Merge pull request #15539 from markdroth/recv_trailing_metadata_ready

move recv_trailing_metadata into its own callback, don't use on_complete for recv_ops

src/core/ext/filters/deadline/deadline_filter.cc

@@ -128,21 +128,25 @@ static void cancel_timer_if_needed(grpc_deadline_state* deadline_state) {
   }
 }
 
-// Callback run when the call is complete.
-static void on_complete(void* arg, grpc_error* error) {
+// Callback run when we receive trailing metadata.
+static void recv_trailing_metadata_ready(void* arg, grpc_error* error) {
   grpc_deadline_state* deadline_state = static_cast<grpc_deadline_state*>(arg);
   cancel_timer_if_needed(deadline_state);
-  // Invoke the next callback.
-  GRPC_CLOSURE_RUN(deadline_state->next_on_complete, GRPC_ERROR_REF(error));
+  // Invoke the original callback.
+  GRPC_CLOSURE_RUN(deadline_state->original_recv_trailing_metadata_ready,
+                   GRPC_ERROR_REF(error));
 }
 
-// Inject our own on_complete callback into op.
-static void inject_on_complete_cb(grpc_deadline_state* deadline_state,
-                                  grpc_transport_stream_op_batch* op) {
-  deadline_state->next_on_complete = op->on_complete;
-  GRPC_CLOSURE_INIT(&deadline_state->on_complete, on_complete, deadline_state,
+// Inject our own recv_trailing_metadata_ready callback into op.
+static void inject_recv_trailing_metadata_ready(
+    grpc_deadline_state* deadline_state, grpc_transport_stream_op_batch* op) {
+  deadline_state->original_recv_trailing_metadata_ready =
+      op->payload->recv_trailing_metadata.recv_trailing_metadata_ready;
+  GRPC_CLOSURE_INIT(&deadline_state->recv_trailing_metadata_ready,
+                    recv_trailing_metadata_ready, deadline_state,
                     grpc_schedule_on_exec_ctx);
-  op->on_complete = &deadline_state->on_complete;
+  op->payload->recv_trailing_metadata.recv_trailing_metadata_ready =
+      &deadline_state->recv_trailing_metadata_ready;
 }
 
 // Callback and associated state for starting the timer after call stack
@@ -226,7 +230,7 @@ void grpc_deadline_state_client_start_transport_stream_op_batch(
     // Make sure we know when the call is complete, so that we can cancel
     // the timer.
     if (op->recv_trailing_metadata) {
-      inject_on_complete_cb(deadline_state, op);
+      inject_recv_trailing_metadata_ready(deadline_state, op);
     }
   }
 }
@@ -323,7 +327,7 @@ static void server_start_transport_stream_op_batch(
     // the client never sends trailing metadata, because this is the
     // hook that tells us when the call is complete on the server side.
     if (op->recv_trailing_metadata) {
-      inject_on_complete_cb(&calld->base.deadline_state, op);
+      inject_recv_trailing_metadata_ready(&calld->base.deadline_state, op);
     }
   }
   // Chain to next filter.

src/core/ext/filters/deadline/deadline_filter.h

@@ -37,12 +37,12 @@ typedef struct grpc_deadline_state {
   grpc_deadline_timer_state timer_state;
   grpc_timer timer;
   grpc_closure timer_callback;
-  // Closure to invoke when the call is complete.
+  // Closure to invoke when we receive trailing metadata.
   // We use this to cancel the timer.
-  grpc_closure on_complete;
-  // The original on_complete closure, which we chain to after our own
-  // closure is invoked.
-  grpc_closure* next_on_complete;
+  grpc_closure recv_trailing_metadata_ready;
+  // The original recv_trailing_metadata_ready closure, which we chain to
+  // after our own closure is invoked.
+  grpc_closure* original_recv_trailing_metadata_ready;
 } grpc_deadline_state;
 
 //

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

@@ -55,8 +55,8 @@ struct call_data {
   grpc_closure recv_initial_metadata_ready;
   // State for handling recv_trailing_metadata ops.
   grpc_metadata_batch* recv_trailing_metadata;
-  grpc_closure* original_recv_trailing_metadata_on_complete;
-  grpc_closure recv_trailing_metadata_on_complete;
+  grpc_closure* original_recv_trailing_metadata_ready;
+  grpc_closure recv_trailing_metadata_ready;
   // State for handling send_message ops.
   grpc_transport_stream_op_batch* send_message_batch;
   size_t send_message_bytes_read;
@@ -153,8 +153,7 @@ static void recv_initial_metadata_ready(void* user_data, grpc_error* error) {
   GRPC_CLOSURE_RUN(calld->original_recv_initial_metadata_ready, error);
 }
 
