Add test case for xsgetn in basic_filebuf (#167937)

This is the promised follow-up to #167779. It simply adds a test case
provided by philnik777

Author: Sterling-Augustine

libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/xsgetn.pass.cpp

@@ -0,0 +1,73 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// FILE_DEPENDENCIES: xsgetn.test.dat
+
+// <fstream>
+
+// template <class charT, class traits = char_traits<charT> >
+// class basic_ifstream
+
+// streamsize xsgetn(char_type*, streamsize) override;
+
+// This isn't a required override by the standard, but most implementations
+// override it, since it allows for significantly improved performance in some
+// cases. All of this code is required to work, so this isn't a libc++ extension
+
+#include <cassert>
+#include <fstream>
+#include <vector>
+
+#include "test_macros.h"
+
+int main(int, char**) {
+  std::vector<char> stream_buffer(10);
+  std::ifstream fs("xsgetn.test.dat");
+  std::filebuf* fb = fs.rdbuf();
+  fb->pubsetbuf(stream_buffer.data(), 10);
+
+  // Ensure that the buffer is set up
+  assert(fb->sgetc() == 't');
+
+  std::vector<char> test_buffer(5);
+  test_buffer[0] = '\0';
+
+  { // Check that a read smaller than the buffer works fine
+    assert(fb->sgetn(test_buffer.data(), 5) == 5);
+    assert(std::string(test_buffer.data(), 5) == "this ");
+  }
+  { // Check that reading up to the buffer end works fine
+    assert(fb->sgetn(test_buffer.data(), 5) == 5);
+    assert(std::string(test_buffer.data(), 5) == "is so");
+  }
+  { // Check that reading from an empty buffer, but more than the buffer can
+    // hold works fine
+    test_buffer.resize(12);
+    assert(fb->sgetn(test_buffer.data(), 12) == 12);
+    assert(std::string(test_buffer.data(), 12) == "me random da");
+  }
+  { // Check that reading from a non-empty buffer, and more than the buffer can
+    // hold works fine Fill the buffer up
+    test_buffer.resize(2);
+    assert(fb->sgetn(test_buffer.data(), 2) == 2);
+    assert(std::string(test_buffer.data(), 2) == "ta");
+
+    // Do the actual check
+    test_buffer.resize(12);
+    assert(fb->sgetn(test_buffer.data(), 12) == 12);
+    assert(std::string(test_buffer.data(), 12) == " to be able ");
+  }
+  { // Check that trying to read more than the file size works fine
+    test_buffer.resize(30);
+    assert(fb->sgetn(test_buffer.data(), 30) == 24);
+    test_buffer.resize(24);
+    assert(std::string(test_buffer.data(), 24) == "to test buffer behaviour");
+  }
+
+  return 0;
+}

libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/xsgetn.test.dat

@@ -0,0 +1 @@
+this is some random data to be able to test buffer behaviour