Add SeekEnd to MemoryFixSizeBuffer. (#109)

* Don't assert buffer size.

test/cpp/CMakeLists.txt

@@ -2,7 +2,10 @@ find_package(GTest REQUIRED)
 
 add_executable(
   unit_tests
-  allreduce_base_test.cc allreduce_robust_test.cc allreduce_mock_test.cc
+  test_io.cc
+  allreduce_robust_test.cc
+  allreduce_base_test.cc
+  allreduce_mock_test.cc
   test_main.cpp)
 
 target_link_libraries(

test/cpp/test_io.cc

@@ -0,0 +1,18 @@
+/*!
+ *  Copyright (c) 2019 by Contributors
+ */
+#include <gtest/gtest.h>
+#include <rabit/internal/io.h>
+
+#include <vector>
+
+namespace rabit {
+TEST(MemoryFixSizeBuffer, Seek) {
+  size_t constexpr kSize { 64 };
+  std::vector<int32_t> memory( kSize );
+  utils::MemoryFixSizeBuffer buf(memory.data(), memory.size());
+  buf.Seek(utils::MemoryFixSizeBuffer::SeekEnd);
+  size_t end = buf.Tell();
+  ASSERT_EQ(end, kSize);
+}
+}  // namespace rabit