-static void recv_trailing_metadata_on_complete(void* user_data,
-                                               grpc_error* error) {
+static void recv_trailing_metadata_ready(void* user_data, grpc_error* error) {
   grpc_call_element* elem = static_cast<grpc_call_element*>(user_data);
   call_data* calld = static_cast<call_data*>(elem->call_data);
   if (error == GRPC_ERROR_NONE) {
@@ -163,7 +162,7 @@ static void recv_trailing_metadata_on_complete(void* user_data,
   } else {
     GRPC_ERROR_REF(error);
   }
-  GRPC_CLOSURE_RUN(calld->original_recv_trailing_metadata_on_complete, error);
+  GRPC_CLOSURE_RUN(calld->original_recv_trailing_metadata_ready, error);
 }
 
 static void send_message_on_complete(void* arg, grpc_error* error) {
@@ -312,8 +311,10 @@ static void hc_start_transport_stream_op_batch(
     /* substitute our callback for the higher callback */
     calld->recv_trailing_metadata =
         batch->payload->recv_trailing_metadata.recv_trailing_metadata;
-    calld->original_recv_trailing_metadata_on_complete = batch->on_complete;
-    batch->on_complete = &calld->recv_trailing_metadata_on_complete;
+    calld->original_recv_trailing_metadata_ready =
+        batch->payload->recv_trailing_metadata.recv_trailing_metadata_ready;
+    batch->payload->recv_trailing_metadata.recv_trailing_metadata_ready =
+        &calld->recv_trailing_metadata_ready;
   }
 
   grpc_error* error = GRPC_ERROR_NONE;
@@ -420,8 +421,8 @@ static grpc_error* init_call_elem(grpc_call_element* elem,
   GRPC_CLOSURE_INIT(&calld->recv_initial_metadata_ready,
                     recv_initial_metadata_ready, elem,
                     grpc_schedule_on_exec_ctx);
-  GRPC_CLOSURE_INIT(&calld->recv_trailing_metadata_on_complete,
-                    recv_trailing_metadata_on_complete, elem,
+  GRPC_CLOSURE_INIT(&calld->recv_trailing_metadata_ready,
+                    recv_trailing_metadata_ready, elem,
                     grpc_schedule_on_exec_ctx);
   GRPC_CLOSURE_INIT(&calld->send_message_on_complete, send_message_on_complete,
                     elem, grpc_schedule_on_exec_ctx);

src/core/ext/transport/chttp2/transport/chttp2_transport.cc

@@ -1149,12 +1149,10 @@ static void maybe_start_some_streams(grpc_chttp2_transport* t) {
   }
 }
 
-/* Flag that this closure barrier wants stats to be updated before finishing */
-#define CLOSURE_BARRIER_STATS_BIT (1 << 0)
 /* Flag that this closure barrier may be covering a write in a pollset, and so
    we should not complete this closure until we can prove that the write got
    scheduled */
-#define CLOSURE_BARRIER_MAY_COVER_WRITE (1 << 1)
+#define CLOSURE_BARRIER_MAY_COVER_WRITE (1 << 0)
 /* First bit of the reference count, stored in the high order bits (with the low
    bits being used for flags defined above) */
 #define CLOSURE_BARRIER_FIRST_REF_BIT (1 << 16)
@@ -1206,10 +1204,6 @@ void grpc_chttp2_complete_closure_step(grpc_chttp2_transport* t,
         grpc_error_add_child(closure->error_data.error, error);
   }
   if (closure->next_data.scratch < CLOSURE_BARRIER_FIRST_REF_BIT) {
-    if (closure->next_data.scratch & CLOSURE_BARRIER_STATS_BIT) {
-      grpc_transport_move_stats(&s->stats, s->collecting_stats);
-      s->collecting_stats = nullptr;
-    }
     if ((t->write_state == GRPC_CHTTP2_WRITE_STATE_IDLE) ||
         !(closure->next_data.scratch & CLOSURE_BARRIER_MAY_COVER_WRITE)) {
       GRPC_CLOSURE_RUN(closure, closure->error_data.error);
@@ -1351,9 +1345,14 @@ static void perform_stream_op_locked(void* stream_op,
   }
 
   grpc_closure* on_complete = op->on_complete;
+  // TODO(roth): This is a hack needed because we use data inside of the
+  // closure itself to do the barrier calculation (i.e., to ensure that
+  // we don't schedule the closure until all ops in the batch have been
+  // completed).  This can go away once we move to a new C++ closure API
+  // that provides the ability to create a barrier closure.
   if (on_complete == nullptr) {
-    on_complete =
-        GRPC_CLOSURE_CREATE(do_nothing, nullptr, grpc_schedule_on_exec_ctx);
+    on_complete = GRPC_CLOSURE_INIT(&op->handler_private.closure, do_nothing,
+                                    nullptr, grpc_schedule_on_exec_ctx);
   }
 
   /* use final_data as a barrier until enqueue time; the inital counter is
@@ -1361,12 +1360,6 @@ static void perform_stream_op_locked(void* stream_op,
   on_complete->next_data.scratch = CLOSURE_BARRIER_FIRST_REF_BIT;
   on_complete->error_data.error = GRPC_ERROR_NONE;
 
-  if (op->collect_stats) {
-    GPR_ASSERT(s->collecting_stats == nullptr);
-    s->collecting_stats = op_payload->collect_stats.collect_stats;
-    on_complete->next_data.scratch |= CLOSURE_BARRIER_STATS_BIT;
-  }
-
   if (op->cancel_stream) {
     GRPC_STATS_INC_HTTP2_OP_CANCEL();
     grpc_chttp2_cancel_stream(t, s, op_payload->cancel_stream.cancel_error);
@@ -1600,8 +1593,11 @@ static void perform_stream_op_locked(void* stream_op,
 
