Fix: Assertion failed: (!m_first_die || m_first_die == m_die_array.front()), function ExtractDIEsRWLocked

[jankratochvil]

Summary

xbolva00 bugreported $subj
It can happen only from the line:

m_die_array.back().SetEmptyChildren(true);

In the case DW_TAG_compile_unit has DW_CHILDREN_yes but there is only 0 (end of list, no children present). Therefore the assertion can fortunately happen only with a hand-crafted DWARF or with DWARF from some suboptimal compilers.
I could not find a reproducer with unpatched LLDB code, could you provide what LLDB commands could reproduce that? A debuggee binary which can reproduce it is: lldb-repro.tar.xz

[xbolva00]

reproducer archive - 404 not found

[jankratochvil]

In D53255#1264782, @xbolva00 wrote:

reproducer archive - 404 not found

Fixed, sorry.

[xbolva00]

In our case (yes, we have some "additional dwarf", we are gonna fix it on our side), it crashed with "process launch -s".

[xbolva00]

assert fix looks fine

[aprantl]

Out of curiosity: why do we allow for both an empty array and an empty children sentinel? Is that distinction useful?

[jankratochvil]

In D53255#1265520, @aprantl wrote:

Out of curiosity: why do we allow for both an empty array and an empty children sentinel? Is that distinction useful?

Filed for that new D53321.

[labath]

It looks like it should be easy to write a test for this by hand-crafting a minimal .s file and feeding it to lldb-test.

[jankratochvil]

In D53255#1267850, @labath wrote:

It looks like it should be easy to write a test for this by hand-crafting a minimal .s file and feeding it to lldb-test.

Even with such a crafted file (in lldb-repro.tar.xz) I haven't found a way how to call DWARFUnit::ExtractUnitDIEIfNeeded() first. LLDB will always call DWARFUnit::ExtractDIEsRWLocked() first which cannot reproduce the bug. @xbolva00's file must have something special that process launch -s does call DWARFUnit::ExtractUnitDIEIfNeeded() first, my testcase does not. As I guess @xbolva00 cannot provide that file to us I think it would be enough to see a backtrace to the first call of DWARFUnit::ExtractUnitDIEIfNeeded() to find it out.

lldb/trunk/source/Plugins/SymbolFile/DWARF/DWARFUnit.cpp

if (null_die) {
         die_index_stack.push_back(0);
         ++depth;
       }
       prev_die_had_children = die_has_children;
     }
   }
 
   if (!m_die_array.empty()) {
-    lldbassert(!m_first_die || m_first_die == m_die_array.front());
+    if (m_first_die) {
+      // Only needed for the assertion.
+      m_first_die.SetEmptyChildren(m_die_array.front().GetEmptyChildren());
+      lldbassert(m_first_die == m_die_array.front());
+    }
     m_first_die = m_die_array.front();
   }
 
   m_die_array.shrink_to_fit();
 
   ExtractDIEsEndCheck(offset);
 
   if (m_dwo_symbol_file) {