Use the libc clone wrapper in sys_clone.

sandbox/linux/services/syscall_wrappers_unittest.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "sandbox/linux/services/syscall_wrappers.h"
 
 #include <sys/syscall.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <unistd.h>
@@ skipping 21 matching lines @@
 TEST(SyscallWrappers, CloneParentSettid) {
   pid_t ptid = 0;
   pid_t child = sys_clone(CLONE_PARENT_SETTID | SIGCHLD, nullptr, &ptid,
                           nullptr, nullptr);
   TestUtils::HandlePostForkReturn(child);
   EXPECT_LT(0, child);
   EXPECT_EQ(child, ptid);
 }
 
 TEST(SyscallWrappers, CloneChildSettid) {
+  // Warm up the libc pid cache, if there is one.
+  ASSERT_EQ(sys_getpid(), getpid());
+
   pid_t ctid = 0;
   pid_t pid =
       sys_clone(CLONE_CHILD_SETTID | SIGCHLD, nullptr, nullptr, &ctid, nullptr);
 
   const int kSuccessExit = 0;
   if (0 == pid) {
-    // In child.
-    if (sys_getpid() == ctid)
+    // In child.  Check both the raw getpid syscall and the libc getpid wrapper
+    // (which may rely on a pid cache).
+    if (sys_getpid() == ctid && getpid() == ctid)
       _exit(kSuccessExit);
     _exit(1);
   }
 
   ASSERT_NE(-1, pid);
   int status = 0;
   ASSERT_EQ(pid, HANDLE_EINTR(waitpid(pid, &status, 0)));
   ASSERT_TRUE(WIFEXITED(status));
   EXPECT_EQ(kSuccessExit, WEXITSTATUS(status));
 }
 
 }  // namespace
 
 }  // namespace sandbox