NONE;
 }
 

src/core/ext/filters/rbac/rbac_filter.cc

@@ -134,15 +134,17 @@ grpc_error_handle RbacFilter::Init(grpc_channel_element* elem,
   if (auth_context == nullptr) {
     return GRPC_ERROR_CREATE_FROM_STATIC_STRING("No auth context found");
   }
-  if (args->optional_transport == nullptr) {
+  auto* transport = grpc_channel_args_find_pointer<grpc_transport>(
+      args->channel_args, GRPC_ARG_TRANSPORT);
+  if (transport == nullptr) {
     // This should never happen since the transport is always set on the server
     // side.
     return GRPC_ERROR_CREATE_FROM_STATIC_STRING("No transport configured");
   }
   new (elem->channel_data) RbacFilter(
       grpc_channel_stack_filter_instance_number(args->channel_stack, elem),
-      EvaluateArgs::PerChannelArgs(
-          auth_context, grpc_transport_get_endpoint(args->optional_transport)));
+      EvaluateArgs::PerChannelArgs(auth_context,
+                                   grpc_transport_get_endpoint(transport)));
   return GRPC_ERROR_NONE;
 }
 

src/core/lib/channel/channel_stack.cc

@@ -104,8 +104,8 @@ grpc_call_element* grpc_call_stack_element(grpc_call_stack* call_stack,
 grpc_error_handle grpc_channel_stack_init(
     int initial_refs, grpc_iomgr_cb_func destroy, void* destroy_arg,
     const grpc_channel_filter** filters, size_t filter_count,
-    const grpc_channel_args* channel_args, grpc_transport* optional_transport,
-    const char* name, grpc_channel_stack* stack) {
+    const grpc_channel_args* channel_args, const char* name,
+    grpc_channel_stack* stack) {
   if (grpc_trace_channel_stack.enabled()) {
     gpr_log(GPR_INFO, "CHANNEL_STACK: init %s", name);
     for (size_t i = 0; i < filter_count; i++) {
@@ -134,7 +134,6 @@ grpc_error_handle grpc_channel_stack_init(
   for (i = 0; i < filter_count; i++) {
     args.channel_stack = stack;
     args.channel_args = channel_args;
-    args.optional_transport = optional_transport;
     args.is_first = i == 0;
     args.is_last = i == (filter_count - 1);
     elems[i].filter = filters[i];

src/core/lib/channel/channel_stack.h

@@ -68,11 +68,11 @@ typedef struct grpc_call_element grpc_call_element;
 typedef struct grpc_channel_stack grpc_channel_stack;
 typedef struct grpc_call_stack grpc_call_stack;
 
+#define GRPC_ARG_TRANSPORT "grpc.internal.transport"
+
 struct grpc_channel_element_args {
   grpc_channel_stack* channel_stack;
   const grpc_channel_args* channel_args;
-  /** Transport, iff it is known */
-  grpc_transport* optional_transport;
   int is_first;
   int is_last;
 };
@@ -237,8 +237,7 @@ size_t grpc_channel_stack_size(const grpc_channel_filter** filters,
 grpc_error_handle grpc_channel_stack_init(
     int initial_refs, grpc_iomgr_cb_func destroy, void* destroy_arg,
     const grpc_channel_filter** filters, size_t filter_count,
-    const grpc_channel_args* args, grpc_transport* optional_transport,
-    const char* name, grpc_channel_stack* stack);
+    const grpc_channel_args* args, const char* name, grpc_channel_stack* stack);
 /* Destroy a channel stack */
 void grpc_channel_stack_destroy(grpc_channel_stack* stack);
 

src/core/lib/channel/channel_stack_builder.cc

@@ -25,6 +25,7 @@
 #include <grpc/support/alloc.h>
 #include <grpc/support/string_util.h>
 
+#include "src/core/lib/channel/channel_args.h"
 #include "src/core/lib/channel/channel_stack_builder.h"
 #include "src/core/lib/gprpp/memory.h"
 