   if (op->recv_trailing_metadata) {
     GRPC_STATS_INC_HTTP2_OP_RECV_TRAILING_METADATA();
+    GPR_ASSERT(s->collecting_stats == nullptr);
+    s->collecting_stats = op_payload->recv_trailing_metadata.collect_stats;
     GPR_ASSERT(s->recv_trailing_metadata_finished == nullptr);
-    s->recv_trailing_metadata_finished = add_closure_barrier(on_complete);
+    s->recv_trailing_metadata_finished =
+        op_payload->recv_trailing_metadata.recv_trailing_metadata_ready;
     s->recv_trailing_metadata =
         op_payload->recv_trailing_metadata.recv_trailing_metadata;
     s->final_metadata_requested = true;
@@ -1960,11 +1956,12 @@ void grpc_chttp2_maybe_complete_recv_trailing_metadata(grpc_chttp2_transport* t,
     }
     if (s->read_closed && s->frame_storage.length == 0 && !pending_data &&
         s->recv_trailing_metadata_finished != nullptr) {
+      grpc_transport_move_stats(&s->stats, s->collecting_stats);
+      s->collecting_stats = nullptr;
       grpc_chttp2_incoming_metadata_buffer_publish(&s->metadata_buffer[1],
                                                    s->recv_trailing_metadata);
-      grpc_chttp2_complete_closure_step(
-          t, s, &s->recv_trailing_metadata_finished, GRPC_ERROR_NONE,
-          "recv_trailing_metadata_finished");
+      null_then_run_closure(&s->recv_trailing_metadata_finished,
+                            GRPC_ERROR_NONE);
     }
   }
 }

src/core/ext/transport/cronet/transport/cronet_transport.cc

@@ -925,6 +925,10 @@ static bool op_can_be_run(grpc_transport_stream_op_batch* curr_op,
       result = false;
     }
     /* Check if every op that was asked for is done. */
+    /* TODO(muxi): We should not consider the recv ops here, since they
+     * have their own callbacks.  We should invoke a batch's on_complete
+     * as soon as all of the batch's send ops are complete, even if
+     * there are still recv ops pending. */
     else if (curr_op->send_initial_metadata &&
              !stream_state->state_callback_received[OP_SEND_INITIAL_METADATA]) {
       CRONET_LOG(GPR_DEBUG, "Because");
@@ -1280,12 +1284,20 @@ static enum e_op_result execute_stream_op(struct op_and_state* oas) {
              op_can_be_run(stream_op, s, &oas->state,
                            OP_RECV_TRAILING_METADATA)) {
     CRONET_LOG(GPR_DEBUG, "running: %p  OP_RECV_TRAILING_METADATA", oas);
-    if (oas->s->state.rs.trailing_metadata_valid) {
+    grpc_error* error = GRPC_ERROR_NONE;
+    if (stream_state->state_op_done[OP_CANCEL_ERROR]) {
+      error = GRPC_ERROR_REF(stream_state->cancel_error);
+    } else if (stream_state->state_op_done[OP_FAILED]) {
+      error = make_error_with_desc(GRPC_STATUS_UNAVAILABLE, "Unavailable.");
+    } else if (oas->s->state.rs.trailing_metadata_valid) {
       grpc_chttp2_incoming_metadata_buffer_publish(
           &oas->s->state.rs.trailing_metadata,
           stream_op->payload->recv_trailing_metadata.recv_trailing_metadata);
       stream_state->rs.trailing_metadata_valid = false;
     }
+    GRPC_CLOSURE_SCHED(
+        stream_op->payload->recv_trailing_metadata.recv_trailing_metadata_ready,
+        error);
     stream_state->state_op_done[OP_RECV_TRAILING_METADATA] = true;
     result = ACTION_TAKEN_NO_CALLBACK;
   } else if (stream_op->cancel_stream &&
@@ -1398,6 +1410,11 @@ static void perform_stream_op(grpc_transport* gt, grpc_stream* gs,
       GRPC_CLOSURE_SCHED(op->payload->recv_message.recv_message_ready,
                          GRPC_ERROR_CANCELLED);
     }
+    if (op->recv_trailing_metadata) {
+      GRPC_CLOSURE_SCHED(
+          op->payload->recv_trailing_metadata.recv_trailing_metadata_ready,
+          GRPC_ERROR_CANCELLED);
+    }
     GRPC_CLOSURE_SCHED(op->on_complete, GRPC_ERROR_CANCELLED);
     return;
   }

src/core/ext/transport/inproc/inproc_transport.cc

@@ -120,7 +120,6 @@ typedef struct inproc_stream {
   struct inproc_stream* stream_list_next;
 } inproc_stream;
 
-static grpc_closure do_nothing_closure;
 static bool cancel_stream_locked(inproc_stream* s, grpc_error* error);
 static void op_state_machine(void* arg, grpc_error* error);
 
