test::Multiprocess: exit cleanly from children with _exit().

Child exits were using exit(), which caused crashes on 10.10 for
Multiprocess-based tests that ran after HTTPTransport tests. The crashes
occurred while running exit-time destructors. exit() was never correct
in this situation, this should have used _exit() all along.

TEST=crashpad_util_test
BUG=crashpad:17
R=rsesek@chromium.org

Review URL: https://codereview.chromium.org/1000653002

util/test/multiprocess.h

@@ -164,9 +164,9 @@ class Multiprocess {
   //! \brief Runs the child side of the test.
   //!
   //! This method establishes the child’s environment, calls
-  //! MultiprocessChild(), and exits cleanly. However, if any failure (via fatal
+  //! MultiprocessChild(), and exits cleanly by calling `_exit(0)`. However, if
-  //! or nonfatal gtest assertion) is detected, the child will exit with a
+  //! any failure (via fatal or nonfatal gtest assertion) is detected, the child
-  //! failure status.
+  //! will exit with a failure status.
   void RunChild();
 
   //! \brief The subclass-provided parent routine.
@@ -186,6 +186,11 @@ class Multiprocess {
   //! `FAIL()`, etc.
   //!
   //! Subclasses must implement this method to define how the child operates.
+  //! Subclasses may exit with a failure status by using `LOG(FATAL)`,
+  //! `abort()`, or similar. They may exit cleanly by returning from this method
+  //! or by calling `_exit(0)`. Under no circumstances may `exit()` be called
+  //! by the child without having the child process `exec()`. Use
+  //! MultiprocessExec if the child should call `exec()`.
   virtual void MultiprocessChild() = 0;
 
   internal::MultiprocessInfo* info_;

util/test/multiprocess_posix.cc

@@ -212,7 +212,8 @@ void Multiprocess::RunChild() {
     return;
   }
 
-  exit(0);
+  // In a forked child, exit() is unsafe. Use _exit() instead.
+  _exit(EXIT_SUCCESS);
 }
 
 }  // namespace test