[Lldb-commits] [lldb] [lldb] Fix libstdc++ std::string formatter after #147835 (PR #152993)

[David Spickett via lldb-commits]

https://github.com/DavidSpickett created https://github.com/llvm/llvm-project/pull/152993

https://github.com/llvm/llvm-project/pull/147835 made changes that caused libstdc++ std::string tests to fail:
```
======================================================================
FAIL: test_unavailable_summary_libstdcxx_dwo (TestDataFormatterStdString.StdStringDataFormatterTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/tcwg-buildbot/worker/lldb-aarch64-ubuntu/llvm-project/lldb/packages/Python/lldbsuite/test/lldbtest.py", line 1805, in test_method
    return attrvalue(self)
  File "/home/tcwg-buildbot/worker/lldb-aarch64-ubuntu/llvm-project/lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/string/TestDataFormatterStdString.py", line 223, in test_unavailable_summary_libstdcxx
    self.do_test_summary_unavailable()
  File "/home/tcwg-buildbot/worker/lldb-aarch64-ubuntu/llvm-project/lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/string/TestDataFormatterStdString.py", line 213, in do_test_summary_unavailable
    self.assertEqual(summary, "Summary Unavailable", "No summary for bad value")
AssertionError: '(null)' != 'Summary Unavailable'
- (null)
+ Summary Unavailable
 : No summary for bad value
Config=aarch64-/home/tcwg-buildbot/worker/lldb-aarch64-ubuntu/build/bin/clang
----------------------------------------------------------------------
```

This test constructs an invalid std::string by starting with a null pointer.

Somehow this improvement in Clang uncovered a bug in the formatter. The formatter tries to access `_M_p` and checked whether the resulting ValueObjectSP was null, but not that it did not contain an error value.

I think that error value can be there if you are able to access one part of the path, `_M_dataplus`, but another part fails. Since the layout looks like this:
```
      struct _Alloc_hider :
      {
	      pointer _M_p; // The actual data.
      };

      _Alloc_hider	_M_dataplus;

      void
      _M_data(pointer __p)
      { _M_dataplus._M_p = __p; }
```
So I think we were able to read `_M_dataplus` just by offset, but then failed because it contains, or points to something containing nulls or a null pointer.

Or perhaps an error value means we know what the class member is, but could not read from it.

I found this by comparing with the libcxx formatter, so I've copied the same handling from there to fix the issue.

>From facece685fe39b8fbeca3261183a5aeac8dcee4c Mon Sep 17 00:00:00 2001
From: David Spickett <david.spickett at linaro.org>
Date: Mon, 11 Aug 2025 10:43:37 +0000
Subject: [PATCH] [lldb] Fix libstdc++ std::string formatter after #147835

https://github.com/llvm/llvm-project/pull/147835 made changes that
caused libstdc++ std::string tests to fail:
```
======================================================================
FAIL: test_unavailable_summary_libstdcxx_dwo (TestDataFormatterStdString.StdStringDataFormatterTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/tcwg-buildbot/worker/lldb-aarch64-ubuntu/llvm-project/lldb/packages/Python/lldbsuite/test/lldbtest.py", line 1805, in test_method
    return attrvalue(self)
  File "/home/tcwg-buildbot/worker/lldb-aarch64-ubuntu/llvm-project/lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/string/TestDataFormatterStdString.py", line 223, in test_unavailable_summary_libstdcxx
    self.do_test_summary_unavailable()
  File "/home/tcwg-buildbot/worker/lldb-aarch64-ubuntu/llvm-project/lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/string/TestDataFormatterStdString.py", line 213, in do_test_summary_unavailable
    self.assertEqual(summary, "Summary Unavailable", "No summary for bad value")
AssertionError: '(null)' != 'Summary Unavailable'
- (null)
+ Summary Unavailable
 : No summary for bad value
Config=aarch64-/home/tcwg-buildbot/worker/lldb-aarch64-ubuntu/build/bin/clang
----------------------------------------------------------------------
```

This test constructs an invalid std::string by starting with a null pointer.

Somehow this improvement in Clang uncovered a bug in the formatter. The formatter
tries to access `_M_p` and checked whether the resulting ValueObjectSP was null,
but not that it did not contain an error value.

I think that error value can be there if you are able to access one part
of the path, `_M_dataplus`, but another part fails. Since the layout looks like this:
```
      struct _Alloc_hider :
      {
	      pointer _M_p; // The actual data.
      };

      _Alloc_hider	_M_dataplus;

      void
      _M_data(pointer __p)
      { _M_dataplus._M_p = __p; }
```
So I think we were able to read `_M_dataplus` just by offset, but
then failed because it contains, or points to something containing
nulls or a null pointer.

Or perhaps an error value means we know what the class member is,
but could not read from it.

I found this by comparing with the libcxx formatter, so I've copied
the same handling from there to fix the issue.
---
 lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp b/lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp
index 595e835b37df9..f4a695e036999 100644
--- a/lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp
+++ b/lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp
@@ -241,10 +241,11 @@ VectorIteratorSyntheticFrontEnd::GetIndexOfChildWithName(ConstString name) {
 bool lldb_private::formatters::LibStdcppStringSummaryProvider(
     ValueObject &valobj, Stream &stream, const TypeSummaryOptions &options) {
   ValueObjectSP ptr = valobj.GetChildAtNamePath({"_M_dataplus", "_M_p"});
-  if (!ptr)
-    return false;
+  if (!ptr || !ptr->GetError().Success())
+    stream << "Summary Unavailable";
+  else
+    stream << ptr->GetSummaryAsCString();
 
-  stream << ptr->GetSummaryAsCString();
   return true;
 }