@@ -373,6 +372,10 @@ static void complete_if_batch_end_locked(inproc_stream* s, grpc_error* error,
                                          const char* msg) {
   int is_sm = static_cast<int>(op == s->send_message_op);
   int is_stm = static_cast<int>(op == s->send_trailing_md_op);
+  // TODO(vjpai): We should not consider the recv ops here, since they
+  // have their own callbacks.  We should invoke a batch's on_complete
+  // as soon as all of the batch's send ops are complete, even if there
+  // are still recv ops pending.
   int is_rim = static_cast<int>(op == s->recv_initial_md_op);
   int is_rm = static_cast<int>(op == s->recv_message_op);
   int is_rtm = static_cast<int>(op == s->recv_trailing_md_op);
@@ -496,6 +499,11 @@ static void fail_helper_locked(inproc_stream* s, grpc_error* error) {
     s->send_trailing_md_op = nullptr;
   }
   if (s->recv_trailing_md_op) {
+    INPROC_LOG(GPR_INFO, "fail_helper %p scheduling trailing-metadata-ready %p",
+               s, error);
+    GRPC_CLOSURE_SCHED(s->recv_trailing_md_op->payload->recv_trailing_metadata
+                           .recv_trailing_metadata_ready,
+                       GRPC_ERROR_REF(error));
     INPROC_LOG(GPR_INFO, "fail_helper %p scheduling trailing-md-on-complete %p",
                s, error);
     complete_if_batch_end_locked(
@@ -638,6 +646,12 @@ static void op_state_machine(void* arg, grpc_error* error) {
       }
       s->trailing_md_sent = true;
       if (!s->t->is_client && s->trailing_md_recvd && s->recv_trailing_md_op) {
+        INPROC_LOG(GPR_INFO,
+                   "op_state_machine %p scheduling trailing-metadata-ready", s);
+        GRPC_CLOSURE_SCHED(
+            s->recv_trailing_md_op->payload->recv_trailing_metadata
+                .recv_trailing_metadata_ready,
+            GRPC_ERROR_NONE);
         INPROC_LOG(GPR_INFO,
                    "op_state_machine %p scheduling trailing-md-on-complete", s);
         GRPC_CLOSURE_SCHED(s->recv_trailing_md_op->on_complete,
@@ -711,6 +725,12 @@ static void op_state_machine(void* arg, grpc_error* error) {
   }
   if (s->recv_trailing_md_op && s->t->is_client && other &&
       other->send_message_op) {
+    INPROC_LOG(GPR_INFO,
+               "op_state_machine %p scheduling trailing-metadata-ready %p", s,
+               GRPC_ERROR_NONE);
+    GRPC_CLOSURE_SCHED(s->recv_trailing_md_op->payload->recv_trailing_metadata
+                           .recv_trailing_metadata_ready,
+                       GRPC_ERROR_NONE);
     maybe_schedule_op_closure_locked(other, GRPC_ERROR_NONE);
   }
   if (s->to_read_trailing_md_filled) {
@@ -766,6 +786,10 @@ static void op_state_machine(void* arg, grpc_error* error) {
         INPROC_LOG(GPR_INFO,
                    "op_state_machine %p scheduling trailing-md-on-complete %p",
                    s, new_err);
+        GRPC_CLOSURE_SCHED(
+            s->recv_trailing_md_op->payload->recv_trailing_metadata
+                .recv_trailing_metadata_ready,
+            GRPC_ERROR_REF(new_err));
         GRPC_CLOSURE_SCHED(s->recv_trailing_md_op->on_complete,
                            GRPC_ERROR_REF(new_err));
         s->recv_trailing_md_op = nullptr;
@@ -859,6 +883,9 @@ static bool cancel_stream_locked(inproc_stream* s, grpc_error* error) {
     // couldn't complete that because we hadn't yet sent out trailing
     // md, now's the chance
     if (!s->t->is_client && s->trailing_md_recvd && s->recv_trailing_md_op) {
+      GRPC_CLOSURE_SCHED(s->recv_trailing_md_op->payload->recv_trailing_metadata
+                             .recv_trailing_metadata_ready,
+                         GRPC_ERROR_REF(s->cancel_self_error));
       complete_if_batch_end_locked(
           s, s->cancel_self_error, s->recv_trailing_md_op,
           "cancel_stream scheduling trailing-md-on-complete");
@@ -873,6 +900,8 @@ static bool cancel_stream_locked(inproc_stream* s, grpc_error* error) {
   return ret;
 }
 
+static void do_nothing(void* arg, grpc_error* error) {}
+
 static void perform_stream_op(grpc_transport* gt, grpc_stream* gs,
                               grpc_transport_stream_op_batch* op) {
   INPROC_LOG(GPR_INFO, "perform_stream_op %p %p %p", gt, gs, op);
@@ -892,8 +921,14 @@ static void perform_stream_op(grpc_transport* gt, grpc_stream* gs,
   }
   grpc_error* error = GRPC_ERROR_NONE;
   grpc_closure* on_complete = op->on_complete;
+  // TODO(roth): This is a hack needed because we use data inside of the
+  // closure itself to do the barrier calculation (i.e., to ensure that
+  // we don't schedule the closure until all ops in the batch have been
+  // completed).  This can go away once we move to a new C++ closure API
+  // that provides the ability to create a barrier closure.
   if (on_complete == nullptr) {
-    on_complete = &do_nothing_closure;
+    on_complete = GRPC_CLOSURE_INIT(&op->handler_private.closure, do_nothing,
+                                    nullptr, grpc_schedule_on_exec_ctx);
   }
 
   if (op->cancel_stream) {
@@ -1026,6 +1061,15 @@ static void perform_stream_op(grpc_transport* gt, grpc_stream* gs,
         GRPC_CLOSURE_SCHED(op->payload->recv_message.recv_message_ready,
                            GRPC_ERROR_REF(error));
       }
+      if (op->recv_trailing_metadata) {
+        INPROC_LOG(
+            GPR_INFO,
+            "perform_stream_op error %p scheduling trailing-metadata-ready %p",
+            s, error);
+        GRPC_CLOSURE_SCHED(
+            op->payload->recv_trailing_metadata.recv_trailing_metadata_ready,
+            GRPC_ERROR_REF(error));
+      }
     }
     INPROC_LOG(GPR_INFO, "perform_stream_op %p scheduling on_complete %p", s,
                error);
@@ -1129,12 +1173,8 @@ static grpc_endpoint* get_endpoint(grpc_transport* t) { return nullptr; }
 /*******************************************************************************
  * GLOBAL INIT AND DESTROY
  */
-static void do_nothing(void* arg, grpc_error* error) {}
-
 void grpc_inproc_transport_init(void) {
   grpc_core::ExecCtx exec_ctx;
-  GRPC_CLOSURE_INIT(&do_nothing_closure, do_nothing, nullptr,
-                    grpc_schedule_on_exec_ctx);
   g_empty_slice = grpc_slice_from_static_buffer(nullptr, 0);
 
