Make breakpad_unittests work with Chrome's test runner instead of gtest's

Chrome's test runner on Linux installs its own StackDumpSignalHandler
which swallows signals and doesn't re-raise them. This is sloppy, but
apparently there are reasons (https://crbug.com/551681). For
breakpad_unittests, it causes problems where a test process expects (via
waitpid()) to observe a child crash.  Deal with those cases by
explicitly restoring the default signal handler.

In another case, Chrome's test runner seems to have been arriving at the
conclusion that it was to expect output from a child. Transitioning from
exit() to _exit() fixes this problem, and it's not necessarily a bad
idea to do this in post-fork() children without an execve() anyway.

Bug: chromium:949098
Change-Id: I5a6af0c2a09cd8eac9998358f6d5ea665288236f
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/1575670
Reviewed-by: Mark Mentovai <mark@chromium.org>

src/client/linux/handler/exception_handler_unittest.cc

@@ -421,6 +421,10 @@ TEST(ExceptionHandlerTest, RedeliveryToDefaultHandler) {
 
   const pid_t child = fork();
   if (child == 0) {
+    // Custom signal handlers, which may have been installed by a test launcher,
+    // are undesirable in this child.
+    signal(SIGSEGV, SIG_DFL);
+
     CrashWithCallbacks(FilterCallbackReturnFalse, NULL, temp_dir.path());
   }
 

src/client/linux/minidump_writer/linux_ptrace_dumper_unittest.cc

@@ -144,7 +144,7 @@ class LinuxPtraceDumperChildTest : public testing::Test {
     if (child_pid_ == 0) {
       // child process
       RealTestBody();
-      exit(HasFatalFailure() ? kFatalFailure :
+      _exit(HasFatalFailure() ? kFatalFailure :
             (HasNonfatalFailure() ? kNonFatalFailure : 0));
     }
 
@@ -250,7 +250,7 @@ void LinuxPtraceDumperMappingsTest::SetUp() {
   helper_path_ = GetHelperBinary();
   if (helper_path_.empty()) {
     FAIL() << "Couldn't find helper binary";
-    exit(1);
+    _exit(1);
   }
 
   // mmap two segments out of the helper binary, one

src/common/linux/tests/crash_generator.cc

@@ -184,6 +184,12 @@ bool CrashGenerator::CreateChildCrash(
 
   pid_t pid = fork();
   if (pid == 0) {
+    // Custom signal handlers, which may have been installed by a test launcher,
+    // are undesirable in this child.
+    if (signal(crash_signal, SIG_DFL) == SIG_ERR) {
+      perror("CrashGenerator: signal");
+      exit(1);
+    }
     if (chdir(temp_dir_.path().c_str()) == -1) {
       perror("CrashGenerator: Failed to change directory");
       exit(1);