Allow API fuzzer to send multiple slices (#27993)

* Allow API fuzzer to send multiple slices

* fixes

src/core/lib/transport/byte_stream.cc

@@ -40,6 +40,10 @@ SliceBufferByteStream::SliceBufferByteStream(grpc_slice_buffer* slice_buffer,
   GPR_ASSERT(slice_buffer->length <= UINT32_MAX);
   grpc_slice_buffer_init(&backing_buffer_);
   grpc_slice_buffer_swap(slice_buffer, &backing_buffer_);
+  if (backing_buffer_.count == 0) {
+    grpc_slice_buffer_add_indexed(&backing_buffer_, grpc_empty_slice());
+    GPR_ASSERT(backing_buffer_.count > 0);
+  }
 }
 
 SliceBufferByteStream::~SliceBufferByteStream() {}

test/core/end2end/fuzzers/api_fuzzer.cc

@@ -379,9 +379,12 @@ class Call : public std::enable_shared_from_this<Call> {
           *batch_is_ok = false;
         } else {
           *batch_ops |= 1 << GRPC_OP_SEND_MESSAGE;
-          auto send = ReadSlice(batch_op.send_message().message());
+          std::vector<grpc_slice> slices;
+          for (const auto& m : batch_op.send_message().message()) {
+            slices.push_back(ReadSlice(m));
+          }
           send_message_ = op.data.send_message.send_message =
-              grpc_raw_byte_buffer_create(&send, 1);
+              grpc_raw_byte_buffer_create(slices.data(), slices.size());
           unwinders->push_back([this]() {
             grpc_byte_buffer_destroy(send_message_);
             send_message_ = nullptr;

test/core/end2end/fuzzers/api_fuzzer.proto

@@ -110,7 +110,7 @@ message SendInitialMetadata {
 }
 
 message SendMessage {
-    ByteSlice message = 1;
+    repeated ByteSlice message = 1;
 }
 
 message SendCloseFromClient {