   grpc_slice key_tmp = grpc_slice_from_static_string(":path");

src/core/lib/channel/connected_channel.cc

@@ -51,6 +51,7 @@ typedef struct connected_channel_call_data {
   callback_state on_complete[6];  // Max number of pending batches.
   callback_state recv_initial_metadata_ready;
   callback_state recv_message_ready;
+  callback_state recv_trailing_metadata_ready;
 } call_data;
 
 static void run_in_call_combiner(void* arg, grpc_error* error) {
@@ -111,6 +112,12 @@ static void con_start_transport_stream_op_batch(
     intercept_callback(calld, state, false, "recv_message_ready",
                        &batch->payload->recv_message.recv_message_ready);
   }
+  if (batch->recv_trailing_metadata) {
+    callback_state* state = &calld->recv_trailing_metadata_ready;
+    intercept_callback(
+        calld, state, false, "recv_trailing_metadata_ready",
+        &batch->payload->recv_trailing_metadata.recv_trailing_metadata_ready);
+  }
   if (batch->cancel_stream) {
     // There can be more than one cancellation batch in flight at any
     // given time, so we can't just pick out a fixed index into
@@ -121,7 +128,7 @@ static void con_start_transport_stream_op_batch(
         static_cast<callback_state*>(gpr_malloc(sizeof(*state)));
     intercept_callback(calld, state, true, "on_complete (cancel_stream)",
                        &batch->on_complete);
-  } else {
+  } else if (batch->on_complete != nullptr) {
     callback_state* state = get_state_for_batch(calld, batch);
     intercept_callback(calld, state, false, "on_complete", &batch->on_complete);
   }

src/core/lib/iomgr/call_combiner.h

@@ -26,6 +26,7 @@
 #include <grpc/support/atm.h>
 
 #include "src/core/lib/gpr/mpscq.h"
+#include "src/core/lib/gprpp/inlined_vector.h"
 #include "src/core/lib/iomgr/closure.h"
 
 // A simple, lock-free mechanism for serializing activity related to a
@@ -109,4 +110,83 @@ void grpc_call_combiner_set_notify_on_cancel(grpc_call_combiner* call_combiner,
 void grpc_call_combiner_cancel(grpc_call_combiner* call_combiner,
                                grpc_error* error);
 
+namespace grpc_core {
+
+// Helper for running a list of closures in a call combiner.
+//
+// Each callback running in the call combiner will eventually be
+// returned to the surface, at which point the surface will yield the
+// call combiner.  So when we are running in the call combiner and have
+// more than one callback to return to the surface, we need to re-enter
+// the call combiner for all but one of those callbacks.
+class CallCombinerClosureList {
+ public:
+  CallCombinerClosureList() {}
+
+  // Adds a closure to the list.  The closure must eventually result in
+  // the call combiner being yielded.
+  void Add(grpc_closure* closure, grpc_error* error, const char* reason) {
+    closures_.emplace_back(closure, error, reason);
+  }
+
+  // Runs all closures in the call combiner and yields the call combiner.
+  //
+  // All but one of the closures in the list will be scheduled via
+  // GRPC_CALL_COMBINER_START(), and the remaining closure will be
+  // scheduled via GRPC_CLOSURE_SCHED(), which will eventually result in
+  // yielding the call combiner.  If the list is empty, then the call
+  // combiner will be yielded immediately.
+  void RunClosures(grpc_call_combiner* call_combiner) {
+    for (size_t i = 1; i < closures_.size(); ++i) {
+      auto& closure = closures_[i];
+      GRPC_CALL_COMBINER_START(call_combiner, closure.closure, closure.error,
+                               closure.reason);
+    }
+    if (closures_.size() > 0) {
+      if (grpc_call_combiner_trace.enabled()) {
+        gpr_log(GPR_INFO,
+                "CallCombinerClosureList executing closure while already "
+                "holding call_combiner %p: closure=%p error=%s reason=%s",
+                call_combiner, closures_[0].closure,
+                grpc_error_string(closures_[0].error), closures_[0].reason);
+      }
+      // This will release the call combiner.
+      GRPC_CLOSURE_SCHED(closures_[0].closure, closures_[0].error);
+    } else {
+      GRPC_CALL_COMBINER_STOP(call_combiner, "no closures to schedule");
+    }
+    closures_.clear();
+  }
+
+  // Runs all closures in the call combiner, but does NOT yield the call
+  // combiner.  All closures will be scheduled via GRPC_CALL_COMBINER_START().
+  void RunClosuresWithoutYielding(grpc_call_combiner* call_combiner) {
+    for (size_t i = 0; i < closures_.size(); ++i) {
+      auto& closure = closures_[i];
+      GRPC_CALL_COMBINER_START(call_combiner, closure.closure, closure.error,
+                               closure.reason);
+    }
+    closures_.clear();
+  }
+
+  size_t size() const { return closures_.size(); }
+
+ private:
+  struct CallCombinerClosure {
+    grpc_closure* closure;
+    grpc_error* error;
+    const char* reason;
+
+    CallCombinerClosure(grpc_closure* closure, grpc_error* error,
+                        const char* reason)
+        : closure(closure), error(error), reason(reason) {}
+  };
+
+  // There are generally a maximum of 6 closures to run in the call
+  // combiner, one for each pending op.
+  InlinedVector<CallCombinerClosure, 6> closures_;
+};
+
+}  // namespace grpc_core
+
 #endif /* GRPC_CORE_LIB_IOMGR_CALL_COMBINER_H */

