win: Capture some CRITICAL_SECTION debugging data

client/crashpad_client_win.cc

Index: client/crashpad_client_win.cc
diff --git a/client/crashpad_client_win.cc b/client/crashpad_client_win.cc
index 195916004198e2a517ff7b8fe30aa4f244552da9..fe79d2d0950c6d61ff46649c37a257e7fc37132f 100644
--- a/client/crashpad_client_win.cc
+++ b/client/crashpad_client_win.cc
@@ -48,6 +48,13 @@ base::Lock* g_non_crash_dump_lock;
 // dump.
 crashpad::ExceptionInformation g_non_crash_exception_information;
 
+// A `CRITICAL_SECTION` initialized with
+// `RTL_CRITICAL_SECTION_FLAG_FORCE_DEBUG_INFO` to force it to be allocated with
+// a valid .DebugInfo field. The address of this critical section is given to
+// the handler. All critical sections with debug info are linked in a
+// doubly-linked list, so this allows the handler to capture all of them.
+CRITICAL_SECTION g_critical_section_with_debug_info;
+
 LONG WINAPI UnhandledExceptionHandler(EXCEPTION_POINTERS* exception_pointers) {
   // Tracks whether a thread has already entered UnhandledExceptionHandler.
   static base::subtle::AtomicWord have_crashed;
@@ -94,6 +101,18 @@ LONG WINAPI UnhandledExceptionHandler(EXCEPTION_POINTERS* exception_pointers) {
   return EXCEPTION_CONTINUE_SEARCH;
 }
 
+BOOL CrashpadInitializeCriticalSectionEx(
+    CRITICAL_SECTION* critical_section,
+    DWORD spin_count,
+    DWORD flags) {
+  static decltype(InitializeCriticalSectionEx)* initialize_critical_section_ex =
+      reinterpret_cast<decltype(InitializeCriticalSectionEx)*>(GetProcAddress(
+          LoadLibrary(L"kernel32.dll"), "InitializeCriticalSectionEx"));
+  if (!initialize_critical_section_ex)
+    return false;
+  return InitializeCriticalSectionEx(critical_section, spin_count, flags);
+}
+
 }  // namespace
 
 namespace crashpad {
@@ -129,6 +148,14 @@ bool CrashpadClient::SetHandler(const std::string& ipc_port) {
   message.registration.non_crash_exception_information =
       reinterpret_cast<WinVMAddress>(&g_non_crash_exception_information);
 
+  if (CrashpadInitializeCriticalSectionEx(
+          &g_critical_section_with_debug_info,
+          0,
+          RTL_CRITICAL_SECTION_FLAG_FORCE_DEBUG_INFO)) {
+    message.registration.critical_section_address =

[Mark Mentovai, 2015/10/12 14:08:59]

I’m not sure this is necessary.

Sure, I understand how you’re using it here to give the handler a hint about
where to find the memory it needs for this. But the debugger doesn’t have access
to this particular critical_section_address, and !locks seems to work there just
fine. You don’t have to tell it where a CRITICAL_SECTION is in order for that to
work. This hint is never written to the minidump anywhere.

Our ProcessReader must already have all of the information it needs to find
where the the CRITICAL_SECTIONs with debug info reside, and this must also
already be making it into the minidumps we write. Maybe it’s hiding out
somewhere in the PEB?

(To be clear, I know that we need to write the block of CRITICAL_SECTIONs out as
ExtraMemory, I just don’t see how this hint is necessary at registration time.)

+        reinterpret_cast<WinVMAddress>(&g_critical_section_with_debug_info);
+  }
+
   ServerToClientMessage response = {0};
 
   if (!SendToCrashHandlerServer(