Fix tuple getter in std unique pointer pretty-printer

[aleksandr.urakov]

Summary

Check case when _M_t child member is not present.

[labath]

Did you investigate when does this happen? I take it it can happen if a user has a custom std::unique_ptr definition that kinda looks like the libstdc++ one, but not really, but I am wondering if we're not missing something else here..

[aleksandr.urakov]

This happened with MSVC's std::unique_ptr implementation. But I stepped through whole std::make_unique, and this was the only place, where lldb crashed.

[labath]

Aha I see.

I think the root cause here is that this data formatter even gets invoked for MSVC's std::unique_ptr. Unfortunately, it seems that won't be able to differentiate easily, as the libstdc++ unique_ptr comes out simply as std::unique_ptr, without any tell-tale signs like __1 namespace that libc++ data types tend to have. If we ever get to implement unique_ptr formatter for MSVC type, we will probably have to have a single class that supports both. But this should be good enough for now.

[tberghammer]

BTW this can also happen if we are missing some debug info.

[aleksandr.urakov]

Please, commit this for me? I have no commit access.

[labath]

Committed as r334411. I also squeezed in a quick test.

lldb/trunk/packages/Python/lldbsuite/test/functionalities/data-formatter/data-formatter-stl/libstdcpp/unique_ptr/invalid/TestDataFormatterInvalidStdUniquePtr.py

+import lldbsuite.test.lldbinline as lldbinline
+from lldbsuite.test.decorators import *
+
+lldbinline.MakeInlineTest(__file__, globals(), [no_debug_info_test])
+

lldb/trunk/packages/Python/lldbsuite/test/functionalities/data-formatter/data-formatter-stl/libstdcpp/unique_ptr/invalid/main.cpp

+// Test that we don't crash when trying to pretty-print structures that don't
+// have the layout our data formatters expect.
+namespace std {
+template<typename T, typename Deleter = void>
+class unique_ptr {};
+}
+
+int main() {
+  std::unique_ptr<int> U;
+  return 0; //% self.expect("frame variable U", substrs=["unique_ptr", "{}"])
+}

lldb/trunk/source/Plugins/Language/CPlusPlus/LibStdcppUniquePointer.cpp

if (!valobj_backend_sp)
     return nullptr;
 
   ValueObjectSP valobj_sp = valobj_backend_sp->GetNonSyntheticValue();
   if (!valobj_sp)
     return nullptr;
 
   ValueObjectSP obj_child_sp =
       valobj_sp->GetChildMemberWithName(ConstString("_M_t"), true);
+  if (!obj_child_sp)
+      return nullptr;
 
   ValueObjectSP obj_subchild_sp =
       obj_child_sp->GetChildMemberWithName(ConstString("_M_t"), true);
 
   // if there is a _M_t subchild, the tuple is found in the obj_subchild_sp
   // (for libstdc++ 6.0.23).
   if (obj_subchild_sp) {
     return obj_subchild_sp;