src/core/lib/iomgr/closure.h

@@ -283,9 +283,10 @@ inline void grpc_closure_sched(grpc_closure* c, grpc_error* error) {
     if (c->scheduled) {
       gpr_log(GPR_ERROR,
               "Closure already scheduled. (closure: %p, created: [%s:%d], "
-              "previously scheduled at: [%s: %d] run?: %s",
+              "previously scheduled at: [%s: %d], newly scheduled at [%s: %d], "
+              "run?: %s",
               c, c->file_created, c->line_created, c->file_initiated,
-              c->line_initiated, c->run ? "true" : "false");
+              c->line_initiated, file, line, c->run ? "true" : "false");
       abort();
     }
     c->scheduled = true;

src/core/lib/surface/call.cc

@@ -233,6 +233,7 @@ struct grpc_call {
   grpc_closure receiving_slice_ready;
   grpc_closure receiving_stream_ready;
   grpc_closure receiving_initial_metadata_ready;
+  grpc_closure receiving_trailing_metadata_ready;
   uint32_t test_only_last_message_flags;
 
   grpc_closure release_call;
@@ -1209,7 +1210,6 @@ static void post_batch_completion(batch_control* bctl) {
 
   if (bctl->op.send_initial_metadata) {
     grpc_metadata_batch_destroy(
-
         &call->metadata_batch[0 /* is_receiving */][0 /* is_trailing */]);
   }
   if (bctl->op.send_message) {
@@ -1217,14 +1217,9 @@ static void post_batch_completion(batch_control* bctl) {
   }
   if (bctl->op.send_trailing_metadata) {
     grpc_metadata_batch_destroy(
-
         &call->metadata_batch[0 /* is_receiving */][1 /* is_trailing */]);
   }
   if (bctl->op.recv_trailing_metadata) {
-    grpc_metadata_batch* md =
-        &call->metadata_batch[1 /* is_receiving */][1 /* is_trailing */];
-    recv_trailing_filter(call, md);
-
     /* propagate cancellation to any interested children */
     gpr_atm_rel_store(&call->received_final_op_atm, 1);
     parent_call* pc = get_parent_call(call);
@@ -1246,7 +1241,6 @@ static void post_batch_completion(batch_control* bctl) {
       }
       gpr_mu_unlock(&pc->child_list_mu);
     }
-
     if (call->is_client) {
       get_final_status(call, set_status_value_directly,
                        call->final_op.client.status,
@@ -1256,7 +1250,6 @@ static void post_batch_completion(batch_control* bctl) {
       get_final_status(call, set_cancelled_value,
                        call->final_op.server.cancelled, nullptr, nullptr);
     }
-
     GRPC_ERROR_UNREF(error);
     error = GRPC_ERROR_NONE;
   }
@@ -1538,6 +1531,19 @@ static void receiving_initial_metadata_ready(void* bctlp, grpc_error* error) {
   finish_batch_step(bctl);
 }
 