@@ -80,10 +81,32 @@ grpc_error_handle ChannelStackBuilder::Build(size_t prefix_bytes,
   // fetch a pointer to the channel stack
   grpc_channel_stack* channel_stack = reinterpret_cast<grpc_channel_stack*>(
       static_cast<char*>(*result) + prefix_bytes);
+
+  const grpc_channel_args* final_args;
+  if (transport_ != nullptr) {
+    static const grpc_arg_pointer_vtable vtable = {
+        // copy
+        [](void* p) { return p; },
+        // destroy
+        [](void*) {},
+        // cmp
+        [](void* a, void* b) { return QsortCompare(a, b); },
+    };
+    grpc_arg arg = grpc_channel_arg_pointer_create(
+        const_cast<char*>(GRPC_ARG_TRANSPORT), transport_, &vtable);
+    final_args = grpc_channel_args_copy_and_add(args_, &arg, 1);
+  } else {
+    final_args = args_;
+  }
+
   // and initialize it
   grpc_error_handle error = grpc_channel_stack_init(
       initial_refs, destroy, destroy_arg == nullptr ? *result : destroy_arg,
-      filters.data(), filters.size(), args_, transport_, name_, channel_stack);
+      filters.data(), filters.size(), final_args, name_, channel_stack);
+
+  if (final_args != args_) {
+    grpc_channel_args_destroy(final_args);
+  }
 
   if (error != GRPC_ERROR_NONE) {
     grpc_channel_stack_destroy(channel_stack);

test/core/channel/channel_stack_test.cc

@@ -112,7 +112,7 @@ static void test_create_channel_stack(void) {
   GPR_ASSERT(GRPC_LOG_IF_ERROR(
       "grpc_channel_stack_init",
       grpc_channel_stack_init(1, free_channel, channel_stack, &filters, 1,
-                              &chan_args, nullptr, "test", channel_stack)));
+                              &chan_args, "test", channel_stack)));
   GPR_ASSERT(channel_stack->count == 1);
   channel_elem = grpc_channel_stack_element(channel_stack, 0);
   channel_data = static_cast<int*>(channel_elem->channel_data);

test/cpp/microbenchmarks/bm_call_create.cc

@@ -451,6 +451,19 @@ static const grpc_transport_vtable phony_transport_vtable = {0,
 
 static grpc_transport phony_transport = {&phony_transport_vtable};
 
+grpc_arg Arg() {
+  static const grpc_arg_pointer_vtable vtable = {
+      // copy
+      [](void* p) { return p; },
+      // destroy
+      [](void*) {},
+      // cmp
+      [](void* a, void* b) { return grpc_core::QsortCompare(a, b); },
+  };
+  return grpc_channel_arg_pointer_create(const_cast<char*>(GRPC_ARG_TRANSPORT),
+                                         &phony_transport, &vtable);
+}
+
 }  // namespace phony_transport
 
 class NoOp {
@@ -508,7 +521,10 @@ static void BM_IsolatedFilter(benchmark::State& state) {
           &fake_client_channel_factory),
       StringArg(GRPC_ARG_SERVER_URI, "localhost"),
   };
-  grpc_channel_args channel_args = {args.size(), &args[0]};
+  if (fixture.flags & REQUIRES_TRANSPORT) {
+    args.push_back(phony_transport::Arg());
+  }
+  grpc_channel_args channel_args = {args.size(), args.data()};
 
   std::vector<const grpc_channel_filter*> filters;
   if (fixture.filter != nullptr) {
@@ -528,11 +544,8 @@ static void BM_IsolatedFilter(benchmark::State& state) {
       "channel_stack_init",
       grpc_channel_stack_init(1, FilterDestroy, channel_stack,
                               filters.empty() ? nullptr : &filters[0],
-                              filters.size(), &channel_args,
-                              fixture.flags & REQUIRES_TRANSPORT
-                                  ? &phony_transport::phony_transport
-                                  : nullptr,
-                              "CHANNEL", channel_stack)));
+                              filters.size(), &channel_args, "CHANNEL",
+                              channel_stack)));
   grpc_core::ExecCtx::Get()->Flush();
   grpc_call_stack* call_stack =
       static_cast<grpc_call_stack*>(gpr_zalloc(channel_stack->call_stack_size));