[libc] Add write(2) implementation for Linux and FDReader test utility

Summary: Adds `write` for Linux and FDReader utility which should be useful for some stdio tests as well.

Reviewers: sivachandra, PaulkaToast

Reviewed By: sivachandra

Subscribers: mgorny, tschuett, libc-commits

Differential Revision: https://reviews.llvm.org/D78184

libc/utils/testutils/FDReaderUnix.cpp

@@ -0,0 +1,41 @@
+//===-- FDReader.cpp ------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "FDReader.h"
+#include "llvm/Support/MemoryBuffer.h"
+#include <cassert>
+#include <cstring>
+#include <unistd.h>
+
+namespace __llvm_libc {
+namespace testutils {
+
+FDReader::FDReader() {
+  int err = ::pipe(pipefd);
+  assert(!err && "pipe(2) failed");
+}
+
+FDReader::~FDReader() {
+  ::close(pipefd[0]);
+  ::close(pipefd[1]);
+}
+
+bool FDReader::matchWritten(const char *str) {
+  llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> bufOrErr =
+      llvm::MemoryBuffer::getOpenFile(pipefd[0], "<pipe>",
+                                      /* FileSize (irrelevant) */ 0);
+  if (!bufOrErr) {
+    assert(0 && "Error reading from pipe");
+    return false;
+  }
+  const llvm::MemoryBuffer &buf = **bufOrErr;
+  return !std::strncmp(buf.getBufferStart(), str, buf.getBufferSize());
+}
+
+} // namespace testutils
+} // namespace __llvm_libc