+static void receiving_trailing_metadata_ready(void* bctlp, grpc_error* error) {
+  batch_control* bctl = static_cast<batch_control*>(bctlp);
+  grpc_call* call = bctl->call;
+  GRPC_CALL_COMBINER_STOP(&call->call_combiner, "recv_trailing_metadata_ready");
+  add_batch_error(bctl, GRPC_ERROR_REF(error), false);
+  if (error == GRPC_ERROR_NONE) {
+    grpc_metadata_batch* md =
+        &call->metadata_batch[1 /* is_receiving */][1 /* is_trailing */];
+    recv_trailing_filter(call, md);
+  }
+  finish_batch_step(bctl);
+}
+
 static void finish_batch(void* bctlp, grpc_error* error) {
   batch_control* bctl = static_cast<batch_control*>(bctlp);
   grpc_call* call = bctl->call;
@@ -1558,7 +1564,8 @@ static grpc_call_error call_start_batch(grpc_call* call, const grpc_op* ops,
   size_t i;
   const grpc_op* op;
   batch_control* bctl;
-  int num_completion_callbacks_needed = 1;
+  bool has_send_ops = false;
+  int num_recv_ops = 0;
   grpc_call_error error = GRPC_CALL_OK;
   grpc_transport_stream_op_batch* stream_op;
   grpc_transport_stream_op_batch_payload* stream_op_payload;
@@ -1664,6 +1671,7 @@ static grpc_call_error call_start_batch(grpc_call* call, const grpc_op* ops,
           stream_op_payload->send_initial_metadata.peer_string =
               &call->peer_string;
         }
+        has_send_ops = true;
         break;
       }
       case GRPC_OP_SEND_MESSAGE: {
@@ -1693,6 +1701,7 @@ static grpc_call_error call_start_batch(grpc_call* call, const grpc_op* ops,
             &op->data.send_message.send_message->data.raw.slice_buffer, flags);
         stream_op_payload->send_message.send_message.reset(
             call->sending_stream.get());
+        has_send_ops = true;
         break;
       }
       case GRPC_OP_SEND_CLOSE_FROM_CLIENT: {
@@ -1713,6 +1722,7 @@ static grpc_call_error call_start_batch(grpc_call* call, const grpc_op* ops,
         call->sent_final_op = true;
         stream_op_payload->send_trailing_metadata.send_trailing_metadata =
             &call->metadata_batch[0 /* is_receiving */][1 /* is_trailing */];
+        has_send_ops = true;
         break;
       }
       case GRPC_OP_SEND_STATUS_FROM_SERVER: {
@@ -1777,6 +1787,7 @@ static grpc_call_error call_start_batch(grpc_call* call, const grpc_op* ops,
         }
         stream_op_payload->send_trailing_metadata.send_trailing_metadata =
             &call->metadata_batch[0 /* is_receiving */][1 /* is_trailing */];
+        has_send_ops = true;
         break;
       }
       case GRPC_OP_RECV_INITIAL_METADATA: {
@@ -1804,7 +1815,7 @@ static grpc_call_error call_start_batch(grpc_call* call, const grpc_op* ops,
           stream_op_payload->recv_initial_metadata.peer_string =
               &call->peer_string;
         }
-        num_completion_callbacks_needed++;
+        ++num_recv_ops;
         break;
       }
       case GRPC_OP_RECV_MESSAGE: {
@@ -1826,7 +1837,7 @@ static grpc_call_error call_start_batch(grpc_call* call, const grpc_op* ops,
                           grpc_schedule_on_exec_ctx);
         stream_op_payload->recv_message.recv_message_ready =
             &call->receiving_stream_ready;
-        num_completion_callbacks_needed++;
+        ++num_recv_ops;
         break;
       }
       case GRPC_OP_RECV_STATUS_ON_CLIENT: {
@@ -1852,11 +1863,16 @@ static grpc_call_error call_start_batch(grpc_call* call, const grpc_op* ops,
         call->final_op.client.error_string =
             op->data.recv_status_on_client.error_string;
         stream_op->recv_trailing_metadata = true;
-        stream_op->collect_stats = true;
         stream_op_payload->recv_trailing_metadata.recv_trailing_metadata =
             &call->metadata_batch[1 /* is_receiving */][1 /* is_trailing */];
-        stream_op_payload->collect_stats.collect_stats =
+        stream_op_payload->recv_trailing_metadata.collect_stats =
             &call->final_info.stats.transport_stream_stats;
+        GRPC_CLOSURE_INIT(&call->receiving_trailing_metadata_ready,
+                          receiving_trailing_metadata_ready, bctl,
+                          grpc_schedule_on_exec_ctx);
+        stream_op_payload->recv_trailing_metadata.recv_trailing_metadata_ready =
+            &call->receiving_trailing_metadata_ready;
+        ++num_recv_ops;
         break;
       }
       case GRPC_OP_RECV_CLOSE_ON_SERVER: {
@@ -1877,11 +1893,16 @@ static grpc_call_error call_start_batch(grpc_call* call, const grpc_op* ops,
         call->final_op.server.cancelled =
             op->data.recv_close_on_server.cancelled;
         stream_op->recv_trailing_metadata = true;
-        stream_op->collect_stats = true;
         stream_op_payload->recv_trailing_metadata.recv_trailing_metadata =
             &call->metadata_batch[1 /* is_receiving */][1 /* is_trailing */];
-        stream_op_payload->collect_stats.collect_stats =
+        stream_op_payload->recv_trailing_metadata.collect_stats =
             &call->final_info.stats.transport_stream_stats;
+        GRPC_CLOSURE_INIT(&call->receiving_trailing_metadata_ready,
+                          receiving_trailing_metadata_ready, bctl,
+                          grpc_schedule_on_exec_ctx);
+        stream_op_payload->recv_trailing_metadata.recv_trailing_metadata_ready =
+            &call->receiving_trailing_metadata_ready;
+        ++num_recv_ops;
         break;
       }
     }
@@ -1891,13 +1912,15 @@ static grpc_call_error call_start_batch(grpc_call* call, const grpc_op* ops,
   if (!is_notify_tag_closure) {
     GPR_ASSERT(grpc_cq_begin_op(call->cq, notify_tag));
   }
-  gpr_ref_init(&bctl->steps_to_complete, num_completion_callbacks_needed);
+  gpr_ref_init(&bctl->steps_to_complete, (has_send_ops ? 1 : 0) + num_recv_ops);
 
+  if (has_send_ops) {
     GRPC_CLOSURE_INIT(&bctl->finish_batch, finish_batch, bctl,
                       grpc_schedule_on_exec_ctx);
     stream_op->on_complete = &bctl->finish_batch;
-  gpr_atm_rel_store(&call->any_ops_sent_atm, 1);
+  }
 
+  gpr_atm_rel_store(&call->any_ops_sent_atm, 1);
   execute_batch(call, stream_op, &bctl->start_batch);
 
 done:

