Add the MultiprocessExec test and its test

util/test/multiprocess_exec_test_child.cc

+// Copyright 2014 The Crashpad Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <errno.h>
+#include <limits.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include <algorithm>
+
+int main(int argc, char* argv[]) {
+  // Make sure that there’s nothing open at any FD higher than 3. All FDs other
+  // than stdin, stdout, and stderr should have been closed prior to or at
+  // exec().
+  int max_fd = std::max(static_cast<int>(sysconf(_SC_OPEN_MAX)), OPEN_MAX);
+  max_fd = std::max(max_fd, getdtablesize());
+  for (int fd = STDERR_FILENO + 1; fd < max_fd; ++fd) {
+    if (close(fd) == 0 || errno != EBADF) {
+      abort();
+    }
+  }
+
+  // Read a byte from stdin, expecting it to be a specific value.
+  char c;
+  ssize_t rv = read(STDIN_FILENO, &c, 1);
+  if (rv != 1 || c != 'z') {
+    abort();
+  }
+
+  // Write a byte to stdout.
+  c = 'Z';
+  rv = write(STDOUT_FILENO, &c, 1);
+  if (rv != 1) {
+    abort();
+  }
+
+  return 0;
+}