Handle race in external/breakpad

...that hit once in a UBSan build in UITest_calc_tests4:

> soffice.bin: /data/sbergman/lo-san/core/include/com/sun/star/uno/Reference.h:420: interface_type *com::sun::star::uno::Reference<com::sun::star::xml::crypto::XXMLSecurityContext>::operator->() const [interface_type = com::sun::star::xml::crypto::XXMLSecurityContext]: Assertion `_pInterface != NULL' failed.
> warn:desktop:11041:11425:desktop/source/app/sofficemain.cxx:82: minidump generated: /data/sbergman/lo-san/core/workdir/UITest/calc_tests4/user/crash//0585280b-6f1e-fa99-4b711485-60baa6f1.dmp
> src/client/linux/handler/exception_handler.cc:367:34: runtime error: member call on null pointer of type 'std::vector<google_breakpad::ExceptionHandler *, std::allocator<google_breakpad::ExceptionHandler *> >'
> warn:sal.osl.mutex:11041:11041:sal/osl/unx/mutex.cxx:82: pthread_mutex_destroy failed: Device or resource busy
>  #0 in google_breakpad::ExceptionHandler::SignalHandler(int, siginfo_t*, void*) at <null> (instdir/program/libsofficeapp.so +0x963904)
> warn:unotools.config:11041:11041:unotools/source/config/configmgr.cxx:169: ConfigManager not empty
>  #1  at <null> (/lib64/libpthread.so.0 +0x11fbf)

Change-Id: Icf46adea879c0541a297e5f54f9ce936caa9858b
Reviewed-on: https://gerrit.libreoffice.org/59157
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

external/breakpad/0001-Handle-race-between-ExceptionHandler-SignalHandler-a.patch.1

+From caa6f1ea462d0f0c612b871106e3e309fe0290f5 Mon Sep 17 00:00:00 2001
+From: Stephan Bergmann <sbergman@redhat.com>
+Date: Thu, 16 Aug 2018 09:04:35 +0200
+Subject: [PATCH] Handle race between ExceptionHandler::SignalHandler and
+ ~ExceptionHandler
+
+...where thread A is blocked locking g_handler_stack_mutex_ in SignalHandler
+while thread B executes ~ExceptionHandler and sets g_handler_stack to null, but
+which thread A didn't expect to be null once it acquired the lock.
+---
+ src/client/linux/handler/exception_handler.cc | 6 ++++--
+ 1 file changed, 4 insertions(+), 2 deletions(-)
+
+diff --git a/src/client/linux/handler/exception_handler.cc b/src/client/linux/handler/exception_handler.cc
+index b895f6d7..4d58e510 100644
+--- a/src/client/linux/handler/exception_handler.cc
++++ b/src/client/linux/handler/exception_handler.cc
+@@ -372,8 +372,10 @@ void ExceptionHandler::SignalHandler(int sig, siginfo_t* info, void* uc) {
+   }
+ 
+   bool handled = false;
+-  for (int i = g_handler_stack_->size() - 1; !handled && i >= 0; --i) {
+-    handled = (*g_handler_stack_)[i]->HandleSignal(sig, info, uc);
++  if (g_handler_stack_ != nullptr) {
++    for (int i = g_handler_stack_->size() - 1; !handled && i >= 0; --i) {
++      handled = (*g_handler_stack_)[i]->HandleSignal(sig, info, uc);
++    }
+   }
+ 
+   // Upon returning from this signal handler, sig will become unmasked and then
+-- 
+2.17.1
+

external/breakpad/UnpackedTarball_breakpad.mk

@@ -13,12 +13,16 @@ $(eval $(call gb_UnpackedTarball_set_patchlevel,breakpad,0))
 
 $(eval $(call gb_UnpackedTarball_set_tarball,breakpad,$(BREAKPAD_TARBALL)))
 
+# external/breakpad/0001-Handle-race-between-ExceptionHandler-SignalHandler-a.patch upstreamed at
+#  <https://chromium-review.googlesource.com/c/breakpad/breakpad/+/1176811> "Handle race between
+#  ExceptionHandler::SignalHandler and ~ExceptionHandler":
 $(eval $(call gb_UnpackedTarball_add_patches,breakpad,\
 	external/breakpad/breakpad-use-correct-http-header.patch.1 \
 	external/breakpad/breakpad-wshadow.patch.1 \
 	external/breakpad/breakpad-wshadow2.patch.1 \
 	external/breakpad/breakpad-stackwalk.patch.1 \
 	external/breakpad/ucontext.patch \
+	external/breakpad/0001-Handle-race-between-ExceptionHandler-SignalHandler-a.patch.1 \
 ))
 
 ifeq ($(COM_IS_CLANG),TRUE)