Fix race with fetching data and writing it in chttp2

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

@@ -836,8 +836,8 @@ static void continue_fetching_send_locked(grpc_exec_ctx *exec_ctx,
     return;
   }
   if (s->fetched_send_message_length == s->fetching_send_message->length) {
-    ssize_t notify_offset = s->fetching_slice_end_offset;
-    if (notify_offset <= 0) {
+    int64_t notify_offset = s->next_message_end_offset;
+    if (notify_offset <= s->flow_controlled_bytes_written) {
       grpc_chttp2_complete_closure_step(
           exec_ctx, t, s, &s->fetching_send_message_finished, GRPC_ERROR_NONE,
           "fetching_send_message_finished");
@@ -848,7 +848,7 @@ static void continue_fetching_send_locked(grpc_exec_ctx *exec_ctx,
       } else {
         t->write_cb_pool = cb->next;
       }
-      cb->call_at_byte = (size_t)notify_offset;
+      cb->call_at_byte = notify_offset;
       cb->closure = s->fetching_send_message_finished;
       s->fetching_send_message_finished = NULL;
       cb->next = s->on_write_finished_cbs;
@@ -1005,13 +1005,14 @@ static void perform_stream_op_locked(grpc_exec_ctx *exec_ctx, void *stream_op,
       frame_hdr[4] = (uint8_t)(len);
       s->fetching_send_message = op->send_message;
       s->fetched_send_message_length = 0;
-      s->fetching_slice_end_offset =
-          (ssize_t)s->flow_controlled_buffer.length + (ssize_t)len;
+      s->next_message_end_offset = s->flow_controlled_bytes_written +
+                                   (int64_t)s->flow_controlled_buffer.length +
+                                   (int64_t)len;
       s->complete_fetch_covered_by_poller = op->covered_by_poller;
       if (flags & GRPC_WRITE_BUFFER_HINT) {
         /* allow up to 64kb to be buffered */
         /* TODO(ctiller): make this configurable */
-        s->fetching_slice_end_offset -= 65536;
+        s->next_message_end_offset -= 65536;
       }
       continue_fetching_send_locked(exec_ctx, t, s);
       if (s->id != 0) {

src/core/ext/transport/chttp2/transport/internal.h

@@ -147,7 +147,7 @@ typedef struct grpc_chttp2_outstanding_ping {
 } grpc_chttp2_outstanding_ping;
 
 typedef struct grpc_chttp2_write_cb {
-  size_t call_at_byte;
+  int64_t call_at_byte;
   grpc_closure *closure;
   struct grpc_chttp2_write_cb *next;
 } grpc_chttp2_write_cb;
@@ -353,7 +353,8 @@ struct grpc_chttp2_stream {
   grpc_byte_stream *fetching_send_message;
   uint32_t fetched_send_message_length;
   gpr_slice fetching_slice;
-  int64_t fetching_slice_end_offset;
+  int64_t next_message_end_offset;
+  int64_t flow_controlled_bytes_written;
   bool complete_fetch_covered_by_poller;
   grpc_closure complete_fetch;
   grpc_closure complete_fetch_locked;

src/core/ext/transport/chttp2/transport/writing.c

@@ -56,16 +56,16 @@ static void finish_write_cb(grpc_exec_ctx *exec_ctx, grpc_chttp2_transport *t,
 }
 
 static void update_list(grpc_exec_ctx *exec_ctx, grpc_chttp2_transport *t,
-                        grpc_chttp2_stream *s, size_t send_bytes,
+                        grpc_chttp2_stream *s, int64_t send_bytes,
                         grpc_chttp2_write_cb **list, grpc_error *error) {
   grpc_chttp2_write_cb *cb = *list;
   *list = NULL;
+  s->flow_controlled_bytes_written += send_bytes;
   while (cb) {
     grpc_chttp2_write_cb *next = cb->next;
-    if (cb->call_at_byte <= send_bytes) {
+    if (cb->call_at_byte <= s->flow_controlled_bytes_written) {
       finish_write_cb(exec_ctx, t, s, cb, GRPC_ERROR_REF(error));
     } else {
-      cb->call_at_byte -= send_bytes;
       add_to_write_list(list, cb);
     }
     cb = next;
@@ -236,8 +236,8 @@ void grpc_chttp2_end_write(grpc_exec_ctx *exec_ctx, grpc_chttp2_transport *t,
           GRPC_ERROR_REF(error), "send_initial_metadata_finished");
     }
     if (s->sending_bytes != 0) {
-      update_list(exec_ctx, t, s, s->sending_bytes, &s->on_write_finished_cbs,
-                  GRPC_ERROR_REF(error));
+      update_list(exec_ctx, t, s, (int64_t)s->sending_bytes,
+                  &s->on_write_finished_cbs, GRPC_ERROR_REF(error));
       s->sending_bytes = 0;
     }
     if (s->sent_trailing_metadata) {