src/core/lib/transport/transport.cc

@@ -212,21 +212,32 @@ void grpc_transport_stream_op_batch_finish_with_failure(
   if (batch->send_message) {
     batch->payload->send_message.send_message.reset();
   }
-  if (batch->recv_message) {
-    GRPC_CALL_COMBINER_START(
-        call_combiner, batch->payload->recv_message.recv_message_ready,
-        GRPC_ERROR_REF(error), "failing recv_message_ready");
+  if (batch->cancel_stream) {
+    GRPC_ERROR_UNREF(batch->payload->cancel_stream.cancel_error);
   }
+  // Construct a list of closures to execute.
+  grpc_core::CallCombinerClosureList closures;
   if (batch->recv_initial_metadata) {
-    GRPC_CALL_COMBINER_START(
-        call_combiner,
+    closures.Add(
         batch->payload->recv_initial_metadata.recv_initial_metadata_ready,
         GRPC_ERROR_REF(error), "failing recv_initial_metadata_ready");
   }
-  GRPC_CLOSURE_SCHED(batch->on_complete, error);
-  if (batch->cancel_stream) {
-    GRPC_ERROR_UNREF(batch->payload->cancel_stream.cancel_error);
+  if (batch->recv_message) {
+    closures.Add(batch->payload->recv_message.recv_message_ready,
+                 GRPC_ERROR_REF(error), "failing recv_message_ready");
+  }
+  if (batch->recv_trailing_metadata) {
+    closures.Add(
+        batch->payload->recv_trailing_metadata.recv_trailing_metadata_ready,
+        GRPC_ERROR_REF(error), "failing recv_trailing_metadata_ready");
+  }
+  if (batch->on_complete != nullptr) {
+    closures.Add(batch->on_complete, GRPC_ERROR_REF(error),
+                 "failing on_complete");
   }
+  // Execute closures.
+  closures.RunClosures(call_combiner);
+  GRPC_ERROR_UNREF(error);
 }
 
 typedef struct {

src/core/lib/transport/transport.h

@@ -122,9 +122,15 @@ typedef struct grpc_transport_stream_op_batch_payload
 /* Transport stream op: a set of operations to perform on a transport
    against a single stream */
 typedef struct grpc_transport_stream_op_batch {
-  /** Should be enqueued when all requested operations (excluding recv_message
-      and recv_initial_metadata which have their own closures) in a given batch
-      have been completed. */
+  /** Should be scheduled when all of the non-recv operations in the batch
+      are complete.
+
+      The recv ops (recv_initial_metadata, recv_message, and
+      recv_trailing_metadata) each have their own callbacks.  If a batch
+      contains both recv ops and non-recv ops, on_complete should be
+      scheduled as soon as the non-recv ops are complete, regardless of
+      whether or not the recv ops are complete.  If a batch contains
+      only recv ops, on_complete can be null. */
   grpc_closure* on_complete;
 
   /** Values for the stream op (fields set are determined by flags above) */
@@ -149,9 +155,6 @@ typedef struct grpc_transport_stream_op_batch {
    */
   bool recv_trailing_metadata : 1;
 
-  /** Collect any stats into provided buffer, zero internal stat counters */
-  bool collect_stats : 1;
-
   /** Cancel this stream with the provided error */
   bool cancel_stream : 1;
 
@@ -219,11 +222,10 @@ struct grpc_transport_stream_op_batch_payload {
 
   struct {
     grpc_metadata_batch* recv_trailing_metadata;
-  } recv_trailing_metadata;
-
-  struct {
     grpc_transport_stream_stats* collect_stats;
-  } collect_stats;
+    /** Should be enqueued when initial metadata is ready to be processed. */
+    grpc_closure* recv_trailing_metadata_ready;
+  } recv_trailing_metadata;
 
   /** Forcefully close this stream.
       The HTTP2 semantics should be:

src/core/lib/transport/transport_op_string.cc

@@ -120,13 +120,6 @@ char* grpc_transport_stream_op_batch_string(
     gpr_strvec_add(&b, tmp);
   }
 
-  if (op->collect_stats) {
-    gpr_strvec_add(&b, gpr_strdup(" "));
-    gpr_asprintf(&tmp, "COLLECT_STATS:%p",
-                 op->payload->collect_stats.collect_stats);
-    gpr_strvec_add(&b, tmp);
-  }
-
   out = gpr_strvec_flatten(&b, nullptr);
   gpr_strvec_destroy(&b);
 

test/cpp/microbenchmarks/bm_call_create.cc

@@ -621,18 +621,26 @@ typedef struct {
 static void StartTransportStreamOp(grpc_call_element* elem,
                                    grpc_transport_stream_op_batch* op) {
   call_data* calld = static_cast<call_data*>(elem->call_data);
+  // Construct list of closures to return.
+  grpc_core::CallCombinerClosureList closures;
   if (op->recv_initial_metadata) {
-    GRPC_CALL_COMBINER_START(
-        calld->call_combiner,
-        op->payload->recv_initial_metadata.recv_initial_metadata_ready,
+    closures.Add(op->payload->recv_initial_metadata.recv_initial_metadata_ready,
                  GRPC_ERROR_NONE, "recv_initial_metadata");
   }
   if (op->recv_message) {
-    GRPC_CALL_COMBINER_START(calld->call_combiner,
-                             op->payload->recv_message.recv_message_ready,
-                             GRPC_ERROR_NONE, "recv_message");
+    closures.Add(op->payload->recv_message.recv_message_ready, GRPC_ERROR_NONE,
+                 "recv_message");
   }
-  GRPC_CLOSURE_SCHED(op->on_complete, GRPC_ERROR_NONE);
+  if (op->recv_trailing_metadata) {
+    closures.Add(
+        op->payload->recv_trailing_metadata.recv_trailing_metadata_ready,
+        GRPC_ERROR_NONE, "recv_trailing_metadata");
+  }
+  if (op->on_complete != nullptr) {
+    closures.Add(op->on_complete, GRPC_ERROR_NONE, "on_complete");
+  }
+  // Execute closures.
+  closures.RunClosures(calld->call_combiner);
 }
 
 static void StartTransportOp(grpc_channel_element* elem,